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diff --git a/doc/queues.html b/doc/queues.html new file mode 100644 index 0000000..80641d8 --- /dev/null +++ b/doc/queues.html @@ -0,0 +1,350 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> +<html><head> +<meta http-equiv="Content-Language" content="de"> +<title>Understanding rsyslog queues</title></head> +<body> + +<h1>Understanding rsyslog Queues</h1> +<p>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.</p> +<p>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).</p> +<h1>Queue Modes</h1> +<p>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 "<i>$<object>QueueType</i>" +config directive.</p> +<h2>Direct Queues</h2> +<p>Direct queues are <b>non</b>-queuing queues. A queue in direct mode does +neither queue nor buffer any of the queue elements but rather passes the element +directly (and immediately) from the producer to the consumer. This sounds strange, +but there is a good reason for this queue type.</p> +<p>Direct mode queues allow to use queues generically, even in places where +queuing is not always desired. A good example is the queue in front of output +actions. While it makes perfect sense to buffer forwarding actions or database +writes, it makes only limited sense to build up a queue in front of simple local +file writes. Yet, rsyslog still has a queue in front of every action. So for +file writes, the queue mode can simply be set to "direct", in which case no +queuing happens.</p> +<p>Please note that a direct queue also is the only queue type that passes back +the execution return code (success/failure) from the consumer to the producer. +This, for example, is needed for the backup action logic. Consequently, backup +actions require the to-be-checked action to use a "direct" mode queue.</p> +<p>To create a direct queue, use the "<i>$<object>QueueType Direct</i>" config +directive.</p> +<h2>Disk Queues</h2> +<p>Disk queues use disk drives for buffering. The important fact is that the +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.</p> +<p>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 "<i>$<object>QueueFilename</i>" +config directive) and followed by a 7-digit number (starting at one and +incremented for each file). Chunks are 10mb by default, a different size can be +set via the"<i>$<object>QueueMaxFileSize</i>" 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 but +(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.</p> +<p>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.</p> +<p>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.</p> +<p>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.</p> +<p>Each queue can be placed on a different disk for best performance and/or +isolation. This is currently selected by specifying different <i>$WorkDirectory</i> +config directives before the queue creation statement.</p> +<p>To create a disk queue, use the "<i>$<object>QueueType Disk</i>" config +directive. Checkpoint intervals can be specified via "<i>$<object>QueueCheckpointInterval</i>", +with 0 meaning no checkpoints. </p> +<h2>In-Memory Queues</h2> +<p>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. </p> +<p>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. </p> +<p>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).</p> +<p>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.</p> +<p><b>In general, it is advised to use LinkedList mode if in doubt</b>. The +processing overhead compared to FixedArray is low and may be +<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman',serif;" lang="EN-US"> +outweigh </span>by the reduction in memory use. Paging in most-often-unused +pointer array pages can be much slower than dynamically allocating them.</p> +<p>To create an in-memory queue, use the "<i>$<object>QueueType LinkedList</i>" +or "<i>$<object>QueueType FixedArray</i>" config directive.</p> +<h3>Disk-Assisted Memory Queues</h3> +<p>If a disk queue name is defined for in-memory queues (via <i> +$<object>QueueFileName</i>), they automatically +become "disk-assisted" (DA). In that mode, data is written to disk (and read +back) on an as-needed basis.</p> +<p>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.</p> +<p>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].</p> +<p>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 <a href="rsyslog_high_database_rate.html"> +massive database inserts</a> which nicely describes this use case. It may even +be a good read if you do not intend to use databases.</p> +<p>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!).</p> +<p>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.</p> +<p>The water marks can be set via the "<i>$<object>QueueHighWatermark</i>" and +"<i>$<object>QueueHighWatermark</i>" configuration file directives. Note that +these are actual numbers, not precentages. Be sure they make sense (also in +respect to "<i>$<object>QueueSize</i>"), as rsyslodg does currently not perform +any checks on the numbers provided. It is easy to screw up the system here (yes, +a feature enhancement request is filed ;)).</p> +<h1>Limiting the Queue Size</h1> +<p>All queues, including disk queues, have a limit of the number of elements +they can enqueue. This is set via the "<i>$<object>QueueSize</i>" 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).</p> +<p>Disk assisted queues are special in that they do <b>not</b> 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 "<i>$<object>QueueMaxDiskSpace</i>" +configuration parameter. If it is not set, the limit is only available free +space (and reaching this limit is currently not very gracefully handled, so +avoid running into it!). If a limit is set, the queue can not grow larger than +it. Note, however, that the limit is approximate. The engine always writes +complete records. As such, it is possible that slightly more than the set limit +is used (usually less than 1k, given the average message size). Keeping strictly +on the limit would be a performance hurt, and thus the design decision was to +favour performance. If you don't like that policy, simply specify a slightly +lower limit (e.g. 999,999K instead of 1G).</p> +<p>In general, it is a good idea to limit the pysical disk space even if you +dedicate a whole disk to rsyslog. That way, you prevent it from running out of +space (future version will have an auto-size-limit logic, that then kicks in in +such situations).</p> +<h1>Worker Thread Pools</h1> +<p>Each queue (except in "direct" mode) has an associated pool of worker +threads. Worker threads carry out the action to be performed on the data +elements enqueued. As an actual sample, the main message queue's worker task is +to apply filter logic to each incoming message and enqueue them to the relevant +output queues (actions).</p> +<p>Worker threads are started and stopped on an as-needed basis. On a system +without activity, there may be no worker at all running. One is automatically +started when a message comes in. Similarily, additional workers are started if +the queue grows above a specific size. The "<i>$<object>QueueWorkerThreadMinimumMessages</i>" +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. Similarily, a third worker will be +started when there are at least 300 messages, a forth when reaching 400 and so +on.</p> +<p>It, however, does not make sense to have too many worker threads running in +parall. Thus, the upper limit ca be set via "<i>$<object>QueueWorkerThreads</i>". +If it, for example, is set to four, no more than four workers will ever be +started, no matter how many elements are enqueued. </p> +<p>Worker threads that have been started are kept running until an inactivity +timeout happens. The timeout can be set via "<i>$<object>QueueWorkerTimeoutShutdown</i>" +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.</p> +<h2>Discarding Messages</h2> +<p>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 loose. 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.</p> +<p>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...</p> +<p>The discard watermark is set via the "<i>$<object>QueueDiscardMark</i>" +directive. The priority of messages to be discarded is set via "<i>$<object>QueueDiscardSeverity</i>". +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:</p> +<pre> Numerical Severity<br> Code<br><br> 0 Emergency: system is unusable<br> 1 Alert: action must be taken immediately<br> 2 Critical: critical conditions<br> 3 Error: error conditions<br> 4 Warning: warning conditions<br> 5 Notice: normal but significant condition<br> 6 Informational: informational messages<br> 7 Debug: debug-level messages</pre> +<p>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" <i>$<object>QueueDiscardSeverity</i>" to an actual value.</p> +<p>An interesting application is with disk-assisted queues: if the discard +watermark is set lower than the high watermark, message discarding will start +before the queue becomes disk-assisted. This may be a good thing if you would +like to switch to disk-assisted mode only in cases where it is absolutely +unavoidable and you prefer to discard less important messages first.</p> +<h1>Filled-Up Queues</h1> +<p>If the queue has either reached its configured maximum number of entries or +disk space, it is finally full. If so, rsyslogd throttles the data element +submitter. If that, for example, is a reliable input (TCP, local log socket), +that will slow down the message originator which is a good +<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman',serif;" lang="EN-US"> +resolution </span>for this scenario.</p> +<p>During +<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman',serif;" lang="EN-US"> +throtteling</span>, a disk-assisted queue continues to write to disk and +messages are also discarded based on severity as well as regular dequeuing and +processing continues. So chances are good the situation will be resolved by +simply throttling. Note, though, that throtteling is highly undesirable for +unreliable sources, like UDP message reception. So it is not a good thing to run +into throtteling mode at all.</p> +<p>We can not hold processing +<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman',serif;" lang="EN-US"> +infinitely</span>, not even when throtteling. For example, throtteling the local +log socket too long would cause the system at whole come to a standstill. To +prevent this, rsyslogd times out after a configured period ("<i>$<object>QueueTimeoutEnqueue</i>", +specified in milliseconds) if no space becomes available. As a last resort, it +then discards the newly arrived message.</p> +<p>If you do not like throtteling, 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.</p> +<h2>Rate Limiting</h2> +<p>Rate limiting provides a way to prevent rsyslogd from processing things too +fast. It can, for example, prevent overruning a receiver system.</p> +<p>Currently, there are only limited rate-limiting features available. The "<i>$<object>QueueDequeueSlowdown</i>" +directive allows to specify how long (in microseconds) dequeueing should be +delayed. While simple, it still is powerful. For example, using a +DequeueSlowdown delay of 1,000 microseconds on a UDP send action ensures that no +more than 1,000 messages can be sent within a second (actually less, as there is +also some time needed for the processing itself). </p> +<h2>Terminating Queues</h2> +<p>Terminating a process sounds easy, but can be complex. +<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman',serif;" lang="EN-US"> +Terminating </span>a running queue is in fact the most complex operation a queue +object can perform. You don't see that from a user's point of view, but its +quite hard work for the developer to do everything in the right order.</p> +<p>The complexity arises when the queue has still data enqueued when it +finishes. Rsyslog tries to preserve as much of it as possible. As a first +measure, there is a regular queue time out ("<i>$<object>QueueTimeoutShutdown</i>", +specified in milliseconds): the queue workers are given that time period to +finish processing the queue.</p> +<p>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 ("<i>$<object>QueueTimeoutActionCompletion</i>", +also specified in milliseconds) that specifies how long the workers have to +finish the current element. If that timeout expires, any remaining workers are +cancelled and the queue is brought down.</p> +<p>If you do not like to lose data on shutdown, the "<i>$<object>QueueSaveOnShutdown</i>" +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.</p> +<h1>Where are Queues Used?</h1> +<p> Currently, queues are used for the main message queue and for the +actions.</p> +<p>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.</p> +<p>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.</p> +<p>Future versions of rsyslog will most probably utilize queues at other places, +too.</p> +<p> +<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman',serif;" lang="EN-US"> +Wherever </span>"<i><object></i>" was used above in the config file +statements, substitute "<i><object></i>" with either "MainMsg" or "Action". The +former will set main message queue +<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman',serif;" lang="EN-US"> +parameters</span>, 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 <i>$MainMsgQueueSaveOnShutdown on</i>".</p> +<p>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).</p> +<p>Not all queues necessarily support the full set of queue configuration +parameters, because not all are applicable. For example, in current output +module design, actions do not support multi-threading. Consequently, the number +of worker threads is fixed to one for action queues and can not be changed.</p> + +</body></html>
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