.. _concepts-queues:

Understanding rsyslog Queues
============================

.. index::
   single: queue
   single: queue; main queue
   single: queue; ruleset queue
   single: queue; action queue
   single: queue; disk-assisted
   single: queue; direct

.. meta::
   :description: Conceptual overview of rsyslog queue types, configuration scope, and operational behaviors for main, ruleset, and action queues.
   :keywords: rsyslog, queue, main queue, ruleset queue, action queue, disk queue, disk assisted, linked list, fixed array, rainerScript

.. summary-start

Explains how rsyslog queues decouple producers and consumers, the differences between direct, memory, disk, and disk-assisted modes, and how main, ruleset, and per-action queues are configured with modern RainerScript syntax.

.. summary-end

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. The engine
relies on three queue scopes:

- A **main message queue** sits after inputs and feeds the primary ruleset.
- **Ruleset queues** optionally buffer messages bound to a specific ruleset
  to isolate processing between different pipelines.
- **Per-action queues** sit in front of each configured action and can be
  tuned individually.

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 be a better starting
point; once you have understood that document, the material here will be
much easier to grasp and look much more natural.

.. seealso::

   - :doc:`action() object reference <../configuration/actions>`
   - :doc:`ruleset() usage and binding <../concepts/multi_ruleset>`
   - :doc:`main_queue() and shared queue parameter reference <../rainerscript/queue_parameters>`
   - :doc:`log pipeline stages <log_pipeline/stages>` for end-to-end stage context

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. A ruleset queue, if
configured, buffers that ruleset’s workload before handing messages to its
actions. 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).

Queue scope and configuration syntax
------------------------------------

rsyslog operates one main queue and can attach additional queues at each
ruleset and action:

- 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 in ``rsyslog.conf`` and dispatches them to the
  relevant ruleset (and thus action) queues. Once a message is in a
  downstream queue, it is deleted from the main queue.
- Optional **ruleset queues** isolate a ruleset's workload. They absorb
  bursts and provide per-pipeline flow control before the ruleset's
  actions run.
- 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 can be tuned independently for the
  given use case.

Queue parameters are expressed with modern RainerScript syntax inside
the object that owns the queue:

- ``main_queue()`` defines the main message queue.
- ``ruleset()`` can define a dedicated queue for that ruleset.
- ``action()`` embeds an action-specific queue.

Parameters are written as ``queue.<parameter>=<value>`` attributes inside
the respective object. For example, to make the main queue persistent
across restarts and to size its memory buffer, use::

    global(workDirectory="/var/spool/rsyslog")
    main_queue(
        queue.type="LinkedList"
        queue.size="200000"
        queue.filename="mainq"
        queue.saveOnShutdown="on"
    )

Action queue parameters live inside the action statement and apply only
to that action::

    action(
        type="omfwd" target="10.0.0.5" protocol="tcp"
        queue.type="LinkedList"
        queue.dequeueBatchSize="1000"
        queue.workerThreads="4"
    )

Ruleset queues follow the same syntax inside the ``ruleset()`` block and
allow you to tune burst handling for a specific processing pipeline::

    ruleset(name="net_ingest"
        queue.type="LinkedList"
        queue.size="500000"
        queue.maxDiskSpace="20g"
    )

Queue parameters are reset to defaults for each action definition. The
main queue is created after rsyslog has parsed the complete
configuration (including any ``include`` statements).

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. Attempts 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 (``type``)
is set via the ``queue.type`` parameter.

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 rsyslog to keep a unified queueing model while
avoiding unnecessary buffering in front of simple actions such as local
file writes. Direct mode is also the only queue type that passes back
execution return codes (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
``queue.type="Direct"`` queue.

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
``queue.filename`` parameter) 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 ``queue.maxFileSize``. 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 can be slightly larger
than configured.

Writing in chunks is used so that processed data can quickly be deleted
while at the same time keeping no artificial upper limit on disk space
used. If a disk quota is set (via ``queue.maxDiskSpace``), 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.

The disk queue by default does not update its housekeeping structures
every time it writes to disk. For durability, tune ``queue.checkpointInterval``
and ``queue.syncqueuefiles``. Each queue can be placed on a different
disk for best performance and/or isolation by setting a dedicated
``queue.spoolDirectory`` on that queue definition. To create a disk
queue, set ``queue.type="Disk"``.

If you happen to lose or otherwise need the housekeeping structures and
have all your queue chunks you can use the ``recover_qi.pl`` script
included in rsyslog to regenerate them::

    recover_qi.pl -w /var/spool/rsyslog -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 a 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.

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
higher-than-normal 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, set ``queue.type="LinkedList"`` or
``queue.type="FixedArray"``.

Disk-Assisted Memory Queues
^^^^^^^^^^^^^^^^^^^^^^^^^^^

If a disk queue name is defined for in-memory queues (via
``queue.filename``), 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.

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
:doc:`massive database inserts </tutorials/high_database_rate>` which
nicely describes this use case. It may even be a good read if you do not
intend to use databases.

With DA queues, rsyslog uses a "high watermark" and a "low watermark"
to control spillover. Both specify numbers of queued items. If the queue
size reaches ``queue.highWatermark`` elements, the queue begins to write
data elements to disk. It does so until it reaches ``queue.lowWatermark``
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
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.

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 ``queue.size`` parameter.
Note that the size is specified in number of enqueued elements, not
actual memory size.

Disk assisted queues are special in that they do **not** have any size
limit. They enqueue an unlimited amount of elements. To prevent running
out of space, disk and disk-assisted queues can be size-limited via the
``queue.maxDiskSpace`` configuration parameter. If it is not set, the
limit is only available free space. 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 hurt
performance, so specify a slightly lower limit (e.g. ``999999K`` instead
of ``1G``) if you need a sharp cutoff.

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.

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
``queue.workerThreadMinimumMessages`` parameter controls worker startup
and interacts with ``queue.workerThreads``, the maximum number of worker
threads. For example, if ``queue.workerThreadMinimumMessages="1000"``, a
second worker starts once the queue holds more than 1000 messages, a
third at 2000, and so on.

Worker threads that have been started are kept running until an
inactivity timeout happens. The timeout can be set via
``queue.timeoutWorkerthreadShutdown`` 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 they are kept
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 the discard watermark, 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. The
watermark is set via ``queue.discardMark``. The priority of messages to
be discarded is set via ``queue.discardSeverity``. 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 ``queue.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 ``queue.discardSeverity``
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 (``queue.timeoutEnqueue``, 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
``queue.dequeueSlowDown`` directive allows to specify how long (in
microseconds) dequeueing should be delayed. While simple, it still is
powerful. For example, using a ``queue.dequeueSlowDown="1000"`` delay on
a UDP send action ensures that no more than roughly 1,000 messages can
be sent within a second (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 it can only be
specified by the hour. There are two configuration directives, both
should be used together or results are unpredictable: ``queue.dequeueTimeBegin``
and ``queue.dequeueTimeEnd``. 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 (``queue.dequeueBatchSize``). 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 receives a 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
``queue.dequeueBatchSize`` * ``queue.workerThreads`` 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 (``queue.timeoutshutdown``,
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
(``queue.timeoutActionCompletion``, 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
``queue.saveOnShutdown`` 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 that were 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.

.. _concept-model-concepts-queues:

Conceptual model
----------------

- Queues decouple producers and consumers so message flow continues even when downstream actions stall.
- Each queue instance is typed (direct, memory, disk, disk-assisted) to trade throughput, latency, and durability.
- Direct queues act as synchronous pass-through channels, enabling return-code propagation for backup actions.
- Memory queues prioritize low latency and throughput, with LinkedList vs. FixedArray implementations balancing footprint and CPU cost.
- Disk queues persist every element to chunked files, favoring durability over performance and bypassing in-memory buffering.
- Disk-assisted queues layer a memory primary queue with a spillover disk queue, governed by high/low watermarks to avoid unnecessary writes.
- Worker thread pools dequeue elements in batches, scaling concurrency based on backlog while respecting configured limits.
- Flow control uses enqueue timeouts, discard watermarks, and optional rate limiting to bound resource use under pressure.
- Shutdown sequencing uses timeouts and optional save-on-shutdown persistence to minimize data loss during termination.

Related references
------------------
- :doc:`Log pipeline overview <log_pipeline/index>` for how queues interact with inputs, rulesets, and actions.
- :doc:`Queue parameters reference <../rainerscript/queue_parameters>` for full RainerScript options.
- :doc:`Queue analogy whitepaper <../whitepapers/queues_analogy>` for a narrative introduction.
