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diff --git a/source/concepts/queues.rst b/source/concepts/queues.rst new file mode 100644 index 0000000..bb5647c --- /dev/null +++ b/source/concepts/queues.rst @@ -0,0 +1,524 @@ +Understanding rsyslog Queues +============================ + +Rsyslog uses queues whenever two activities need to be loosely coupled. +With a queue, one part of the system "produces" something while another +part "consumes" this something. The "something" is most often syslog +messages, but queues may also be used for other purposes. + +This document provides a good insight into technical details, operation +modes and implications. In addition to it, an :doc:`rsyslog queue concepts +overview <../whitepapers/queues_analogy>` document exists which tries to explain +queues with the help of some analogies. This may probably be a better +place to start reading about queues. I assume that once you have +understood that document, the material here will be much easier to grasp +and look much more natural. + +The most prominent example is the main message queue. Whenever rsyslog +receives a message (e.g. locally, via UDP, TCP or in whatever else way), +it places these messages into the main message queue. Later, it is +dequeued by the rule processor, which then evaluates which actions are +to be carried out. In front of each action, there is also a queue, which +potentially de-couples the filter processing from the actual action +(e.g. writing to file, database or forwarding to another host). + +Where are Queues Used? +---------------------- + +Currently, queues are used for the main message queue and for the +actions. + +There is a single main message queue inside rsyslog. Each input module +delivers messages to it. The main message queue worker filters messages +based on rules specified in rsyslog.conf and dispatches them to the +individual action queues. Once a message is in an action queue, it is +deleted from the main message queue. + +There are multiple action queues, one for each configured action. By +default, these queues operate in direct (non-queueing) mode. Action +queues are fully configurable and thus can be changed to whatever is +best for the given use case. + +Future versions of rsyslog will most probably utilize queues at other +places, too. + +Wherever "*<object>*\ " is used in the config file statements, +substitute "*<object>*\ " with either "MainMsg" or "Action". The former +will set main message queue parameters, the later parameters for the +next action that will be created. Action queue parameters can not be +modified once the action has been specified. For example, to tell the +main message queue to save its content on shutdown, use +*$MainMsgQueueSaveOnShutdown on*". + +If the same parameter is specified multiple times before a queue is +created, the last one specified takes precedence. The main message queue +is created after parsing the config file and all of its potential +includes. An action queue is created each time an action selector is +specified. Action queue parameters are reset to default after an action +queue has been created (to provide a clean environment for the next +action). + +Not all queues necessarily support the full set of queue configuration +parameters, because not all are applicable. For example, disk queues +always have exactly one worker thread. This cannot be overridden by +configuration parameters. Tries to do so are ignored. + +Queue Modes +----------- + +Rsyslog supports different queue modes, some with submodes. Each of them +has specific advantages and disadvantages. Selecting the right queue +mode is quite important when tuning rsyslogd. The queue mode (aka +"type") is set via the "*$<object>QueueType*\ " config directive. + +Direct Queues +~~~~~~~~~~~~~ + +Direct queues are **non**-queuing queues. A queue in direct mode does +neither queue nor buffer any of the queue elements but rather passes the +element directly (and immediately) from the producer to the consumer. +This sounds strange, but there is a good reason for this queue type. + +Direct mode queues allow to use queues generically, even in places where +queuing is not always desired. A good example is the queue in front of +output actions. While it makes perfect sense to buffer forwarding +actions or database writes, it makes only limited sense to build up a +queue in front of simple local file writes. Yet, rsyslog still has a +queue in front of every action. So for file writes, the queue mode can +simply be set to "direct", in which case no queuing happens. + +Please note that a direct queue also is the only queue type that passes +back the execution return code (success/failure) from the consumer to +the producer. This, for example, is needed for the backup action logic. +Consequently, backup actions require the to-be-checked action to use a +"direct" mode queue. + +To create a direct queue, use the "*$<object>QueueType Direct*\ " config +directive. + +Disk Queues +~~~~~~~~~~~ + +Disk queues use disk drives for buffering. The important fact is that +they always use the disk and do not buffer anything in memory. Thus, the +queue is ultra-reliable, but by far the slowest mode. For regular use +cases, this queue mode is not recommended. It is useful if log data is +so important that it must not be lost, even in extreme cases. + +When a disk queue is written, it is done in chunks. Each chunk receives +its individual file. Files are named with a prefix (set via the +"*$<object>QueueFilename*\ " config directive) and followed by a 7-digit +number (starting at one and incremented for each file). Chunks are 10mb +by default, a different size can be set via +the"*$<object>QueueMaxFileSize*\ " config directive. Note that the size +limit is not a sharp one: rsyslog always writes one complete queue +entry, even if it violates the size limit. So chunks are actually a +little bit (usually less than 1k) larger then the configured size. Each +chunk also has a different size for the same reason. If you observe +different chunk sizes, you can relax: this is not a problem. + +Writing in chunks is used so that processed data can quickly be deleted +and is free for other uses - while at the same time keeping no +artificial upper limit on disk space used. If a disk quota is set +(instructions further below), be sure that the quota/chunk size allows +at least two chunks to be written. Rsyslog currently does not check that +and will fail miserably if a single chunk is over the quota. + +Creating new chunks costs performance but provides quicker ability to +free disk space. The 10mb default is considered a good compromise +between these two. However, it may make sense to adapt these settings to +local policies. For example, if a disk queue is written on a dedicated +200gb disk, it may make sense to use a 2gb (or even larger) chunk size. + +Please note, however, that the disk queue by default does not update its +housekeeping structures every time it writes to disk. This is for +performance reasons. In the event of failure, data will still be lost +(except when manually is mangled with the file structures). However, +disk queues can be set to write bookkeeping information on checkpoints +(every n records), so that this can be made ultra-reliable, too. If the +checkpoint interval is set to one, no data can be lost, but the queue is +exceptionally slow. + +Each queue can be placed on a different disk for best performance and/or +isolation. This is currently selected by specifying different +*$WorkDirectory* config directives before the queue creation statement. + +To create a disk queue, use the "*$<object>QueueType Disk*\ " config +directive. Checkpoint intervals can be specified via +"*$<object>QueueCheckpointInterval*\ ", with 0 meaning no checkpoints. +Note that disk-based queues can be made very reliable by issuing a +(f)sync after each write operation. Starting with version 4.3.2, this +can be requested via "*<object>QueueSyncQueueFiles on/off* with the +default being off. Activating this option has a performance penalty, so +it should not be turned on without reason. + +If you happen to lose or otherwise need the housekeeping structures and +have all yours queue chunks you can use perl script included in rsyslog +package to generate it. +Usage: recover_qi.pl -w *$WorkDirectory* -f QueueFileName -d 8 > QueueFileName.qi + + +In-Memory Queues +~~~~~~~~~~~~~~~~ + +In-memory queue mode is what most people have on their mind when they +think about computing queues. Here, the enqueued data elements are held +in memory. Consequently, in-memory queues are very fast. But of course, +they do not survive any program or operating system abort (what usually +is tolerable and unlikely). Be sure to use an UPS if you use in-memory +mode and your log data is important to you. Note that even in-memory +queues may hold data for an infinite amount of time when e.g. an output +destination system is down and there is no reason to move the data out +of memory (lying around in memory for an extended period of time is NOT +a reason). Pure in-memory queues can't even store queue elements +anywhere else than in core memory. + +There exist two different in-memory queue modes: LinkedList and +FixedArray. Both are quite similar from the user's point of view, but +utilize different algorithms. + +A FixedArray queue uses a fixed, pre-allocated array that holds pointers +to queue elements. The majority of space is taken up by the actual user +data elements, to which the pointers in the array point. The pointer +array itself is comparatively small. However, it has a certain memory +footprint even if the queue is empty. As there is no need to dynamically +allocate any housekeeping structures, FixedArray offers the best run +time performance (uses the least CPU cycle). FixedArray is best if there +is a relatively low number of queue elements expected and performance is +desired. It is the default mode for the main message queue (with a limit +of 10,000 elements). + +A LinkedList queue is quite the opposite. All housekeeping structures +are dynamically allocated (in a linked list, as its name implies). This +requires somewhat more runtime processing overhead, but ensures that +memory is only allocated in cases where it is needed. LinkedList queues +are especially well-suited for queues where only occasionally a +than-high number of elements need to be queued. A use case may be +occasional message burst. Memory permitting, it could be limited to e.g. +200,000 elements which would take up only memory if in use. A FixedArray +queue may have a too large static memory footprint in such cases. + +**In general, it is advised to use LinkedList mode if in doubt**. The +processing overhead compared to FixedArray is low and may be outweighed by +the reduction in memory use. Paging in most-often-unused pointer array +pages can be much slower than dynamically allocating them. + +To create an in-memory queue, use the "*$<object>QueueType +LinkedList*\ " or "*$<object>QueueType FixedArray*\ " config directive. + +Disk-Assisted Memory Queues +^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +If a disk queue name is defined for in-memory queues (via +*$<object>QueueFileName*), they automatically become "disk-assisted" +(DA). In that mode, data is written to disk (and read back) on an +as-needed basis. + +Actually, the regular memory queue (called the "primary queue") and a +disk queue (called the "DA queue") work in tandem in this mode. Most +importantly, the disk queue is activated if the primary queue is full or +needs to be persisted on shutdown. Disk-assisted queues combine the +advantages of pure memory queues with those of pure disk queues. Under +normal operations, they are very fast and messages will never touch the +disk. But if there is need to, an unlimited amount of messages can be +buffered (actually limited by free disk space only) and data can be +persisted between rsyslogd runs. + +With a DA-queue, both disk-specific and in-memory specific configuration +parameters can be set. From the user's point of view, think of a DA +queue like a "super-queue" which does all within a single queue [from +the code perspective, there is some specific handling for this case, so +it is actually much like a single object]. + +DA queues are typically used to de-couple potentially long-running and +unreliable actions (to make them reliable). For example, it is +recommended to use a disk-assisted linked list in-memory queue in front +of each database and "send via tcp" action. Doing so makes these actions +reliable and de-couples their potential low execution speed from the +rest of your rules (e.g. the local file writes). There is a howto on +`massive database inserts <rsyslog_high_database_rate.html>`_ which +nicely describes this use case. It may even be a good read if you do not +intend to use databases. + +With DA queues, we do not simply write out everything to disk and then +run as a disk queue once the in-memory queue is full. A much smarter +algorithm is used, which involves a "high watermark" and a "low +watermark". Both specify numbers of queued items. If the queue size +reaches high watermark elements, the queue begins to write data elements +to disk. It does so until it reaches the low water mark elements. At +this point, it stops writing until either high water mark is reached +again or the on-disk queue becomes empty, in which case the queue +reverts back to in-memory mode, only. While holding at the low +watermark, new elements are actually enqueued in memory. They are +eventually written to disk, but only if the high water mark is ever +reached again. If it isn't, these items never touch the disk. So even +when a queue runs disk-assisted, there is in-memory data present (this +is a big difference to pure disk queues!). + +This algorithm prevents unnecessary disk writes, but also leaves some +additional buffer space for message bursts. Remember that creating disk +files and writing to them is a lengthy operation. It is too lengthy to +e.g. block receiving UDP messages. Doing so would result in message +loss. Thus, the queue initiates DA mode, but still is able to receive +messages and enqueue them - as long as the maximum queue size is not +reached. The number of elements between the high water mark and the +maximum queue size serves as this "emergency buffer". Size it according +to your needs, if traffic is very bursty you will probably need a large +buffer here. Keep in mind, though, that under normal operations these +queue elements will probably never be used. Setting the high water mark +too low will cause disk-assistance to be turned on more often than +actually needed. + +The water marks can be set via the "*$<object>QueueHighWatermark*\ " +and "*$<object>QueueLowWatermark*\ " configuration file directives. +Note that these are actual numbers, not percentages. Be sure they make +sense (also in respect to "*$<object>QueueSize*\ "). Rsyslogd does +perform some checks on the numbers provided, and issues warning when +numbers are "suspicious". + +Limiting the Queue Size +----------------------- + +All queues, including disk queues, have a limit of the number of +elements they can enqueue. This is set via the "*$<object>QueueSize*\ " +config parameter. Note that the size is specified in number of enqueued +elements, not their actual memory size. Memory size limits can not be +set. A conservative assumption is that a single syslog messages takes up +512 bytes on average (in-memory, NOT on the wire, this \*is\* a +difference). + +Disk assisted queues are special in that they do **not** have any size +limit. The enqueue an unlimited amount of elements. To prevent running +out of space, disk and disk-assisted queues can be size-limited via the +"*$<object>QueueMaxDiskSpace*\ " configuration parameter. If it is not +set, the limit is only available free space (and reaching this limit is +currently not very gracefully handled, so avoid running into it!). If a +limit is set, the queue can not grow larger than it. Note, however, that +the limit is approximate. The engine always writes complete records. As +such, it is possible that slightly more than the set limit is used +(usually less than 1k, given the average message size). Keeping strictly +on the limit would be a performance hurt, and thus the design decision +was to favour performance. If you don't like that policy, simply specify +a slightly lower limit (e.g. 999,999K instead of 1G). + +In general, it is a good idea to limit the physical disk space even if +you dedicate a whole disk to rsyslog. That way, you prevent it from +running out of space (future version will have an auto-size-limit logic, +that then kicks in in such situations). + +Worker Thread Pools +------------------- + +Each queue (except in "direct" mode) has an associated pool of worker +threads. Worker threads carry out the action to be performed on the data +elements enqueued. As an actual sample, the main message queue's worker +task is to apply filter logic to each incoming message and enqueue them +to the relevant output queues (actions). + +Worker threads are started and stopped on an as-needed basis. On a +system without activity, there may be no worker at all running. One is +automatically started when a message comes in. Similarly, additional +workers are started if the queue grows above a specific size. The +"*$<object>QueueWorkerThreadMinimumMessages*\ " config parameter +controls worker startup. If it is set to the minimum number of elements +that must be enqueued in order to justify a new worker startup. For +example, let's assume it is set to 100. As long as no more than 100 +messages are in the queue, a single worker will be used. When more than +100 messages arrive, a new worker thread is automatically started. +Similarly, a third worker will be started when there are at least 300 +messages, a forth when reaching 400 and so on. + +It, however, does not make sense to have too many worker threads running +in parallel. Thus, the upper limit can be set via +"*$<object>QueueWorkerThreads*\ ". If it, for example, is set to four, +no more than four workers will ever be started, no matter how many +elements are enqueued. + +Worker threads that have been started are kept running until an +inactivity timeout happens. The timeout can be set via +"*$<object>QueueWorkerTimeoutThreadShutdown*\ " and is specified in +milliseconds. If you do not like to keep the workers running, simply set +it to 0, which means immediate timeout and thus immediate shutdown. But +consider that creating threads involves some overhead, and this is why +we keep them running. If you would like to never shutdown any worker +threads, specify -1 for this parameter. + +Discarding Messages +~~~~~~~~~~~~~~~~~~~ + +If the queue reaches the so called "discard watermark" (a number of +queued elements), less important messages can automatically be +discarded. This is in an effort to save queue space for more important +messages, which you even less like to lose. Please note that whenever +there are more than "discard watermark" messages, both newly incoming as +well as already enqueued low-priority messages are discarded. The +algorithm discards messages newly coming in and those at the front of +the queue. + +The discard watermark is a last resort setting. It should be set +sufficiently high, but low enough to allow for large message burst. +Please note that it take effect immediately and thus shows effect +promptly - but that doesn't help if the burst mainly consist of +high-priority messages... + +The discard watermark is set via the "*$<object>QueueDiscardMark*\ " +directive. The priority of messages to be discarded is set via +"*$<object>QueueDiscardSeverity*\ ". This directive accepts both the +usual textual severity as well as a numerical one. To understand it, you +must be aware of the numerical severity values. They are defined in RFC +3164: + + ==== ======== + Code Severity + ==== ======== + 0 Emergency: system is unusable + 1 Alert: action must be taken immediately + 2 Critical: critical conditions + 3 Error: error conditions + 4 Warning: warning conditions + 5 Notice: normal but significant condition + 6 Informational: informational messages + 7 Debug: debug-level messages + ==== ======== + +Anything of the specified severity and (numerically) above it is +discarded. To turn message discarding off, simply specify the discard +watermark to be higher than the queue size. An alternative is to specify +the numerical value 8 as DiscardSeverity. This is also the default +setting to prevent unintentional message loss. So if you would like to +use message discarding, you need to set" +*$<object>QueueDiscardSeverity*" to an actual value. + +An interesting application is with disk-assisted queues: if the discard +watermark is set lower than the high watermark, message discarding will +start before the queue becomes disk-assisted. This may be a good thing +if you would like to switch to disk-assisted mode only in cases where it +is absolutely unavoidable and you prefer to discard less important +messages first. + +Filled-Up Queues +---------------- + +If the queue has either reached its configured maximum number of entries +or disk space, it is finally full. If so, rsyslogd throttles the data +element submitter. If that, for example, is a reliable input (TCP, local +log socket), that will slow down the message originator which is a good +resolution for this scenario. + +During throttling, a disk-assisted queue continues to write to disk and +messages are also discarded based on severity as well as regular +dequeuing and processing continues. So chances are good the situation +will be resolved by simply throttling. Note, though, that throttling is +highly undesirable for unreliable sources, like UDP message reception. +So it is not a good thing to run into throttling mode at all. + +We can not hold processing infinitely, not even when throttling. For +example, throttling the local log socket too long would cause the +system at whole come to a standstill. To prevent this, rsyslogd times +out after a configured period ("*$<object>QueueTimeoutEnqueue*\ ", +specified in milliseconds) if no space becomes available. As a last +resort, it then discards the newly arrived message. + +If you do not like throttling, set the timeout to 0 - the message will +then immediately be discarded. If you use a high timeout, be sure you +know what you do. If a high main message queue enqueue timeout is set, +it can lead to something like a complete hang of the system. The same +problem does not apply to action queues. + +Rate Limiting +~~~~~~~~~~~~~ + +Rate limiting provides a way to prevent rsyslogd from processing things +too fast. It can, for example, prevent overrunning a receiver system. + +Currently, there are only limited rate-limiting features available. The +"*$<object>QueueDequeueSlowdown*\ " directive allows to specify how +long (in microseconds) dequeueing should be delayed. While simple, it +still is powerful. For example, using a DequeueSlowdown delay of 1,000 +microseconds on a UDP send action ensures that no more than 1,000 +messages can be sent within a second (actually less, as there is also +some time needed for the processing itself). + +Processing Timeframes +~~~~~~~~~~~~~~~~~~~~~ + +Queues can be set to dequeue (process) messages only during certain +timeframes. This is useful if you, for example, would like to transfer +the bulk of messages only during off-peak hours, e.g. when you have only +limited bandwidth on the network path to the central server. + +Currently, only a single timeframe is supported and, even worse, it can +only be specified by the hour. It is not hard to extend rsyslog's +capabilities in this regard - it was just not requested so far. So if +you need more fine-grained control, let us know and we'll probably +implement it. There are two configuration directives, both should be +used together or results are unpredictable:" +*$<object>QueueDequeueTimeBegin <hour>*" +and "*$<object>QueueDequeueTimeEnd <hour>*\ ". The hour parameter must +be specified in 24-hour format (so 10pm is 22). A use case for this +parameter can be found in the `rsyslog +wiki <http://wiki.rsyslog.com/index.php/OffPeakHours>`_. + +Performance +~~~~~~~~~~~ + +The locking involved with maintaining the queue has a potentially large +performance impact. How large this is, and if it exists at all, depends +much on the configuration and actual use case. However, the queue is +able to work on so-called "batches" when dequeueing data elements. With +batches, multiple data elements are dequeued at once (with a single +locking call). The queue dequeues all available elements up to a +configured upper limit (*<object>DequeueBatchSize <number>*). It is +important to note that the actual upper limit is dictated by +availability. The queue engine will never wait for a batch to fill. So +even if a high upper limit is configured, batches may consist of fewer +elements, even just one, if there are no more elements waiting in the +queue. + +Batching can improve performance considerably. Note, however, that it +affects the order in which messages are passed to the queue worker +threads, as each worker now receive as batch of messages. Also, the +larger the batch size and the higher the maximum number of permitted +worker threads, the more main memory is needed. For a busy server, large +batch sizes (around 1,000 or even more elements) may be useful. Please +note that with batching, the main memory must hold BatchSize \* +NumOfWorkers objects in memory (worst-case scenario), even if running in +disk-only mode. So if you use the default 5 workers at the main message +queue and set the batch size to 1,000, you need to be prepared that the +main message queue holds up to 5,000 messages in main memory **in +addition** to the configured queue size limits! + +The queue object's default maximum batch size is eight, but there exists +different defaults for the actual parts of rsyslog processing that +utilize queues. So you need to check these object's defaults. + +Terminating Queues +~~~~~~~~~~~~~~~~~~ + +Terminating a process sounds easy, but can be complex. Terminating a +running queue is in fact the most complex operation a queue object can +perform. You don't see that from a user's point of view, but its quite +hard work for the developer to do everything in the right order. + +The complexity arises when the queue has still data enqueued when it +finishes. Rsyslog tries to preserve as much of it as possible. As a +first measure, there is a regular queue time out +("*$<object>QueueTimeoutShutdown*\ ", specified in milliseconds): the +queue workers are given that time period to finish processing the queue. + +If after that period there is still data in the queue, workers are +instructed to finish the current data element and then terminate. This +essentially means any other data is lost. There is another timeout +("*$<object>QueueTimeoutActionCompletion*\ ", also specified in +milliseconds) that specifies how long the workers have to finish the +current element. If that timeout expires, any remaining workers are +cancelled and the queue is brought down. + +If you do not like to lose data on shutdown, the +"*$<object>QueueSaveOnShutdown*\ " parameter can be set to "on". This +requires either a disk or disk-assisted queue. If set, rsyslogd ensures +that any queue elements are saved to disk before it terminates. This +includes data elements there were begun being processed by workers that +needed to be cancelled due to too-long processing. For a large queue, +this operation may be lengthy. No timeout applies to a required shutdown +save. |