Oracle database reliable message waits

This blog is about the oracle database wait event ‘reliable message’. It should be noted that normally, this should not be a prominent wait, and if it does so, the most logical cause would be something that is not working as it should, either by oversubscribing something or simply because of a bug.

The reliable message note on My Oracle Support provides a good description:
https://support.oracle.com/epmos/faces/DocContentDisplay?_afrLoop=34595019638128&id=69088.1&_afrWindowMode=0&_adf.ctrl-state=15ief27plw_97

It does tell something about what a reliable message is:
– A message send using the KSR intra-instance broadcast service.
– The message publisher waits on this wait event until all subscribers have consumed the message.
– The publisher waits for up to one second and then retests if all subscribers have consumed the message.

So the first thing to establish is that a reliable message means that the publisher of the message does wait for all subscribers of the channel it published its message on to have finished consuming the message. That is what ‘reliable’ means with reliable message: all subscribers are guaranteed to have consumed the message. Not all messages in the Oracle executable are ‘reliable’; if you take the log writer notification during redo generation in kcrfw_redo_gen_ext>kcrf_commit_force_int for example, it will only signal the log writer if it isn’t signalled before in the first place, and if it does, it’s just a nudge for the log writer to start its task, there is nothing stateful about the message and in the message itself, like an SCN to write up to.

The second thing to think about is that the KSR/reliable message mechanism to my knowledge itself does not suffer from issues or widespread bugs. This means that, for what I’ve seen (for what that’s worth :-D), the KSR code for reliable messaging is unlikely to be an issue itself if you see this event. It is probably the function of the message that the KSR code is trying to publish on the channel to its subscribers that is causing the reliable message wait.

For this to dive deeper into, I first need to restate what I’ve said at the beginning: in order to sensibly investigate long times spend in a wait event, any wait event really, the very first thing you need to look for is if the system the database is running on is functioning in a normal way. In other words: if the issue you see being represented by a wait event is not an issue outside of the database, for which the database has made your work easier because it has timed the manifestation of the issue outside of the database for you, instead of being an issue inside the database.

For the reliable message to be investigated, you need to understand the logic of reliable messaging in the oracle database. Let me take a truncate table as an example; a ‘truncate table ‘ requires the current blocks to be written to disk. To do that, it sends a reliable message to the checkpointer via the ‘kcbo’ channel, for which the description is ‘obj broadcast channel’ channel.

If you freeze the checkpointer process, it will not be able to do anything, and thus it cannot consume the message that is send by the process that executes a truncate. One way of freezing a process is to attach to it with a debugger, like gdb. Of course this only ever should be done in a test environment and not in production, because freezing (background) processes can have all kinds of bad side effects, and it will almost certainly influence anything else that is done.

If you look at the truncating process, you will see the wait event ‘reliable message’. Take the p1 value from the waiting process and execute the following SQL:

-- replace <p1> with the p1 value of the reliable message wait event
select des.id_ksrcdes, des.name_ksrcdes
from x$ksrcctx ctx, x$ksrcdes des 
where to_number(ctx.addr,'XXXXXXXXXXXXXXXX') = <p1> 
and ctx.name_ksrcctx = des.indx;

This shows the channel and the description of the channel the reliable message is waiting for:

ID_KSRCDES                        NAME_KSRCDES
--------------------------------- ---------------------------------------------
kcbo                              obj broadcast channel

To understand more you can use the x$ksrchdl view and link these with the publishers and subscribers of the channel context:

-- inspired by and largely taken from Tanel Poder's channels2.sql script (https://github.com/tanelpoder/tpt-oracle/blob/master/channel2.sql)
-- replace <p1> with the p1 value of the reliable message wait event
select case when bitand(c.flags_ksrchdl,1)=1 then 'PUB ' end || 
       case when bitand(c.flags_ksrchdl,2)=2 then 'SUB ' end || 
       case when bitand(c.flags_ksrchdl,16)=16 then 'INA ' end flags,
       s.sid,
       p.program,
       cd.name_ksrcdes channel_name, 
       cd.id_ksrcdes, 
       c.ctxp_ksrchdl
from   x$ksrchdl c , 
       v$process p, 
       v$session s, 
       x$ksrcctx ctx, 
       x$ksrcdes cd
where 1=1
and to_number(c.ctxp_ksrchdl,'XXXXXXXXXXXXXXXX') = <p1>
and s.paddr(+)=c.owner_ksrchdl 
and p.addr(+)=c.owner_ksrchdl
and c.ctxp_ksrchdl=ctx.addr
and cd.indx=ctx.name_ksrcctx
;

In my case, this shows the following information:

FLAGS          SID PROGRAM                                                      CHANNEL_NAME                             ID_KSRCDES CTXP_KSRCHDL
------------ ----- ------------------------------------------------------------ ---------------------------------------- ---------- ----------------
SUB            258 oracle@oracle-database.local (CKPT)                          obj broadcast channel                    kcbo       000000007AAB3020
PUB            141 oracle@oracle-database.local (M001)                          obj broadcast channel                    kcbo       000000007AAB3020
PUB            142 oracle@oracle-database.local (W005)                          obj broadcast channel                    kcbo       000000007AAB3020
PUB            145 oracle@oracle-database.local (W007)                          obj broadcast channel                    kcbo       000000007AAB3020
PUB            396 oracle@oracle-database.local (TNS V1-V3)                     obj broadcast channel                    kcbo       000000007AAB3020

The important bit here is that this channel; “kcbo” has 4 publishers (“my” session, and the background processes M001, W005 and W007) and one subscriber: CKPT, the checkpoint processes. In this case, the information about the process we are waiting for could be guessed, not only because I caused the waiting situation myself, but because the “final_blocking_session” field in v$session would point you to it. But in the case of multiple subscribers, you can use this query to find all the processes that are currently participating in the channel.

A subscribing session essentially does 3 things of importance to the reliable message wait:
1. It finds a published message in the channel it is subscribed to, and starts acting on the message.
2. It performs the task or tasks that should be done as a subscriber to that channel.
3. It ‘consumes’ the message, leading to end the reliable message wait event for the publisher.

In the case of a truncate, the foreground process executing the truncate finds the handle, adds itself as publisher to the handle, adds the message and then posts the subscriber(s) using ‘semop’ to activate them. Then it goes into a function called ‘ksrpubwait_ctx’, and waits for the subscribers to do their work in the ‘reliable message’ wait event. The wait is implemented as a semaphore wait (semtimedop) with a 1 second timeout, as the MOS note indicates.

The subscriber, which is the checkpointer process, will be woken from its semtimedop “sleep” if idle, and goes off performing its work cycle. One of the calls in the checkpointer work cycle is ksb_act_run>ksb_act_run_int>kcbo_subch, which performs the task of inspecting the kcbo “KSR channel” to see if work is published.

If so, it performs the task of setting the RO enqueues and add a checkpoint to the checkpoint queue of type 10: object reuse checkpoint and post the logwriter, after which the checkpointer is done, which means it will post the publisher.

Once the publisher is posted, which is the foreground, it will stop waiting for the ‘reliable message’ wait event, and continue on working. Depending on the speed of all the processes and lots of other things, the next thing that happens might be that the foreground now will try to uniquely obtain the RO enqueues and will not be able to do that, or the background processes might have finished their work and released the RO enqueues. But these dealings are not the point of this blogpost.

What I do hope you understand when you’ve reached this part, is that the ‘reliable message’ event is part of the database infrastructure that serves different functions, and that you should look into the specific function for which the KSR infrastructure is used instead of the KSR infrastructure itself.

Oracle database processes and waiting long in the supermarket

Hopefully I got your interest by the weird name of this blogpost. This blogpost is about sensible usage of an Oracle database. Probably, there are a lot of blog posts like this, but I need to get this off my chest.

A quote any starwars fan would recognise is ‘I sense a disturbance in the force’. I do, and I have felt it for a long time. This disturbance is the usage of the number of connections for a database. Because of my profession, this is the oracle database, but this really applies to the server-side of any client/server server processor running on at least (but probably not limited to) intel Xeon processors.

The disturbance is the oversubscription or sometimes even excessive oversubscription of database connections from application servers, or any other means of database processes that acts as clients. This means this does not exclude parallel query processes, in other words: this applies to parallel query processes too.

What is oversubscription of database connections? Any good consultant would be able to tell you this: it depends. In principle, oversubscription means more subscribers than a system can handle. This means ‘oversubscription’ is a multidimensional beast, which can apply to CPU, memory, disk IO, network IO. That makes it hard.

This blogpost is about CPU oversubscription. The way a modern CPU, Intel Xeon in this case, works is not simple, and thus this will and cannot be an exhaustive description. What I want to try is provide a basic, simplistic description to provide an idea and give guidance.

An Intel Xeon CPU provides a number of processing units, called ‘processor’ in /proc/cpuinfo, called ‘cpu’ in the top utility, etc, which are the execution contexts for processes. These execution contexts can be hyperthreads, which for Intel Xeon are two threads per core, or an execution context for a single core. Not all Xeon CPUs provide hyperthreading, and hyperthreading, if available in the CPU, can be disabled in a system’s BIOS. A hyperthread can not, and does not do any processing, that is what a core does.

So why do hyperthreads exist then? I think two prominent reasons are:
1. Not all processes are active all the time. By increasing the number of execution contexts, the switching for processes between execution contexts is reduced, which means a reduction of time spend on context switches.
2. Cores are incredibly powerful, and given that probably not all processes are active all the time, combining two processes on the same core will be “reasonably unnoticeable”.

The next question which then comes to mind is: is hyperthreading a good thing for a database? I cannot give an exhaustive answer for this, partially because there is massive difference between system usage for different types of database usage. I would say that with reasonable usage, hyperthreading in modern Intel Xeon CPUs does provide more benefits, reduced context switching, than it gives downsides, like variances in latency of CPU usage.

This means that for looking at the CPU processing power of a database server the ‘actual’ processing power sits logically between the number of core’s and the number of threads. But wait! Didn’t I just say that threads don’t process, only core’s do? Yes, but let me explain how I look at this: if your processes do processing requiring ACTUAL, ON CPU processing, it means they depend on the core to be able to handle this, versus processes that work by running into waiting really quickly, like doing disk IO, network IO or waiting for users to make them active, which still might appear as running all the time, but in reality are actually processing on the core occasionally.

The first type, doing the actual, on cpu processing should calculate CPU power more towards core count, and the second type, doing lots of stalls, should calculate CPU more towards thread count. Overall, this is quite simply about using a core as efficient as possible.

Now that we gone through CPUs and their cores and threads, and oversubscription in general, the next question is: so how much processes should be allocated on a database server?

The answer is simple: if you want a high performing database for your application servers, the number of processes IN TOTAL should not exceed a number sitting somewhere between CPU core count and CPU thread count.

“Isn’t that incredibly low?”

Yes, for most of the deployments that I see this would be shockingly low. But just because the number of processes is set very high somewhere doesn’t make it right. It just means it’s set that way.

“But why is it set too high everywhere?”

I don’t know. I don’t understand why lots and lots of people do allocate high, up to sometimes ASTRONOMICAL numbers of database processes, and then expect that to be the best tuned way, while there is NO LOGICAL EXPLANATION that I can see for this to make sense. In fact: the explanation why this doesn’t make sense is this blogpost.

To make the comparison with a supermarket and the number of tills: if you go shopping in a supermarket and want to pay and leave as soon as possible, there should be a ready, idle till available, or else you have to wait. For Intel Xeon hyperthreading, you could make the comparison with a till that serves two lanes with persons that want to pay at the same time, because it takes time to put all the items from the shopping basket onto the desk, and the more time that takes, the more efficient a till serving two lanes would be (an Intel Xeon CPU can actually serve two threads at the same time, optimising runtime on the single core).

“Okay, but the majority of the processes is not actually doing anything.”

Well, if the processes are actually not doing anything, why have them in the first place? If that is really true, it doesn’t make sense to have them. And don’t forget: what looks like an idle connection from both an application and a database perspective still is an actual live, running operating system process, and a running database process that has memory allocated, occupies a network socket, has breakable parse locks out, etc, and requires CPU time to maintain the connection.

In fact, by having huge numbers of database connections, you have setup the application to be able to cause “the perfect storm” on the database!

With this, I mean that what I normally see, is that indeed the majority of the database connections are not used. However… if things get worse, and the database gets active and starts lacking CPU processing power, more database connections get active. That is logical, right? The database connections that normally would be active will take longer time because of the increased activity, so with a constant amount of work, new work cannot use an existing connection because that is still active, and thus take another connection that normally would sit idle. However, serving more connections will increase the amount of CPU required even further, which was already lacking, so the waiting time increases further. Now because the waiting time gets higher, more connections are needed, etc.

And then I didn’t talk about dynamically increasing connection pools!

What I mean with that is that I until now talked about STATIC connection pools. Static means the minimal number of connections is the maximal number of connections in the pool. A dynamic connection pool will have a certain amount of connections, and when there is a need for more, which means all the connections are busy, add more connections.

Especially with Oracle, this is really a bad idea. Let me explain. Outside of too much connections in the first place, which is a bad idea already, having an expanding connection pool means not only idle connections are put to work, but instead the database is given EVEN MORE work by initialising new connections. An oracle database connection is not lightweight, it requires initialising memory, which is an expensive operation. And the whole reason the connection is created is because the connection pool established all the connections were busy, which almost certainly is because the database was busy (!!!!).

I hope a lot of people will make it to the end, and then realise that high numbers of connections does not make any sense. If you do have an explanation that makes sense, please comment. Please mind that a tuned setup requires an application server to be reasonably setup too, you cannot have one part setup for ultimate processing power, and another part be just a shipwreck.

What’s new with Oracle database 11.2.0.4.200714 versus 11.2.0.4.201020

This blogpost takes a look at the technical differences between Oracle database 11.2.0.4 PSU 200714 (july 2020) and PSU 201020 (october 2020). This gives technical specialists an idea of the differences, and gives them the ability to assess if the PSU impacts anything.

Functions

code symbol names unique in version 11.2.0.4.200714 versus 11.2.0.4.201020

NAME                                                         RESOLVE                                            ANNOTATION                                                                                 ARCHIVE_OBJECT
------------------------------------------------------------ -------------------------------------------------- ------------------------------------------------------------------------------------------ ------------------------------------------------------------
kglrfSetNiv                                                  (kgl)rfSetNiv                                      kernel generic library cache management ??                                                 libgeneric11.a:kgl2.o

code symbol names unique in version 11.2.0.4.201020 versus 11.2.0.4.200714

NAME                                                         RESOLVE                                            ANNOTATION                                                                                 ARCHIVE_OBJECT
------------------------------------------------------------ -------------------------------------------------- ------------------------------------------------------------------------------------------ ------------------------------------------------------------
__init_0.0                                                   __init_0.0                                         ??
jox_get_cbrls_lock                                           (jox)_get_cbrls_lock                               java jit compiler ??
kglNivHTComp                                                 (kgl)NivHTComp                                     kernel generic library cache management ??                                                 libgeneric11.a:kgl2.o
kglNivHTHash                                                 (kgl)NivHTHash                                     kernel generic library cache management ??                                                 libgeneric11.a:kgl2.o
kglrfAddNivHT                                                (kgl)rfAddNivHT                                    kernel generic library cache management ??                                                 libgeneric11.a:kgl2.o
kglrfDestroyNivHT                                            (kgl)rfDestroyNivHT                                kernel generic library cache management ??                                                 libgeneric11.a:kgl2.o
kglrfSetNivHT                                                (kgl)rfSetNivHT                                    kernel generic library cache management ??                                                 libgeneric11.a:kgl2.o
next_marker                                                  (n)ext_marker                                      network ??                                                                                 libjavavm11.a:jdmarker.o
slrac_end_lock                                               (slrac)_end_lock                                   operating system (dependent) library verify (memory) read access ??                        libgeneric11.a:slrac.o

This is a poor, yet one of the only ways, to understand what is going on inside the oracle executable.
There aren’t that many things removed or added. Of course this is version 11.2.0.4, for which this PSU might be the last one because of 11.2.0.4’s expiring extended support state.

Quite striking here too is the absence of all the changed libjava related functions, for which I don’t know if these function additions have anything to do with security. However, please mind I am not implying that this is not secure, I simply can’t tell.

For the added and removed functions for this version: the only function that was added in version 12.1.0.2 is slrac_end_lock, all the other changes to functions seem to be completely unique to version 11.2.0.4.
If you did read all the functions closely, you might have seen that actually there was one libjava related function changed, and that my name resolving showed its limitations, because it marked it as ‘network’. Well, it seems that certain things, like the java functions do not follow the oracle database layered naming by letters approach.
It also seems that for adding and removing functions, the functions are mainly library cache (kgl) related.

In the data dictionary there weren’t any changes spotted. Of course this overview focusses on availability of parameters and metadata (tables and their columns), for a look into the data dictionary you should look at the work of my colleague Rodrigo Jorge (dissecting the 201020 patches)

What’s new with Oracle database 12.1.0.2.200714 versus 12.1.0.2.201020

This blogpost takes a look at the technical differences between Oracle database 12.1.0.2 PSU 200714 (july 2020) and PSU 201020 (october 2020). This gives technical specialists an idea of the differences, and gives them the ability to assess if the PSU impacts anything.

Functions

code symbol names unique in version 12.1.0.2.200714 versus 12.1.0.2.201020

NAME                                                         RESOLVE                                            ANNOTATION                                                                                 ARCHIVE_OBJECT
------------------------------------------------------------ -------------------------------------------------- ------------------------------------------------------------------------------------------ ------------------------------------------------------------
defCtx_GetRngCtx                                             defCtx_GetRngCtx                                   ??                                                                                         libnnzst12.a:ext_ztrsaadapter.o,libnnz12.a:ztrsaadapter.o
kgiflpop                                                     (kgi)flpop                                         kernel generic instantiation manager ??                                                    libgeneric12.a:kgi.o
kkfdGetQueReason                                             (kkfd)GetQueReason                                 kernel compile fast dataflow (PQ DFO) ??                                                   libserver12.a:kkfd.o
kkodpCheckPrefetch                                           (kkodp)CheckPrefetch                               kernel compile optimizer AQP ??                                                            libserver12.a:kkodp.o
kkodpHJLegal                                                 (kkodp)HJLegal                                     kernel compile optimizer AQP ??                                                            libserver12.a:kkodp.o
kkogimpCheckIMCValidity                                      (kko)gimpCheckIMCValidity                          kernel compile optimizer ??                                                                libserver12.a:kkoop.o
knasnblp2                                                    (knas)nblp2                                        kernel replication apply server ??                                                         libserver12.a:knasn.o
krvxuc                                                       (krvx)uc                                           kernel redo recovery extract ??                                                            libserver12.a:krvx.o
ksfd_estimate_fobpools                                       (ksfd)_estimate_fobpools                           kernel service functions disk IO ??                                                        libserver12.a:ksfd.o
ktcnq_get_typesize                                           (ktc)nq_get_typesize                               kernel transaction control component ??                                                    libserver12.a:ktcnq.o
ktuTempAddExtent_swcbk                                       (ktu)TempAddExtent_swcbk                           kernel transaction undo ??                                                                 libserver12.a:ktu.o
ztchmd5m                                                     (zt)chmd5m                                         security encryption ??                                                                     libnnzst12.a:ext_ztchmd5.o,libnnz12.a:ztchmd5.o
ztcriv                                                       (zt)criv                                           security encryption ??                                                                     libnnzst12.a:ext_ztcr.o,libnnz12.a:ztcr.o
ztcrsg                                                       (zt)crsg                                           security encryption ??                                                                     libnnzst12.a:ext_ztcr.o,libnnz12.a:ztcr.o

code symbol names unique in version 12.1.0.2.201020 versus 12.1.0.2.200714

NAME                                                         RESOLVE                                            ANNOTATION                                                                                 ARCHIVE_OBJECT
------------------------------------------------------------ -------------------------------------------------- ------------------------------------------------------------------------------------------ ------------------------------------------------------------
kcbo_exam_buf                                                (kcbo)_exam_buf                                    kernel cache buffers object queue ??                                                       libserver12.a:kcbo.o
kgnfs_isstaterenewal                                         (kgnfs)_isstaterenewal                             kernel generic network file system ??                                                      libgeneric12.a:kgnfs.o
kjbrnd2dummy                                                 (kjbr)nd2dummy                                     kernel lock management global cache service resource table ??                              libserver12.a:kjbr.o
kkfd_par_all                                                 (kkfd)_par_all                                     kernel compile fast dataflow (PQ DFO) ??                                                   libserver12.a:kkfd.o
kkqjpdClearFroCtx                                            (kkqjpd)ClearFroCtx                                kernel compile query  join analysis predicate push down ??                                 libserver12.a:kkqjpd.o
kksResetPMONSessionState                                     (kks)ResetPMONSessionState                         kernel compile shared objects (cursor) ??                                                  libserver12.a:kks1.o
knasnblp2l                                                   (knas)nblp2l                                       kernel replication apply server ??                                                         libserver12.a:knasn.o
knasnblp2lr                                                  (knas)nblp2lr                                      kernel replication apply server ??                                                         libserver12.a:knasn.o
kokaugi_iobs                                                 (koka)ugi_iobs                                     kernel objects kernel side access ??                                                       libserver12.a:koka.o
krvxpcls                                                     (krvx)pcls                                         kernel redo recovery extract ??                                                            libserver12.a:krvx.o
ktuAddTrsp                                                   (ktu)AddTrsp                                       kernel transaction undo ??                                                                 libserver12.a:ktu.o
ktuFindTrsp                                                  (ktu)FindTrsp                                      kernel transaction undo ??                                                                 libserver12.a:ktu.o
ktuGetTseg                                                   (ktu)GetTseg                                       kernel transaction undo ??                                                                 libserver12.a:ktu.o
ktuTempsoClnUp_pdcbk                                         (ktu)TempsoClnUp_pdcbk                             kernel transaction undo ??                                                                 libserver12.a:ktu.o
ktuTempsoClnUp_single                                        (ktu)TempsoClnUp_single                            kernel transaction undo ??                                                                 libserver12.a:ktu.o
nzosGetCipherDetails                                         (nz)osGetCipherDetails                             network security ??                                                                        libnnz12.a:ext_nzos.o,libnnzst12.a:ext_nzos.o
nzosGetCipherDetails2                                        (nz)osGetCipherDetails2                            network security ??                                                                        libnnz12.a:ext_nzos.o,libnnzst12.a:ext_nzos.o
nzosGetCipherList                                            (nz)osGetCipherList                                network security ??                                                                        libnnz12.a:ext_nzos.o,libnnzst12.a:ext_nzos.o
nzosGetDefaultCipherlist                                     (nz)osGetDefaultCipherlist                         network security ??                                                                        libnnz12.a:ext_nzos.o,libnnzst12.a:ext_nzos.o
qksceCEContainerMismatch                                     (qksce)CEContainerMismatch                         query kernel sql cursor evaluation ??                                                      libserver12.a:qksce.o
slrac_end_lock                                               (slrac)_end_lock                                   operating system (dependent) library verify (memory) read access ??                        libgeneric12.a:slrac.o

This is a poor, yet one of the only ways, to understand what is going on inside the oracle executable.
There aren’t that many things removed or added. Of course this is version 12.1, which is not what I believe to be the 12.2 releases of 12.2.0.1, 18 and 19, so a truly different version/release.

Also quite striking is the absence of all the changed libjava related functions, for which I don’t know if these function additions have anything to do with security. However, please mind I am not implying that this is not secure, I simply can’t tell.

Also, the functions that are removed and added do not seem to have any relationship with the added and removed functions in 12.2 and up. That might indicate that the PSU purely focussed on security updates to existing functions and issues that were logged against this version.

In the data dictionary there weren’t any changes spotted. Of course this overview focusses on availability of parameters and metadata (tables and their columns), for a look into the data dictionary you should look at the work of my colleague Rodrigo Jorge (dissecting the 201020 patches)

About the oracle database and compiling and linking.

This blogpost is about how the oracle database executable created or changed during installation and patching. I take linux for the examples, because that is the version that I am almost uniquely working with. I think the linux operating is where the vast majority of linux installations are installed on, and therefore an explanation with linux is helpful to most of the people.

The first thing to understand is the oracle executable is a dynamically linked executable. This is easy to see when you execute the ‘ldd’ utility against the oracle executable:

$ ldd oracle
	linux-vdso.so.1 (0x00007ffd3f5b0000)
	libodm19.so => /u01/app/oracle/product/19/dbhome_1/lib/libodm19.so (0x00007fa693084000)
	libofs.so => /u01/app/oracle/product/19/dbhome_1/lib/libofs.so (0x00007fa692e82000)
	libcell19.so => /u01/app/oracle/product/19/dbhome_1/lib/libcell19.so (0x00007fa692b69000)
	libskgxp19.so => /u01/app/oracle/product/19/dbhome_1/lib/libskgxp19.so (0x00007fa69284d000)
	libskjcx19.so => /u01/app/oracle/product/19/dbhome_1/lib/libskjcx19.so (0x00007fa692604000)
	librt.so.1 => /lib64/librt.so.1 (0x00007fa6923fb000)
	libclsra19.so => /u01/app/oracle/product/19/dbhome_1/lib/libclsra19.so (0x00007fa6921c0000)
	libdbcfg19.so => /u01/app/oracle/product/19/dbhome_1/lib/libdbcfg19.so (0x00007fa691f93000)
	libhasgen19.so => /u01/app/oracle/product/19/dbhome_1/lib/libhasgen19.so (0x00007fa691270000)
	libskgxn2.so => /u01/app/oracle/product/19/dbhome_1/lib/libskgxn2.so (0x00007fa69106d000)
	libocr19.so => /u01/app/oracle/product/19/dbhome_1/lib/libocr19.so (0x00007fa690d49000)
	libocrb19.so => /u01/app/oracle/product/19/dbhome_1/lib/libocrb19.so (0x00007fa690a49000)
	libocrutl19.so => /u01/app/oracle/product/19/dbhome_1/lib/libocrutl19.so (0x00007fa690828000)
	libaio.so.1 => /lib64/libaio.so.1 (0x00007fa690625000)
	libons.so => /u01/app/oracle/product/19/dbhome_1/lib/libons.so (0x00007fa6903d1000)
	libmql1.so => /u01/app/oracle/product/19/dbhome_1/lib/libmql1.so (0x00007fa69016f000)
	libipc1.so => /u01/app/oracle/product/19/dbhome_1/lib/libipc1.so (0x00007fa68fcf5000)
	libdl.so.2 => /lib64/libdl.so.2 (0x00007fa68faf1000)
	libm.so.6 => /lib64/libm.so.6 (0x00007fa68f76f000)
	libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fa68f54f000)
	libnsl.so.1 => /lib64/libnsl.so.1 (0x00007fa68f336000)
	libresolv.so.2 => /lib64/libresolv.so.2 (0x00007fa68f11f000)
	libc.so.6 => /lib64/libc.so.6 (0x00007fa68ed5d000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fa693287000)

The way this works, is that a library is defined in the ELF (the executable format of linux executables) header of the oracle executable. This can be seen using the ‘readelf’ utility:

$ readelf -d oracle

Dynamic section at offset 0x16f03640 contains 45 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [libodm19.so]
 0x0000000000000001 (NEEDED)             Shared library: [libofs.so]
 0x0000000000000001 (NEEDED)             Shared library: [libcell19.so]
 0x0000000000000001 (NEEDED)             Shared library: [libskgxp19.so]
 0x0000000000000001 (NEEDED)             Shared library: [libskjcx19.so]
 0x0000000000000001 (NEEDED)             Shared library: [librt.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libclsra19.so]
 0x0000000000000001 (NEEDED)             Shared library: [libdbcfg19.so]
 0x0000000000000001 (NEEDED)             Shared library: [libhasgen19.so]
 0x0000000000000001 (NEEDED)             Shared library: [libskgxn2.so]
 0x0000000000000001 (NEEDED)             Shared library: [libocr19.so]
 0x0000000000000001 (NEEDED)             Shared library: [libocrb19.so]
 0x0000000000000001 (NEEDED)             Shared library: [libocrutl19.so]
 0x0000000000000001 (NEEDED)             Shared library: [libaio.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libons.so]
 0x0000000000000001 (NEEDED)             Shared library: [libmql1.so]
 0x0000000000000001 (NEEDED)             Shared library: [libipc1.so]
 0x0000000000000001 (NEEDED)             Shared library: [libdl.so.2]
 0x0000000000000001 (NEEDED)             Shared library: [libm.so.6]
 0x0000000000000001 (NEEDED)             Shared library: [libpthread.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libnsl.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libresolv.so.2]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
 0x000000000000000f (RPATH)              Library rpath: [/u01/app/oracle/product/19/dbhome_1/lib]
 0x000000000000000c (INIT)               0xdb0d80
 0x000000000000000d (FINI)               0x12b54b90
 0x0000000000000019 (INIT_ARRAY)         0x17398c20
 0x000000000000001b (INIT_ARRAYSZ)       8 (bytes)
 0x000000006ffffef5 (GNU_HASH)           0x4002d0
 0x0000000000000005 (STRTAB)             0x9ddb10
 0x0000000000000006 (SYMTAB)             0x528128
 0x000000000000000a (STRSZ)              3567352 (bytes)
 0x000000000000000b (SYMENT)             24 (bytes)
 0x0000000000000015 (DEBUG)              0x0
 0x0000000000000003 (PLTGOT)             0x17504000
 0x0000000000000002 (PLTRELSZ)           26136 (bytes)
 0x0000000000000014 (PLTREL)             RELA
 0x0000000000000017 (JMPREL)             0xdaa768
 0x0000000000000007 (RELA)               0xda9328
 0x0000000000000008 (RELASZ)             5184 (bytes)
 0x0000000000000009 (RELAENT)            24 (bytes)
 0x000000006ffffffe (VERNEED)            0xda9188
 0x000000006fffffff (VERNEEDNUM)         7
 0x000000006ffffff0 (VERSYM)             0xd44a08
 0x0000000000000000 (NULL)               0x0

This shows the names of the needed shared libraries ‘(NEEDED)’. Some of the needed shared libraries are oracle shared libraries, such as libodm19.so, libofs.so, libcell19.so, libskgxp19.so and so on. Other libraries are operating system libraries, such as libc.so.6, libresolv.so.2, libnsl.so.1, libpthread.so.0, libm.so.6 and so on. The oracle libraries are found because an RPATH (runpath) is included in the header, in my case /u01/app/oracle/product/19/dbhome_1/lib. The operating system libraries are not included with the oracle installation, they are dynamically obtained from the operating system, for which the selection lies with the operating system.

So, we got the oracle executable, and we found out it’s a dynamically linked executable, which means that it’s using shared libraries for some of its functionality.

Now let’s take one step further. Whenever the oracle database software is installed or patched, it must be linked in order to build the executable with the current state of the software.
You might wonder the what I mean with the phrase ‘is installed’: you probably don’t execute a relink all. And that is sensible, because the installer does that for you, you can validate it in $ORACLE_HOME/install/make.log.
I’ll get to the manual linking in a bit.

The oracle database executable and compilation

The first thing to discuss now is compilation. Compilation is the process of turning text based code into a compiled form, for which a lot of compilers do not create an executable form, but an intermediary form, which is called an object. Turning an object or objects into an executable form is called linking. Compiling on linux is done using a compiler, and the default C compiler with Oracle and RedHat linux is gcc. Since Oracle 12.2, the compiler is not a requirement for installation anymore. It is documented, but I believe many may have missed this.

But isn’t there the $ORACLE_HOME/rdbms/lib/config.c file, which is still there, and still used, and isn’t there the make target config.o (make -f ins_rdbms.mk config.o)? Yes, both of them are still there. And still gcc is not a requirement anymore. If you have a pressing need for changing the config.c file (which lists the dba,oper,asm,backup,dataguard,keymanagement and RAC group names), you can still change it, and when you remove the config.o file which USED to be generated with gcc, will now be generated by the ‘as’ executable (portable GNU assembler). This is visible in the oracle database executable make target (ioracle):

$ mv config.o config.O
$ make --dry-run -f ins_rdbms.mk ioracle
chmod 755 /u01/app/oracle/product/19/dbhome_1/bin
cd /u01/app/oracle/product/19/dbhome_1/rdbms/lib/; \
/usr/bin/as -o config.o `[ -f config.c ] && echo config.c || echo config.s`; \
/usr/bin/ar r /u01/app/oracle/product/19/dbhome_1/lib/libserver19.a /u01/app/oracle/product/19/dbhome_1/rdbms/lib/config.o
echo
echo " - Linking Oracle "
rm -f /u01/app/oracle/product/19/dbhome_1/rdbms/lib/oracle
/u01/app/oracle/product/19/dbhome_1/bin/orald  -o /u01/app/oracle/product/19/dbhome_1/rdbms/lib/oracle -m64 -z noexecstack -Wl,--disable-new-dtags -L/u01/app/oracle/product/19/dbhome_1/rdbms/lib/ -L/u01/app/oracle/product/19/dbhome_1/lib/ -L/u01/app/oracle/product/19/dbhome_1/lib/stubs/   -Wl,-E /u01/app/oracle/product/19/dbhome_1/rdbms/lib/opimai.o /u01/app/oracle/product/19/dbhome_1/rdbms/lib/ssoraed.o /u01/app/oracle/product/19/dbhome_1/rdbms/lib/ttcsoi.o -Wl,--whole-archive -lperfsrv19 -Wl,--no-whole-archive /u01/app/oracle/product/19/dbhome_1/lib/nautab.o /u01/app/oracle/product/19/dbhome_1/lib/naeet.o /u01/app/oracle/product/19/dbhome_1/lib/naect.o /u01/app/oracle/product/19/dbhome_1/lib/naedhs.o /u01/app/oracle/product/19/dbhome_1/rdbms/lib/config.o  -ldmext -lserver19 -lodm19 -lofs -lcell19 -lnnet19 -lskgxp19 -lsnls19 -lnls19  -lcore19 -lsnls19 -lnls19 -lcore19 -lsnls19 -lnls19 -lxml19 -lcore19 -lunls19 -lsnls19 -lnls19 -lcore19 -lnls19 -lclient19  -lvsnst19 -lcommon19 -lgeneric19 -lknlopt -loraolap19 -lskjcx19 -lslax19 -lpls19  -lrt -lplp19 -ldmext -lserver19 -lclient19  -lvsnst19 -lcommon19 -lgeneric19 `if [ -f /u01/app/oracle/product/19/dbhome_1/lib/libavserver19.a ] ; then echo "-lavserver19" ; else echo "-lavstub19"; fi` `if [ -f /u01/app/oracle/product/19/dbhome_1/lib/libavclient19.a ] ; then echo "-lavclient19" ; fi` -lknlopt -lslax19 -lpls19  -lrt -lplp19 -ljavavm19 -lserver19  -lwwg  `cat /u01/app/oracle/product/19/dbhome_1/lib/ldflags`    -lncrypt19 -lnsgr19 -lnzjs19 -ln19 -lnl19 -lngsmshd19 -lnro19 `cat /u01/app/oracle/product/19/dbhome_1/lib/ldflags`    -lncrypt19 -lnsgr19 -lnzjs19 -ln19 -lnl19 -lngsmshd19 -lnnzst19 -lzt19 -lztkg19 -lmm -lsnls19 -lnls19  -lcore19 -lsnls19 -lnls19 -lcore19 -lsnls19 -lnls19 -lxml19 -lcore19 -lunls19 -lsnls19 -lnls19 -lcore19 -lnls19 -lztkg19 `cat /u01/app/oracle/product/19/dbhome_1/lib/ldflags`    -lncrypt19 -lnsgr19 -lnzjs19 -ln19 -lnl19 -lngsmshd19 -lnro19 `cat /u01/app/oracle/product/19/dbhome_1/lib/ldflags`    -lncrypt19 -lnsgr19 -lnzjs19 -ln19 -lnl19 -lngsmshd19 -lnnzst19 -lzt19 -lztkg19   -lsnls19 -lnls19  -lcore19 -lsnls19 -lnls19 -lcore19 -lsnls19 -lnls19 -lxml19 -lcore19 -lunls19 -lsnls19 -lnls19 -lcore19 -lnls19 `if /usr/bin/ar tv /u01/app/oracle/product/19/dbhome_1/rdbms/lib/libknlopt.a | grep "kxmnsd.o" > /dev/null 2>&1 ; then echo " " ; else echo "-lordsdo19 -lserver19"; fi` -L/u01/app/oracle/product/19/dbhome_1/ctx/lib/ -lctxc19 -lctx19 -lzx19 -lgx19 -lctx19 -lzx19 -lgx19 -lclscest19 -loevm -lclsra19 -ldbcfg19 -lhasgen19 -lskgxn2 -lnnzst19 -lzt19 -lxml19 -lgeneric19 -locr19 -locrb19 -locrutl19 -lhasgen19 -lskgxn2 -lnnzst19 -lzt19 -lxml19 -lgeneric19  -lgeneric19 -lorazip -loraz -llzopro5 -lorabz2 -lorazstd -loralz4 -lipp_z -lipp_bz2 -lippdc -lipps -lippcore  -lippcp -lsnls19 -lnls19  -lcore19 -lsnls19 -lnls19 -lcore19 -lsnls19 -lnls19 -lxml19 -lcore19 -lunls19 -lsnls19 -lnls19 -lcore19 -lnls19 -lsnls19 -lunls19  -lsnls19 -lnls19  -lcore19 -lsnls19 -lnls19 -lcore19 -lsnls19 -lnls19 -lxml19 -lcore19 -lunls19 -lsnls19 -lnls19 -lcore19 -lnls19 -lasmclnt19 -lcommon19 -lcore19  -ledtn19 -laio -lons  -lmql1 -lipc1 -lfthread19    `cat /u01/app/oracle/product/19/dbhome_1/lib/sysliblist` -Wl,-rpath,/u01/app/oracle/product/19/dbhome_1/lib -lm    `cat /u01/app/oracle/product/19/dbhome_1/lib/sysliblist` -ldl -lm   -L/u01/app/oracle/product/19/dbhome_1/lib `test -x /usr/bin/hugeedit -a -r /usr/lib64/libhugetlbfs.so && test -r /u01/app/oracle/product/19/dbhome_1/rdbms/lib/shugetlbfs.o && echo -Wl,-zcommon-page-size=2097152 -Wl,-zmax-page-size=2097152 -lhugetlbfs`
rm -f /u01/app/oracle/product/19/dbhome_1/bin/oracle
mv /u01/app/oracle/product/19/dbhome_1/rdbms/lib/oracle /u01/app/oracle/product/19/dbhome_1/bin/oracle
chmod 6751 /u01/app/oracle/product/19/dbhome_1/bin/oracle
(if [ ! -f /u01/app/oracle/product/19/dbhome_1/bin/crsd.bin ]; then \
    getcrshome="/u01/app/oracle/product/19/dbhome_1/srvm/admin/getcrshome" ; \
    if [ -f "$getcrshome" ]; then \
        crshome="`$getcrshome`"; \
        if [ -n "$crshome" ]; then \
            if [ $crshome != /u01/app/oracle/product/19/dbhome_1 ]; then \
                oracle="/u01/app/oracle/product/19/dbhome_1/bin/oracle"; \
                $crshome/bin/setasmgidwrap oracle_binary_path=$oracle; \
            fi \
        fi \
    fi \
fi\
);
$ mv config.O config.o

First of all, I am in the $ORACLE_HOME/rdbms/lib directory already. I moved the config.o file to a different name, config.O (uppercase O). This will trigger the config.o file to be generated during linking via the the makefile, because the make macro for generating the oracle executable checks for the existence of config.o in $ORACLE_HOME/rdbms/lib, and the generation of config.o is triggered by it not existing.
I used make with the ‘–dry-run’ option, which means it will list what it WOULD do, it doesn’t actually do it.
Now that the make macro doesn’t find the $ORACLE_HOME/rdbms/lib/config.o file, it generates it, using ‘as’, the GNU assembler.
After the run, I move the config.O file back to config.o.
Please mind the make target config.o (make -f ins_rdbms.mk config.o) still exists, and this follows the traditional way, so using gcc, to create the object file config.o.

For anything else than config.c, Oracle provides objects (the compiled, intermediary form of C) in object files. This has several advantages. First of all, the server to install oracle on doesn’t require a compiler. That also means that there is no discussion about compiler versions, Oracle knows for a fact which version of a compiler is used. Second, Oracle can use a different compiler than the GNU compiler, as long as it does provide objects in Linux X86_64 (ELF) format. In fact, that is what oracle does: for Oracle 19.9, Oracle used a compiler from intel: Intel(R) C Intel(R) 64 Compiler for applications running on Intel(R) 64, Version 17.0.2.174 Build 20170213
You can obtain that information from the oracle executable using:

$ readelf -p .comment oracle | egrep 'Intel.*Build\ [0-9]*'

I hope that at this point I made it clear that no compiler is needed anymore for oracle installation and making changes to the oracle installation, like patching.

The oracle database executable and objects and object files

It’s probably a good idea to show how the oracle executable is build. The way this happens is using the make target ‘ioracle’, which is visible above (make -f ins_rdbms.mk ioracle).
The macro calls ‘orald’, which actually is a script in $ORACLE_HOME/bin, which calls the operating-system ‘ld’ executable, which is the GNU linker.
The arguments to ‘orald’ are arguments that mostly are put through to ‘ld’. ‘-o’ is the output flag, and that shows the executable to be build by the linker, and besides options being set, what you mainly see is -L (library path) and -l (library) switches adding libraries (=object files and archive files) to build the oracle executable.
There’s a couple of places that are used to get objects to build the oracle executable:
– $ORACLE_HOME/rdbms/lib — oracle database rdbms specific libraries
– $ORACLE_HOME/lib — oracle database general libraries (objects are used by multiple “products” in the $ORACLE_HOME)
– $ORACLE_HOME/lib/stubs — this is a directory with ‘stub objects’, which are versions of operating system libraries that contain no code, but allow the oracle executable to be build, even if a operating system library is missing (see: https://docs.oracle.com/cd/E23824_01/html/819-0690/chapter2-22.html)
– $ORACLE_HOME/ctx/lib — oracle text (not sure why oracle text requires an explicit lookup to $ORACLE_HOME/ctx/lib, while other options are all in $ORACLE_HOME/lib
– /lib64 — operating system libraries

At this point it’s important to realise that the object files for linking oracle are visible in two forms: as plain object files (.o) and as archive files (.a). Archive files are exactly what the name suggests: these are archives of object files. You can look and manipulate an archive file using the ‘ar’ (archiver utility), for which the working strongly resembles how tar and jar work: t=list, x=extract, c=create.
If you take a look at one of the main archives, libserver19.a, you see that it contains 2852 object files:

$ ar -t $ORACLE_HOME/lib/libserver19.a | wc -l
2852

If you do wonder what’s inside, ‘ar -tv’ would be a good way to have an idea:

$ ar -tv $ORACLE_HOME/lib/libserver19.a
rw-r--r-- 54321/54321  11136 Oct 19 21:12 2020 kdr.o
rw-rw-r-- 94110/42424   8376 Apr 17 04:58 2019 upd.o
rw-rw-r-- 94110/42424  41968 Apr 17 04:58 2019 kd.o
...
rw-r--r-- 54321/54321  13248 Oct 19 21:11 2020 qjsntrans.o
rw-r--r-- 54321/54321  20296 Oct 19 21:11 2020 kubsd.o
rw-r--r-- 54321/54321  16720 Oct 19 21:12 2020 kqro.o

The conclusion here is that archive files are logical and sensible, otherwise the library directories would have been swamped with huge numbers of object files.

When object files are linked to an executable, it requires object files, and these are in ‘.o’ files, or grouped in ‘.a’ files. A third type of file is needed for linking linking an executable that is going to be a dynamically linked executable: the libraries (the ‘.so’ files) the executable is dynamically going to use. The linker will validate the libraries, which means it inspects the libraries to find the symbols that the objects that form the executable is calling. A library (‘.so’ file) is an already compiled form, in facts it’s pretty much similar to an executable, only the way it’s invoked is when it’s called by a dynamically linked executable that uses it, instead of directly.

The object files itself

This above text pretty much describe how executables, libraries, object files and archives sit together, and how the linking creates the oracle executable via the makefile. This description describes how this is configured by oracle for creating the oracle executable. However, this is really flexible, and can be done differently, so this is not how it always is or should be, this is how oracle chosen it to do.

We can look one level deeper into how this works. An object file in fact is already an archive, containing one or more compiled versions of functions:

$ nm -A opimai.o
opimai.o:                 U dbkc_free_bs_context
opimai.o:                 U dbkc_init
opimai.o:                 U dbktFlush
opimai.o:                 U __intel_new_feature_proc_init
opimai.o:                 U kgeasnmierr
opimai.o:                 U kge_pop_guard_fr
opimai.o:                 U kge_push_guard_fr
opimai.o:                 U kge_report_17099
opimai.o:                 U kgeresl
opimai.o:                 U kge_reuse_guard_fr
opimai.o:                 U ksdwrf
opimai.o:                 U kseini
opimai.o:                 U ksmdsg
opimai.o:                 U ksmgpg_
opimai.o:                 U ksmlsge_phaseone
opimai.o:                 U ksmsgaattached_
opimai.o:                 U kso_save_arg
opimai.o:                 U kso_spawn_ts_save
opimai.o:                 U ksosp_parse
opimai.o:                 U ksuginpr
opimai.o:                 U lfvinit
opimai.o:0000000000000010 T main
opimai.o:                 U opiinit
opimai.o:0000000000000300 t opimai_init
opimai.o:0000000000000140 T opimai_real
opimai.o:                 U opiterm
opimai.o:                 U sdterm
opimai.o:                 U _setjmp
opimai.o:                 U skge_sign_fr
opimai.o:                 U skgmstack
opimai.o:                 U skgp_done_args
opimai.o:                 U skgp_retrieve_args
opimai.o:                 U slgtds
opimai.o:                 U slgts
opimai.o:                 U slkbpi
opimai.o:                 U slkfpi
opimai.o:                 U sou2o
opimai.o:                 U spargs
opimai.o:                 U ssthrdmain

This example takes the object file $ORACLE_HOME/rdbms/lib/opimai.o, and this object file contains 3 actual functions (shown by an address and the symbol type ‘T’ or ‘t’), and a whole bunch of functions without an address and symbol type ‘U’. The functions with symbol type ‘U’ are undefined functions, which means that these functions are not in this archive, but defined somewhere else.
The important thing to consider is that a single symbol can contain multiple functions.

I chosen this object file, because this is in fact the actual object file where the main function, the starting function, for the oracle executable is in. If you obtain a stack trace of an oracle database process, the first function (called ‘first frame’) in at least recent linux versions (some other operating systems or versions might show earlier functions) will show main as the first function. This is also what the linker uses to build the executable, it follows the symbol information together with the command line switches to resolve and obtain all the functions via the symbol information. The linker will generate an error and not build the executable if it can’t find or resolve the symbols and get all the information it needs.

Patching

At this point you should have an understanding what a dynamically linked executable, libraries, object files and archives are, and that the oracle executable is build using a makefile using the linker.

It might be handy and interesting to look at patching. This information, the information about archives and objects, should give you more background about the specifics of patching. Oracle patching has many forms, and actually can do and change a lot of things in a lot of ways. It is retraceable what a patch does by looking at the contents of the patch. But that is now what this post is about.

Especially with one-off patches, in the case of a patch to fix or change one or more functions in the oracle executable, what the patch provides is the fixed and thus changed versions of these functions. However, oracle does not provide sourcecode. In general what oracle provides, is the object or objects containing the changed functions. In order to get the changed function or functions into the oracle executable, what generally happens is that the current/old versions of the object file are removed from the archive they are in, and saved in $ORACLE_HOME/.patch_storage, and the patched versions of the object file are inserted into the archive.

But, we saw an object file generally contains more or much more functions. Well, this is why patches can be incompatible with other patches: if multiple patches change different (or the same) functions in the same objects, the patch applied latest will undo the changes of the previous patch(es). This is also why you must request merge patches if patches are incompatible.

Dealing with individual object files, extracting them from an archive and saving them in order to be able to restore it into the archive is tedious. Also, the archive itself doesn’t mind whatever you remove from it or insert to it, even if it will break linking the oracle executable. Therefore, oracle has created opatch to perform a great deal of validations and checks, and take the work of dependency checks from you and fully automate it. In fact, in general, you can take a (one-off) patch and try to apply it, if it does, it will allow oracle to be relinked, if there is a conflicting patch it will tell you. Also, if you want to revert your applied patch, you can simply rollback and get opatch to load the previous version into the archive. This is way better than letting us humans deal with it directly.

After the patching changed the archives to contain the updated versions of the objects which contain updated functions, these must make it into the oracle executable. This must be done by relinking the executable, which will take the objects including the changed objects from all the object files and archives, and create a new executable. The oracle executable is never directly touched on linux with recent versions, to my knowledge.

I hope this explanation made sense and made a lot of these things which we are dealing with as oracle DBAs more understandable. Any comments or updates are welcome!

What’s new with Oracle database 12.2.0.1.200714 versus 12.2.0.1.201020

This blogpost takes a look at the technical differences between Oracle database 12.2.0.1 PSU 200714 (july 2020) and PSU 201020 (october 2020). This gives technical specialists an idea of the differences, and gives them the ability to assess if the PSU impacts anything.

Functions

code symbol names unique in version 12.2.0.1.200714 versus 12.2.0.1.201020

NAME                                                         RESOLVE                                                      ANNOTATION                                                                                 ARCHIVE_OBJECT
------------------------------------------------------------ ------------------------------------------------------------ ------------------------------------------------------------------------------------------ ------------------------------------------------------------
NAME                                               RESOLVE                                            ANNOTATION                                                                       ARCHIVE_OBJECT                                                 COUNT(*)
-------------------------------------------------- -------------------------------------------------- -------------------------------------------------------------------------------- ------------------------------------------------------------ ----------
kcbo                                               kcbo                                               kernel cache buffers object queue                                                                                                                      1
kkdl                                               kkdl                                               kernel compile dictionary lookup                                                                                                                       1
kmgs                                               kmgs                                               kernel multi threaded/mman manage (sga) space (?)                                                                                                      1
kzpR                                               (kzp)R                                             kernel security privileges ??                                                                                                                          1
qcso                                               qcso                                               query compile semantic analysis (parser) OBJECTS                                                                                                       1
qeae                                               qeae                                               query execute aggregate (order by) elimination (?)                                                                                                     1
qkaI                                               (qka)I                                             query kernel allocation ??                                                                                                                             1
qkex                                               (qke)x                                             query kernel expressions ??                                                                                                                            1
qksh                                               (qks)h                                             query kernel sql ??                                                                                                                                    1
qksp                                               (qks)p                                             query kernel sql ??                                                                                                                                    1
krvx                                               krvx                                               kernel redo recovery extract                                                                                                                           2
kxtt                                               kxtt                                               kernel execution temporary table operations                                                                                                            2
Java                                               Java                                               java classes                                                                                                                                           6

code symbol names unique in version 12.2.0.1.201020 versus 12.2.0.1.200714

NAME                                               RESOLVE                                            ANNOTATION                                                                       ARCHIVE_OBJECT                                                 COUNT(*)
-------------------------------------------------- -------------------------------------------------- -------------------------------------------------------------------------------- ------------------------------------------------------------ ----------
ECGr                                               (EC)Gr                                             elleptic curve cryptography ??                                                   libjavavm12.a:ecl.o                                                   7
Java                                               Java                                               java classes                                                                     libjavavm12.a:ShapeSpanIterator.o                                     7
ec_G                                               (ec_)G                                             elleptic curve cryptography ??                                                   libjavavm12.a:ecp_jac.o                                               7
mp_c                                               (mp_)c                                             multiple precision cryptography ??                                               libjavavm12.a:mpi.o                                                   7
mp_i                                               (mp_)i                                             multiple precision cryptography ??                                               libjavavm12.a:mpi.o                                                   7
mp_t                                               (mp_)t                                             multiple precision cryptography ??                                               libjavavm12.a:mpi.o                                                   7
mp_b                                               (mp_)b                                             multiple precision cryptography ??                                               libjavavm12.a:mp_gf2m.o                                               8
mp_s                                               (mp_)s                                             multiple precision cryptography ??                                               libjavavm12.a:mpi.o                                                  11
ec_G                                               (ec_)G                                             elleptic curve cryptography ??                                                   libjavavm12.a:ecl_gf.o                                               21
s_mp                                               s_mp                                               cryptography multiple precision                                                  libjavavm12.a:mpi.o                                                  49

This is a poor, yet one of the only ways to understand what is going on inside the oracle executable.
There aren’t that many thing removed, which is understandable, this is an update. 20 functions have been removed in total. The archive:object annotation is not there, I started doing that for the latest PSU.
For the functions that were added, the vast majority comes from libjavavm18, and seem to be cryptographic related functions.

It should be noted that if you compare the added functions and the changed functions with the Oracle 18 ones, a fair share of them are equal, but there are also functions unique to each version. Of course the equal functions probably are related to huge issues that must be fixed in all versions. The functions that are unique to this version are likely to be (minor-)version specific issues.

What’s new with Oracle database 18.11 versus 18.12

This blogpost takes a look at the technical differences between Oracle database 18 RU 11 (july 2020) and RU 12 (october 2020). This gives technical specialists an idea of the differences, and gives them the ability to assess if the RU impacts anything.

Functions

code symbol names unique in version 18.11 versus 18.12 -- so removed functions in RU 12.

NAME                                                         RESOLVE                                                      ANNOTATION                                                                                 ARCHIVE_OBJECT
------------------------------------------------------------ ------------------------------------------------------------ ------------------------------------------------------------------------------------------ ------------------------------------------------------------
Java_sun_java2d_pipe_ShapeSpanIterator_appendCubic           (Java_sun_java2d_pipe_ShapeSpanIterator_)appendCubic         java classes sun. java2d. pipe. ShapeSpanIterator. ??
Java_sun_java2d_pipe_ShapeSpanIterator_appendLine            (Java_sun_java2d_pipe_ShapeSpanIterator_)appendLine          java classes sun. java2d. pipe. ShapeSpanIterator. ??
Java_sun_java2d_pipe_ShapeSpanIterator_appendQuadratic       (Java_sun_java2d_pipe_ShapeSpanIterator_)appendQuadratic     java classes sun. java2d. pipe. ShapeSpanIterator. ??
Java_sun_java2d_pipe_ShapeSpanIterator_beginSubpath          (Java_sun_java2d_pipe_ShapeSpanIterator_)beginSubpath        java classes sun. java2d. pipe. ShapeSpanIterator. ??
Java_sun_java2d_pipe_ShapeSpanIterator_endPath               (Java_sun_java2d_pipe_ShapeSpanIterator_)endPath             java classes sun. java2d. pipe. ShapeSpanIterator. ??
Java_sun_java2d_pipe_ShapeSpanIterator_getCPathConsumer      (Java_sun_java2d_pipe_ShapeSpanIterator_)getCPathConsumer    java classes sun. java2d. pipe. ShapeSpanIterator. ??
kkdlpGetObjn                                                 (kkdl)pGetObjn                                               kernel compile dictionary lookup ??
kxttSCNGet                                                   (kxtt)SCNGet                                                 kernel execution temporary table operations ??
kxttSCNGetBoth                                               (kxtt)SCNGetBoth                                             kernel execution temporary table operations ??
kzpRoleEnabled                                               (kzp)RoleEnabled                                             kernel security privileges ??
qkspmTravInit                                                (qkspm)TravInit                                              query kernel sql plan management ??
skgpgprocstat                                                (skg)pgprocstat                                              operating system dependent kernel generic ??
skgpgthrstat                                                 (skg)pgthrstat                                               operating system dependent kernel generic ??

code symbol names unique in version 18.11 versus 18.12 (top 11) -- so added in the RU 12.

NAME                                               RESOLVE                                            ANNOTATION                                                                       ARCHIVE_OBJECT                                                 COUNT(*)
-------------------------------------------------- -------------------------------------------------- -------------------------------------------------------------------------------- ------------------------------------------------------------ ----------
ECGr                                               (EC)Gr                                             elleptic curve cryptography ??                                                   libjavavm18.a:ecl.o                                                   7
Java                                               Java                                               java classes                                                                     libjavavm18.a:ShapeSpanIterator.o                                     7
ec_G                                               (ec_)G                                             elleptic curve cryptography ??                                                   libjavavm18.a:ecp_jac.o                                               7
mp_c                                               (mp_)c                                             multiple precision cryptography ??                                               libjavavm18.a:mpi.o                                                   7
mp_i                                               (mp_)i                                             multiple precision cryptography ??                                               libjavavm18.a:mpi.o                                                   7
mp_t                                               (mp_)t                                             multiple precision cryptography ??                                               libjavavm18.a:mpi.o                                                   7
mp_b                                               (mp_)b                                             multiple precision cryptography ??                                               libjavavm18.a:mp_gf2m.o                                               8
mp_s                                               (mp_)s                                             multiple precision cryptography ??                                               libjavavm18.a:mpi.o                                                  11
kewr                                               kewr                                               kernel event AWR repository                                                      libserver18.a:kewr.o                                                 14
ec_G                                               (ec_)G                                             elleptic curve cryptography ??                                                   libjavavm18.a:ecl_gf.o                                               21
s_mp                                               s_mp                                               cryptography multiple precision                                                  libjavavm18.a:mpi.o                                                  49

This is a poor, yet one of the only ways to understand what is going on inside the oracle executable.
There aren’t that many thing removed, which is understandable. Therefore I listed all the functions that were removed, not a as a grouped overview. The archive:object annotation is not there, I started doing that for the latest RU.
For the functions that were added, the vast majority comes from libjavavm18, and seem to be cryptographic related functions.

The only group that might show something is kewr (AWR), however I cannot see something in general that is changed, the names seem random:

NAME                                                         RESOLVE                                                      ANNOTATION                                                                                 ARCHIVE_OBJECT
------------------------------------------------------------ ------------------------------------------------------------ ------------------------------------------------------------------------------------------ ------------------------------------------------------------
kewramcs_app_map_condbid_str                                 (kewr)amcs_app_map_condbid_str                               kernel event AWR repository ??                                                             libserver18.a:kewr.o
kewramvn_append_mdb_vvwname                                  (kewr)amvn_append_mdb_vvwname                                kernel event AWR repository ??                                                             libserver18.a:kewr.o
kewraxc_attribute_xcb                                        (kewr)axc_attribute_xcb                                      kernel event AWR repository ??                                                             libserver18.a:kewr.o
kewrbtsg_build_topseg                                        (kewr)btsg_build_topseg                                      kernel event AWR repository ??                                                             libserver18.a:kewr.o
kewrccsq_collect_csql                                        (kewr)ccsq_collect_csql                                      kernel event AWR repository ??                                                             libserver18.a:kewr.o
kewrgcfes_get_cacheid_from_enum_str                          (kewr)gcfes_get_cacheid_from_enum_str                        kernel event AWR repository ??                                                             libserver18.a:kewr.o
kewrmplvl_map_snap_level                                     (kewr)mplvl_map_snap_level                                   kernel event AWR repository ??                                                             libserver18.a:kewr.o
kewrpfbue_pdb_from_buffer_entry                              (kewr)pfbue_pdb_from_buffer_entry                            kernel event AWR repository ??                                                             libserver18.a:kewr.o
kewrptsq_prep_topsql                                         (kewr)ptsq_prep_topsql                                       kernel event AWR repository ??                                                             libserver18.a:kewr.o
kewrrdsi_rank_dstat_item                                     (kewr)rdsi_rank_dstat_item                                   kernel event AWR repository ??                                                             libserver18.a:kewr.o
kewrrtsq_rank_topsql                                         (kewr)rtsq_rank_topsql                                       kernel event AWR repository ??                                                             libserver18.a:kewr.o
kewrsaobn_set_all_objnames                                   (kewr)saobn_set_all_objnames                                 kernel event AWR repository ??                                                             libserver18.a:kewr.o
kewrsonie_set_object_names_in_entry                          (kewr)sonie_set_object_names_in_entry                        kernel event AWR repository ??                                                             libserver18.a:kewr.o
kewrsqlc_sql_iscolored_cb                                    (kewr)sqlc_sql_iscolored_cb                                  kernel event AWR repository ??                                                             libserver18.a:kewr.o

Outside of code changes, there isn’t anything changed in the data dictionary.

What’s new with Oracle database 19.9 versus 19.8

This blogpost takes a look at the technical differences between Oracle database 19 RU 8 (july 2020) and RU 9 (october 2020). This gives technical specialists an idea of the differences, and gives them the ability to assess if the RU impacts anything.

Functions

code symbol names unique in version 19.8 versus 19.9 (top 5) -- so removed in the 19.9 RU

NAME                                               RESOLVE                                            ANNOTATION                                                                       ARCHIVE_OBJECT                                                 COUNT(*)
-------------------------------------------------- -------------------------------------------------- -------------------------------------------------------------------------------- ------------------------------------------------------------ ----------
qsod                                               (qs)od                                             query system ??                                                                                                                                        3
ZSTD                                               ZSTD                                               Zstandard. fast lossless compression algorithm                                                                                                         4
qjsn                                               qjsn                                               query json                                                                                                                                             5
Java                                               Java                                               java classes                                                                                                                                           6
qcpi                                               qcpi                                               query compile parse interim                                                                                                                            6

code symbol names unique in version 19.9 versus 19.8 (top 10) -- so added in the 19.9 RU

NAME                                               RESOLVE                                            ANNOTATION                                                                       ARCHIVE_OBJECT                                                 COUNT(*)
-------------------------------------------------- -------------------------------------------------- -------------------------------------------------------------------------------- ------------------------------------------------------------ ----------
qsod                                               (qs)od                                             query system ??                                                                  libclntst19.a:generic19_qsodax.o,libgeneric19.a:qsodax.o              8
mp_s                                               (mp_)s                                             multiple precision cryptography ??                                               libjavavm19.a:mpi.o                                                  11
qcpi                                               qcpi                                               query compile parse interim                                                      libclntst19.a:generic19_qcpi7.o,libgeneric19.a:qcpi7.o               11
ksac                                               (ksa)c                                             kernel service asynchronous messages ??                                          libserver19.a:ksacl.o                                                13
jznO                                               (jzn)O                                             json ??                                                                          libxml19.a:jznoct.o,libclntst19.a:xml19_jznoct.o                     15
qjsn                                               qjsn                                               query json                                                                       libserver19.a:qjsn.o                                                 15
jznt                                               (jzn)t                                             json ??                                                                          libxml19.a:jzntrans.o,libclntst19.a:xml19_jzntrans.o                 19
jzno                                               (jzn)o                                             json ??                                                                          libxml19.a:jznoct.o,libclntst19.a:xml19_jznoct.o                     20
ec_G                                               (ec_)G                                             elleptic curve cryptography ??                                                   libjavavm19.a:ecl_gf.o                                               21
s_mp                                               s_mp                                               cryptography multiple precision                                                  libjavavm19.a:mpi.o                                                  49

This is a poor, yet one of the only ways to understand what is going on inside the oracle executable.

There aren’t that many thing removed, which is understandable.
I show the the top 11 of function groups that are added, because their numbers are significant (more than 10).
In general, what I see is most functions are related to cryptography, which is understandable and logical in today’s world.
Additionally, there’s a lot of movement with JSON related libraries, this is also understandable, and probably a lot of the functions are related to DBMS_SODA.
Obviously, there is more changed as we can see below from the parameters (which are switches for bug fixes), and other changes, but the function changes doe not indicate huge groupings of functions to be added or removed outside of the groups indicated above.

Parameters

parameters unique in version 19.8 versus 19.9

NAME
--------------------------------------------------
_eleventh_spare_parameter
_nineteenth_spare_parameter
_one-hundred-and-seventy-fourth_spare_parameter
_one-hundred-and-sixty-eighth_spare_parameter
_thirteenth_spare_parameter
_twentieth_spare_parameter
_twenty-eighth_spare_parameter
_twenty-fifth_spare_parameter
_twenty-first_spare_parameter
_twenty-fourth_spare_parameter
_twenty-seventh_spare_parameter
_twenty-sixth_spare_parameter
_twenty-third_spare_parameter

parameters unique in version 19.9 versus 19.8

NAME
--------------------------------------------------
_bug28482048_fb_log_max_rereads
_bug29504103_runtime_index_key_length_check
_bug30159581_cputime_limit_parallel_fptr
_bug30159581_runtime_limit_parallel_fptr
_bug30165506_enable_unified_memory
_bug30165506_unified_pga_gran_leak_timeout
_bug30165506_unified_pga_list_count
_bug30165506_unified_pga_max_pool_size
_bug31747989_cputime_limit_parallel_fptr
_bug31747989_runtime_limit_parallel_fptr
_ipacl_feature_control_flags
_ldap_use_all_direct_groups_only
_pdb_transition_clean_bg_delay

Once again, this is quite consistent, ‘spare’ parameters being swapped for lots of ‘_bugnnnnnnn_description’ parameters, and a few other underscore parameters.
There are no new documented parameters.

Waitevents

waitevents unique in version 19.8 versus 19.9

NAME
----------------------------------------------------------------------------------------------------
second spare wait event

waitevents unique in version 19.9 versus 19.8

NAME
----------------------------------------------------------------------------------------------------
unified memory allocation

A new wait event was added with PSU 19.9! A ‘spare’ wait event was changed to ‘unified memory allocation’. I do not have more information about this, and have not heard about ‘unified memory’ before.

Sysstat

sysstat statistics unique to 19.8 versus 19.9

NAME
----------------------------------------------------------------------------------------------------
cell RDMA reads
cell pmem cache read hits
spare statistic 1
spare statistic 2

sysstat statistics unique to 19.9 versus 19.8

NAME
----------------------------------------------------------------------------------------------------
cell RDMA reads
cell RDMA reads unused
cell pmem cache read hits
cell pmem cache read hits unused

A few statistics have been added! Quite logically this is related to RDMA and pmem (persistent memory). It looks like existing statistics have been moved to a new statistics number, and the ‘old’ ones have gotten an addition saying ‘unused’. It’s understandable and known that oracle is putting efforts in the area’s of RDMA, where it seems the usage of RDMA is expanded all the time to increase performance of RAC cross instance communication, and pmem is new and currently officially implemented for Exadata. I do not know if the above statistic is the exadata related pmem cache, or if this is a local machine pmem implementation.

DBA/CDB views

dba tables unique to 19.8 versus 19.9

dba tables unique to 19.9 versus 19.8

NAME
----------------------------------------------------------------------------------------------------
DBA_SUBSCR_DUR_REGISTRATIONS

cdb tables unique to 19.8 versus 19.9

cdb tables unique to 19.9 versus 19.8

NAME
----------------------------------------------------------------------------------------------------
CDB_SUBSCR_DUR_REGISTRATIONS

It seems there is a DBA and CDB scope view added.

Using Grafana Loki to be able to search and view all logs

This post is about how to make your log files being aggregated in a single place and easy searchable via a convenient web interface.

You might think: wait a minute; doesn’t this exist already? Well, yes and no. Let me explain.
a) traditional log management: traditionally, logs are/were searched for certain strings, typically error messages or parts of error messages and a shell script run by cron, and when matched, a system that was setup to be able to email would send an indicator that the aforementioned string was found.
b) log management via a monitoring tool: the next step was the individual script on each server was exchanged for a monitoring tool, which performed the same task as the shell script. In reality quite often a default monitoring set/template was enabled, instead of the specific strings that were searched for with the shell script. Sometimes this was an improvement, sometimes this meant the specific issues (and thus messages) gone invisible. This is still the way the monitoring works in 90% of the cases, including a general completely standard monitoring template. At least in my experience.
c) other log gathering and indexing: there are many more products that perform this function. The first one that comes to my mind is splunk, and all the options for doing this cloud based (many there!), and a lot of tools based on elasticsearch, like the “ELK stack”.

I think it’s clear “a)” and “b)” are not dynamic and in fact very static. My major concern is it doesn’t allow exploratory investigation, it simply is a warning that is raised, any investigation means you have to log on and start browsing the available information locally. Everybody who worked with HP Openview or Oracle Enterprise Manager will recognise this. Yes, it’s probably all possible with these tools, but it never (ever(!)) is implemented.

For the last category, the likes of splunk, the ELK stack and the cloud based log tools: for the first two I feel definitely serves a function, but it’s aimed at aggregating logs of multiple servers, and is simply too much to setup on a server alongside the processes it is meant to monitor. For the cloud based tools: it might be my conservatism, but getting a subscription and loading up logging feels awkward, especially if it’s for my own test purposes.

This is where Loki comes in. Finally, I would say, there is a tool that can function on small scale (!) and perform log aggegration and provide a searchable database without a huge setup. This is ideal for your own test or development box to able to discover what is going on, and have all the log files at your fingertips without endlessly going through the filesystem performing cd, tail, grep, wc, sort, uniq, et cetera. I think lots of people recognise travelling from log file to log file.

Loki gives you a database that orders log entries based on time, and Grafana provides a web based UI to view and query the loki database. This is how it looks like:

(click to enlarge)
This is an example, what this shows is my test machine, where I decided to see when linux started, as well when the Oracle database instance was started.
* The first query uses the label “rdbms_alert” (indicating it came from the text-based oracle alert.log file, this is a label I added to it), and within the log lines with the label job with value “rdbms_alert”, I added a filter for the string “Starting ORACLE”, which indicates an Oracle database instance start.
* The second query uses the label “messages” (indicating it came from the linux /var/log/messages file, this label is mine too), and within the log lines with the label job with the value “messages”, I added a filter for the string “kernel: Command line”, which indicates linux startup. I additionally added a negative filter for “loki”, because loki logs the queries to the messages file, which I don’t want to see.

I hope you can see the power of having all the logs in a single place, and completely searchable.

This is just a start, this is a very simple proof-of-concept setup, for example the date/time in the log lines is not used, the date/time of the log lines is when it was ingested into loki, it is possible to have loki interpret these.

If you are interested, but are uncertain if this is for you, and would like to test this: I got a couple of Ansible scripts that can setup the combination of:
* promtail (the default loki log streaming tool)
* loki (the database)
* Grafana (the web UI)
The scripts are created on Oracle Linux 7.8.

Install git, and clone the https://gitlab.com/FritsHoogland/loki-setup.git repository using a user that has sudo rights:

git clone https://gitlab.com/FritsHoogland/loki-setup.git

Install ansible (you might have to install the EPEL repository).

In the loki-setup repo you find the setup scripts for loki, promtail and grafana.
You can execute the scripts in that order (loki, promtail and Grafana) by executing the setup_loki.yml, setup_promtail,yml and setup_grafana.yml scripts.
IMPORTANT: do proofread the scripts, and validate the variables for your situation. Don’t worry: the scripts are easy to read.

After installing, you can go to http://yourmachine:3000, login with admin and the password you set in the setup_grafana.yml script, and click on the compass (explore) option, and you can perform your own log browsing.

If you decide you want to remove it, I got a remove script for each of the components, which will remove the specific component entirely. Same applies here too: validate the script.

How to use DBMS_PIPE to halt and continue a PLSQL database session

I posted a message on twitter saying that DBMS_PIPE is an excellent mechanism to make a session run and halt in PLSQL. One response I gotten was asking for an example of that. That is what this post is about.

DBMS_PIPE is an implementation of a pipe inside the Oracle database. A pipe is a mechanism that is not limited to the Oracle database, in fact I assume the implementation is inspired by an operating system level pipe, which can be created using the ‘mknod /path/pipename p’ on unix and unix-like systems. A quick search shows windows has got them too, but not really, or does it? Another implementation is the pipe to redirect output from one command to the next using ‘|’. The essence of a pipe that input to and output from the pipe are separated from each other, and that information is flowing from the inputting process of the pipe to the outputting process.

For the specific purpose this blogpost is about, DBMS_PIPE, I am not using the information that is sent through the pipe, I am mainly using the fact that a receiving session waits for information to be sent, and therefore halts execution.

When the Oracle database executes in PL/SQL mode, there is no simple way to make it pause and request for user input, like you would do with the ‘pause’ or ‘accept’ commands in sqlplus in SQL mode. PL/SQL runs and provides its output when the session quits PL/SQL mode, in other words: there is no interaction with the terminal that executes the PL/SQL procedure or anonymous block. So if you want to make it stop and continue, you must use something else, for which I use DBMS_PIPE. Of course there are other ways, I would encourage people to come up with other mechanisms as a comment.

The use the technique of using DBMS_PIPE, you need two sessions, one which runs the PL/SQL block that you want to pause and continue (using a database pipe), and one to make it continue by sending a message to that pipe. Another thing that you might notice is the lack of explicit setup of the pipe; I just declare waiting for the pipe, and it’ll get created implicitly.

This is how it’s done:

1. Simple example anonymous PL/SQL block:

set serverout on;
declare
 v_result number;
 v_time1 date;
 v_time2 date;
begin
  select sysdate into v_time1 from dual;
  v_result := dbms_pipe.receive_message ( 'test' );
  select sysdate into v_time2 from dual;
  dbms_output.put_line('you waited: '||(24*60*60)*(v_time2-v_time1)||' seconds');
end;
/

If you execute this, it will provide no output, and in fact be waiting for the pipe that I named ‘test’.

In order to make anonymous PL/SQL block continue, I need another session to send a message to the pipe:
2. Send empty message to the pipe:

var v_result number;
exec v_result := dbms_pipe.send_message ( 'test' );

Once you executed that, the PL/SQL block will continue, and then finish and therefore display the output:

you waited: 3 seconds

PL/SQL procedure successfully completed.

There are a couple of other caveats you need to be aware of:
– a pipe is visible to the instance, in the case of a cluster database it’s NOT visible on the other instances in the cluster.
– there can be multiple sessions waiting on the same pipe, but a single message is consumed/received by a single session, based on time.