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diff --git a/source/development/dev_queue.rst b/source/development/dev_queue.rst new file mode 100644 index 0000000..fb5a286 --- /dev/null +++ b/source/development/dev_queue.rst @@ -0,0 +1,299 @@ +The rsyslog queue object +======================== + +This page reflects the status as of 2008-01-17. The documentation is +still incomplete. Target audience is developers and users who would like +to get an in-depth understanding of queues as used in +`rsyslog <http://www.rsyslog.com/>`_. + +**Please note that this document is outdated and does not longer reflect +the specifics of the queue object. However, I have decided to leave it +in the doc set, as the overall picture provided still is quite OK. I +intend to update this document somewhat later when I have reached the +"store-and-forward" milestone.** + +Some definitions +---------------- + +A queue is DA-enabled if it is configured to use disk-assisted mode when +there is need to. A queue is in DA mode (or DA run mode), when it +actually runs disk assisted. + +Implementation Details +---------------------- + +Disk-Assisted Mode +~~~~~~~~~~~~~~~~~~ + +Memory-Type queues may utilize disk-assisted (DA) mode. DA mode is +enabled whenever a queue file name prefix is provided. This is called +DA-enabled mode. If DA-enabled, the queue operates as a regular memory +queue until a high water mark is reached. If that happens, the queue +activates disk assistance (called "runs disk assisted" or "runs DA" - +you can find that often in source file comments). To do so, it creates a +helper queue instance (the DA queue). At that point, there are two +queues running - the primary queue's consumer changes to a +shuffle-to-DA-queue consumer and the original primary consumer is +assigned to the DA queue. Existing and new messages are spooled to the +disk queue, where the DA worker takes them from and passes them for +execution to the actual consumer. In essence, the primary queue has now +become a memory buffer for the DA queue. The primary queue will be +drained until a low water mark is reached. At that point, processing is +held. New messages enqueued to the primary queue will not be processed +but kept in memory. Processing resumes when either the high water mark +is reached again or the DA queue indicates it is empty. If the DA queue +is empty, it is shut down and processing of the primary queue continues +as a regular in-memory queue (aka "DA mode is shut down"). The whole +thing iterates once the high water mark is hit again. + +There is one special case: if the primary queue is shut down and could +not finish processing all messages within the configured timeout +periods, the DA queue is instantiated to take up the remaining messages. +These will be preserved and be processed during the next run. During +that period, the DA queue runs in "enqueue-only" mode and does not +execute any consumer. Draining the primary queue is typically very fast. +If that behaviour is not desired, it can be turned of via parameters. In +that case, any remaining in-memory messages are lost. + +Due to the fact that when running DA two queues work closely together +and worker threads (including the DA worker) may shut down at any time +(due to timeout), processing synchronization and startup and shutdown is +somewhat complex. I'll outline the exact conditions and steps down here. +I also do this so that I know clearly what to develop to, so please be +patient if the information is a bit too in-depth ;) + +DA Run Mode Initialization +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Three cases: + +#. any time during queueEnqObj() when the high water mark is hit +#. at queue startup if there is an on-disk queue present (presence of QI + file indicates presence of queue data) +#. at queue shutdown if remaining in-memory data needs to be persisted + to disk + +In **case 1**, the worker pool is running. When switching to DA mode, +all regular workers are sent termination commands. The DA worker is +initiated. Regular workers may run in parallel to the DA worker until +they terminate. Regular workers shall terminate as soon as their current +consumer has completed. They shall not execute the DA consumer. + +In **case 2**, the worker pool is not yet running and is NOT started. +The DA worker is initiated. + +In **case 3**, the worker pool is already shut down. The DA worker is +initiated. The DA queue runs in enqueue-only mode. + +In all cases, the DA worker starts up and checks if DA mode is already +fully initialized. If not, it initializes it, what most importantly +means construction of the queue. + +Then, regular worker processing is carried out. That is, the queue +worker will wait on empty queue and terminate after an timeout. However, +If any message is received, the DA consumer is executed. That consumer +checks the low water mark. If the low water mark is reached, it stops +processing until either the high water mark is reached again or the DA +queue indicates it is empty (there is a pthread\_cond\_t for this +synchronization). + +In theory, a **case-2** startup could lead to the worker becoming +inactive and terminating while waiting on the primary queue to fill. In +practice, this is highly unlikely (but only for the main message queue) +because rsyslog issues a startup message. HOWEVER, we can not rely on +that, it would introduce a race. If the primary rsyslog thread (the one +that issues the message) is scheduled very late and there is a low +inactivity timeout for queue workers, the queue worker may terminate +before the startup message is issued. And if the on-disk queue holds +only a few messages, it may become empty before the DA worker is +re-initiated again. So it is possible that the DA run mode termination +criteria occurs while no DA worker is running on the primary queue. + +In cases 1 and 3, the DA worker can never become inactive without +hitting the DA shutdown criteria. In **case 1**, it either shuffles +messages from the primary to the DA queue or it waits because it has the +hit low water mark. + +In **case 3**, it always shuffles messages between the queues (because, +that's the sole purpose of that run). In order for this to happen, the +high water mark has been set to the value of 1 when DA run mode has been +initialized. This ensures that the regular logic can be applied to drain +the primary queue. To prevent a hold due to reaching the low water mark, +that mark must be changed to 0 before the DA worker starts. + +DA Run Mode Shutdown +~~~~~~~~~~~~~~~~~~~~ + +In essence, DA run mode is terminated when the DA queue is empty and the +primary worker queue size is below the high water mark. It is also +terminated when the primary queue is shut down. The decision to switch +back to regular (non-DA) run mode is typically made by the DA worker. If +it switches, the DA queue is destructed and the regular worker pool is +restarted. In some cases, the queue shutdown process may initiate the +"switch" (in this case more or less a clean shutdown of the DA queue). + +One might think that it would be more natural for the DA queue to detect +being idle and shut down itself. However, there are some issues +associated with that. Most importantly, all queue worker threads need to +be shut down during queue destruction. Only after that has happened, +final destruction steps can happen (else we would have a myriad of +races). However, it is the DA queues worker thread that detects it is +empty (empty queue detection always happens at the consumer side and +must so). That would lead to the DA queue worker thread to initiate DA +queue destruction which in turn would lead to that very same thread +being canceled (because workers must shut down before the queue can be +destructed). Obviously, this does not work out (and I didn't even +mention the other issues - so let's forget about it). As such, the +thread that enqueues messages must destruct the queue - and that is the +primary queue's DA worker thread. + +There are some subtleties due to thread synchronization and the fact +that the DA consumer may not be running (in a **case-2 startup**). So it +is not trivial to reliably change the queue back from DA run mode to +regular run mode. The priority is a clean switch. We accept the fact +that there may be situations where we cleanly shut down DA run mode, +just to re-enable it with the very next message being enqueued. While +unlikely, this will happen from time to time and is considered perfectly +legal. We can't predict the future and it would introduce too great +complexity to try to do something against that (that would most probably +even lead to worse performance under regular conditions). + +The primary queue's DA worker thread may wait at two different places: + +#. after reaching the low water mark and waiting for either high water + or DA queue empty +#. at the regular pthread\_cond\_wait() on an empty primary queue + +Case 2 is unlikely, but may happen (see info above on a case 2 startup). + +**The DA worker may also not wait at all,** because it is actively +executing and shuffling messages between the queues. In that case, +however, the program flow passes both of the two wait conditions but +simply does not wait. + +**Finally, the DA worker may be inactive**\ (again, with a case-2 +startup). In that case no work(er) at all is executed. Most importantly, +without the DA worker being active, nobody will ever detect the need to +change back to regular mode. If we have this situation, the very next +message enqueued will cause the switch, because then the DA run mode +shutdown criteria is met. However, it may take close to eternal for this +message to arrive. During that time, disk and memory resources for the +DA queue remain allocated. This also leaves processing in a sub-optimal +state and it may take longer than necessary to switch back to regular +queue mode when a message burst happens. In extreme cases, this could +even lead to shutdown of DA run mode, which takes so long that the high +water mark is passed and DA run mode is immediately re-initialized - +while with an immediate switch, the message burst may have been able to +be processed by the in-memory queue without DA support. + +So in short, it is desirable switch to regular run mode as soon as +possible. To do this, we need an active DA worker. The easy solution is +to initiate DA worker startup from the DA queue's worker once it detects +empty condition. To do so, the DA queue's worker must call into a "*DA +worker startup initiation*\ " routine inside the main queue. As a +reminder, the DA worker will most probably not receive the "DA queue +empty" signal in that case, because it will be long sent (in most cases) +before the DA worker even waits for it. So **it is vital that DA run +mode termination checks be done in the DA worker before it goes into any +wait condition**. + +Please note that the "*DA worker startup initiation*\ " routine may be +called concurrently from multiple initiators. **To prevent a race, it +must be guarded by the queue mutex**\ and return without any action (and +no error code!) if the DA worker is already initiated. + +All other cases can be handled by checking the termination criteria +immediately at the start of the worker and then once again for each run. +The logic follows this simplified flow diagram: + +.. |image0| image:: queueWorkerLogic.jpg + +Some of the more subtle aspects of worker processing (e.g. enqueue +thread signaling and other fine things) have been left out in order to +get the big picture. What is called "check DA mode switchback..." right +after "worker init" is actually a check for the worker's termination +criteria. Typically, **the worker termination criteria is a shutdown +request**. However, **for a DA worker, termination is also requested if +the queue size is below the high water mark AND the DA queue is empty**. +There is also a third termination criteria and it is not even on the +chart: that is the inactivity timeout, which exists in all modes. Note +that while the inactivity timeout shuts down a thread, it logically does +not terminate the worker pool (or DA worker): workers are restarted on +an as-needed basis. However, inactivity timeouts are very important +because they require us to restart workers in some situations where we +may expect a running one. So always keep them on your mind. + +Queue Destruction +~~~~~~~~~~~~~~~~~ + +Now let's consider **the case of destruction of the primary +queue.**\ During destruction, our focus is on loosing as few messages as +possible. If the queue is not DA-enabled, there is nothing but the +configured timeouts to handle that situation. However, with a DA-enabled +queue there are more options. + +If the queue is DA-enabled, it may be *configured to persist messages to +disk before it is terminated*. In that case, loss of messages never +occurs (at the price of a potentially lengthy shutdown). Even if that +setting is not applied, the queue should drain as many messages as +possible to the disk. For that reason, it makes no sense to wait on a +low water mark. Also, if the queue is already in DA run mode, it does +not make any sense to switch back to regular run mode during termination +and then try to process some messages via the regular consumer. It is +much more appropriate the try completely drain the queue during the +remaining timeout period. For the same reason, it is preferred that no +new consumers be activated (via the DA queue's worker), as they only +cost valuable CPU cycles and, more importantly, would potentially be +long(er)-running and possibly be needed to be cancelled. To prevent all +of that, **queue parameters are changed for DA-enabled queues:** the +high water mark is to 1 and the low water mark to 0 on the primary +queue. The DA queue is commanded to run in enqueue-only mode. If the +primary queue is *configured to persist messages to disk before it is +terminated*, its SHUTDOWN timeout is changed to to eternal. These +parameters will cause the queue to drain as much as possible to disk +(and they may cause a case 3 DA run mode initiation). Please note that +once the primary queue has been drained, the DA queue's worker will +automatically switch back to regular (non-DA) run mode. **It must be +ensured that no worker cancellation occurs during that switchback**. +Please note that the queue may not switch back to regular run mode if it +is not *configured to persist messages to disk before it is terminated*. +In order to apply the new parameters, **worker threads must be +awakened.** Remember we may not be in DA run mode at this stage. In that +case, the regular workers must be awakened, which then will switch to DA +run mode. No worker may be active, in that case one must be initiated. +If in DA run mode and the DA worker is inactive, the "*DA worker +startup initiation*\ " must be called to activate it. That routine +ensures only one DA worker is started even with multiple concurrent +callers - this may be the case here. The DA queue's worker may have +requested DA worker startup in order to terminate on empty queue (which +will probably not be honored as we have changed the low water mark). + +After all this is done, the queue destructor requests termination of the +queue's worker threads. It will use the normal timeouts and potentially +cancel too-long running worker threads. **The shutdown process must +ensure that all workers reach running state before they are commanded to +terminate**. Otherwise it may run into a race condition that could lead +to a false shutdown with workers running asynchronously. As a few +workers may have just been started to initialize (to apply new parameter +settings), the probability for this race condition is extremely high, +especially on single-CPU systems. + +After all workers have been shut down (or cancelled), the queue may +still be in DA run mode. If so, this must be terminated, which now can +simply be done by destructing the DA queue object. This is not a real +switchback to regular run mode, but that doesn't matter because the +queue object will soon be gone away. + +Finally, the queue is mostly shut down and ready to be actually +destructed. As a last try, the queuePersists() entry point is called. It +is used to persists a non-DA-enabled queue in whatever way is possible +for that queue. There may be no implementation for the specific queue +type. Please note that this is not just a theoretical construct. This is +an extremely important code path when the DA queue itself is destructed. +Remember that it is a queue object in its own right. The DA queue is +obviously not DA-enabled, so it calls into queuePersists() during its +destruction - this is what enables us to persist the disk queue! + +After that point, left over queue resources (mutexes, dynamic memory, +...) are freed and the queue object is actually destructed. + |