[[persistent-queues]]
=== Persistent queues (PQ)

A {ls} persistent queue helps protect against data loss during abnormal termination by storing the in-flight message queue to disk.

[[persistent-queues-benefits]]
==== Benefits of persistent queues

A persistent queue (PQ):

* Helps protect against message loss during a normal shutdown and when Logstash
is terminated abnormally. 
If Logstash is restarted while events are in-flight,
Logstash attempts to deliver messages stored in the persistent queue until
delivery succeeds at least once.  
* Can absorb bursts of events without needing an external buffering mechanism like Redis or Apache Kafka. 

NOTE: Persistent queues are disabled by default.
To enable them, check out <<configuring-persistent-queues>>.

[[persistent-queues-limitations]]
==== Limitations of persistent queues

Persistent queues do not solve these problems:

* Input plugins that do not use a request-response protocol cannot be protected from data loss. Tcp, udp, zeromq push+pull, and many other inputs do not have a mechanism to acknowledge receipt to the sender. 
(Plugins such as beats and http, which *do* have an acknowledgement capability, are well protected by this queue.)
* Data may be lost if an abnormal shutdown occurs before the checkpoint file has been committed.
* A persistent queue does not handle permanent machine failures such as disk corruption, disk failure, and machine loss. 
The data persisted to disk is not replicated.


[[configuring-persistent-queues]]
==== Configuring persistent queues

To configure persistent queues, specify options in the Logstash <<logstash-settings-file,settings file>>:

`queue.type`:: Specify `persisted` to enable persistent queues. By default, persistent queues are disabled (default: `queue.type: memory`).
`path.queue`:: The directory path where the data files will be stored. By default, the files are stored in `path.data/queue`. 
`queue.page_capacity`:: The maximum size of a queue page in bytes. The queue data consists of append-only files called "pages". The default size is 64mb. Changing this value is unlikely to have performance benefits.
`queue.drain`:: Specify `true` if you want Logstash to wait until the persistent queue is drained before shutting down. The amount of time it takes to drain the queue depends on the number of events that have accumulated in the queue. Therefore, you should avoid using this setting unless the queue, even when full, is relatively small and can be drained quickly. 
`queue.max_events`:: The maximum number of events not yet read by the pipeline worker. The default is 0 (unlimited).
We use this setting for internal testing. 
Users generally shouldn't be changing this value.
`queue.max_bytes`:: The total capacity of _each queue_ in number of bytes. 
Unless overridden in `pipelines.yml` or central management, each persistent
queue will be sized at the value of `queue.max_bytes` specified in
`logstash.yml`. 
The default is 1024mb (1gb).
+
Be sure that your disk has sufficient capacity to handle the cumulative total of `queue.max_bytes` across all persistent queues.
The total of `queue.max.bytes` for _all_ queues should be
lower than the capacity of your disk. 
+
TIP: If you are using persistent queues to protect against data loss, but don't
require much buffering, you can set `queue.max_bytes` to a smaller value.
A smaller value produces smaller queues and improves queue performance. 

`queue.checkpoint.acks`:: Sets the number of acked events before forcing a checkpoint. 
Default is `1024`. Set to `0` for unlimited.
`queue.checkpoint.writes`:: Sets the maximum number of written events before a forced checkpoint. 
Default is `1024`. Set to `0` for unlimited.
+
To avoid losing data in the persistent queue, you can set `queue.checkpoint.writes: 1` to force a checkpoint after each event is
written. Keep in mind that disk writes have a resource cost. Setting this value
to `1` ensures maximum durability, but can severely impact performance.
See <<durability-persistent-queues>> to better understand the trade-offs.
`queue.checkpoint.interval`:: Sets the interval in milliseconds when a checkpoint is forced on the head page.
Default is `1000`. Set to `0` to eliminate periodic checkpoints.

[[pq-config-notes]]
==== Configuration notes

Every situation and environment is different, and the "ideal" configuration varies.
If you optimize for performance, you may increase your risk of losing data.
If you optimize for data protection, you may impact performance. 

[[pq-size]]
===== Queue size
You can control queue size with the `queue.max_events` and  `queue.max_bytes` settings.
If both settings are specified, Logstash uses whichever criteria is reached
first. 
See <<backpressure-persistent-queue>> for behavior when queue limits are
reached.

[[pq-lower-max_bytes]]
===== Smaller queue size
If you are using persistent queues to protect against data loss, but don't
require much buffering, you can set `queue.max_bytes` to a smaller value.
A smaller value may produce smaller queues and improves queue performance.

*Sample configuration*

[source, yaml]
-----
queue.type: persisted
queue.max_bytes: 10mb 
-----

[[pq-fewer-checkpoints]]
===== Fewer checkpoints

Setting `queue.checkpoint.writes` and `queue.checkpoint.acks` to `0` may
yield maximum performance, but may have potential impact on durability.

In a situation where Logstash is terminated or there is a hardware-level
failure, any data that has not been checkpointed, is lost. 
See <<durability-persistent-queues>> to better understand the trade-offs.

[[troubleshooting-pqs]]
==== Troubleshooting persistent queues

Symptoms of persistent queue problems include {ls} or one or more pipelines not starting successfully, accompanied by an error message similar to this one.

```
message=>"java.io.IOException: Page file size is too small to hold elements
```

This error indicates that the head page (the oldest in a directory and the one with lowest page id) has a size < 18 bytes, the size of a page header.


To research and resolve the issue: 

. Identify the queue (or queues) that may be corrupt by checking log files, or running the `pqcheck` utility.
. Stop Logstash, and wait for it to shut down. 
. Run `pqrepair <path>` for each of the corrupted queues.

[[pqcheck]]
===== `pqcheck` utility

 the `pqcheck` utility to identify which persistent queue--or queues--have been corrupted. 

From LOGSTASH_HOME, run:

[source,txt]
-----
bin/pqcheck <queue_directory>
-----

where `<queue_directory>` is the fully qualified path to the persistent queue location.

The `pqcheck utility` reads through the checkpoint files in the given directory and outputs information about the current state of those files. 
The utility outputs this information for each checkpoint file:

* Checkpoint file name
* Whether or not the page file has been fully acknowledged.
A fully acknowledged page file indicates that all events have been read and processed.
* Page file name that the checkpoint file is referencing
* Size of the page file. A page file with a size of 0 results in the output `NOT FOUND`. 
In this case, run `pqrepair` against the specified queue directory. 
* Page number
* First unacknowledged page number (only relevant in the head checkpoint)
* First unacknowledged event sequence number in the page
* First event sequence number in the page
* Number of events in the page
* Whether or not the page has been fully acknowledged

*Sample with healthy page file*

This sample represents a healthy queue with three page files. 
In this sample, Logstash is currently writing to `page.2` as referenced by
`checkpoint.head`.
Logstash is reading from `page.0` as referenced by `checkpoint.0`.

[source,txt]
-----
ubuntu@bigger:/usr/share/logstash$ bin/pqcheck /var/lib/logstash/queue/main/
Using bundled JDK: /usr/share/logstash/jdk
OpenJDK 64-Bit Server VM warning: Option UseConcMarkSweepGC was deprecated in version 9.0 and will likely be removed in a future release.
Checking queue dir: /var/lib/logstash/queue/main
checkpoint.1, fully-acked: NO, page.1 size: 67108864 
  pageNum=1, firstUnackedPageNum=0, firstUnackedSeqNum=239675, minSeqNum=239675,
  elementCount=218241, isFullyAcked=no 
checkpoint.head, fully-acked: NO, page.2 size: 67108864
  pageNum=2, firstUnackedPageNum=0, firstUnackedSeqNum=457916, minSeqNum=457916, elementCount=11805, isFullyAcked=no
checkpoint.0, fully-acked: NO, page.0 size: 67108864  <1>
  pageNum=0, firstUnackedPageNum=0, firstUnackedSeqNum=176126, minSeqNum=1,
  elementCount=239674, isFullyAcked=no <2>
-----
<1> Represents `checkpoint.0`, which refers to the page file `page.0`, and has a size of `67108864`. 
<2> Continuing for `checkpoint.0`, these lines indicate that the page number is `0`, the first unacknowledged event is number `176126`, there are `239674` events in the page file, the first event in this page file is event number `1`, and the page file has not been fully acknowledged. That is, there are still events left in the page file that need to be ingested.


*Sample with corrupted page file*

If Logstash doesn't start and/or `pqcheck` shows an anomaly, such as `NOT_FOUND` for a page, run `pqrepair` on the queue directory.

[source,txt]
-----
bin/pqcheck /var/lib/logstash/queue/main/
Using bundled JDK: /usr/share/logstash/jdk
OpenJDK 64-Bit Server VM warning: Option UseConcMarkSweepGC was deprecated in version 9.0 and will likely be removed in a future release.
Checking queue dir: /var/lib/logstash/queue/main
checkpoint.head, fully-acked: NO, page.2 size: NOT FOUND <1>
  pageNum=2, firstUnackedPageNum=2, firstUnackedSeqNum=534041, minSeqNum=457916,
  elementCount=76127, isFullyAcked=no
-----
<1> `NOT FOUND` is an indication of a corrupted page file. Run `pqrepair` against the specified queue directory.

NOTE: If the queue shows `fully-acked: YES` and 0 bytes, you can safely delete the file. 

[[pqrepair]]
===== `pqrepair` utility

The `pqrepair` utility tries to remove corrupt queue segments to bring the queue back into working order. 
It starts searching from the directory where is launched and looks for `data/queue/main`.

NOTE: The queue may lose some data in this operation.

From LOGSTASH_HOME, run:

[source,txt]
-----
bin/pqrepair <queue_directory>
-----

where `<queue_directory>` is the fully qualified path to the persistent queue location.

There is no output if the utility runs properly.  

The `pqrepair` utility requires write access to the directory. 
Folder permissions may cause problems when Logstash is run as a service.
In this situation, use `sudo`.

[source,txt]
-----
/usr/share/logstash$ sudo -u logstash bin/pqrepair /var/lib/logstash/queue/main/
-----

After you run `pqrepair`, restart Logstash to verify that the repair operation was successful. 

 
[[draining-pqs]]
===== Draining the queue

You may encounter situations where you want to drain the queue.
Examples include:

* Pausing new ingestion. There may be situations where you want to stop new ingestion, but still keep a backlog of data. 
* PQ repair. You can drain the queue to route to a different PQ while repairing an old one.
* Data or workflow migration. If you are moving off a disk/hardware and/or migrating to a new data flow, you may want to drain the existing queue.

To drain the persistent queue:

. In the `logstash.yml` file, set `queue.drain: true`.
. Restart Logstash for this setting to take effect.
. Shutdown Logstash (using CTRL+C or SIGTERM), and wait for the queue to empty.

[[persistent-queues-architecture]]
==== How persistent queues work

The queue sits between the input and filter stages in the same
process:

input → queue → filter + output 

When an input has events ready to process, it writes them to the queue. When
the write to the queue is successful, the input can send an acknowledgement to
its data source.

When processing events from the queue, Logstash acknowledges events as
completed, within the queue, only after filters and outputs have completed.
The queue keeps a record of events that have been processed by the pipeline.
An event is recorded as processed (in this document, called "acknowledged" or
"ACKed") if, and only if, the event has been processed completely by the
Logstash pipeline. 

What does acknowledged mean? This means the event has been handled by all
configured filters and outputs. For example, if you have only one output,
Elasticsearch, an event is ACKed when the Elasticsearch output has successfully
sent this event to Elasticsearch. 

During a normal shutdown (*CTRL+C* or SIGTERM), Logstash stops reading
from the queue and finishes processing the in-flight events being processed
by the filters and outputs. Upon restart, Logstash resumes processing the
events in the persistent queue as well as accepting new events from inputs.

If Logstash is abnormally terminated, any in-flight events will not have been
ACKed and will be reprocessed by filters and outputs when Logstash is
restarted. Logstash processes events in batches, so it is possible
that for any given batch, some of that batch may have been successfully
completed, but not recorded as ACKed, when an abnormal termination occurs.

NOTE: If you override the default behavior by setting `drain.queue: true`, Logstash reads from the queue until it is emptied--even after a controlled shutdown. 

For more details specific behaviors of queue writes and acknowledgement, see 
<<durability-persistent-queues>>.


[[backpressure-persistent-queue]]
===== Handling back pressure

When the queue is full, Logstash puts back pressure on the inputs to stall data
flowing into Logstash. This mechanism helps Logstash control the rate of data
flow at the input stage without overwhelming outputs like Elasticsearch.

Use `queue.max_bytes` setting to configure the total capacity of the queue on
disk. The following example sets the total capacity of the queue to 8gb:

[source, yaml]
-----
queue.type: persisted
queue.max_bytes: 8gb
-----

With these settings specified, Logstash buffers events on disk until the
size of the queue reaches 8gb. When the queue is full of unACKed events, and
the size limit has been reached, Logstash no longer accepts new events. 

Each input handles back pressure independently. For example, when the
<<plugins-inputs-beats,beats>> input encounters back pressure, it no longer
accepts new connections and waits until the persistent queue has space to accept
more events. After the filter and output stages finish processing existing
events in the queue and ACKs them, Logstash automatically starts accepting new
events.

[[durability-persistent-queues]]
===== Controlling durability

Durability is a property of storage writes that ensures data will be available after it's written.

When the persistent queue feature is enabled, Logstash stores events on
disk. Logstash commits to disk in a mechanism called _checkpointing_.

The queue itself is a set of pages. There are two kinds of pages: head pages and tail pages. The head page is where new events are written. There is only one head page. When the head page is of a certain size (see `queue.page_capacity`), it becomes a tail page, and a new head page is created. Tail pages are immutable, and the head page is append-only. 
Second, the queue records details about itself (pages, acknowledgements, etc) in a separate file called a checkpoint file.

When recording a checkpoint, Logstash:

* Calls `fsync` on the head page.
* Atomically writes to disk the current state of the queue.

The process of checkpointing is atomic, which means any update to the file is saved if successful.

IMPORTANT: If Logstash is terminated, or if there is a hardware-level failure,
any data that is buffered in the persistent queue, but not yet checkpointed, is
lost.

You can force Logstash to checkpoint more frequently by setting
`queue.checkpoint.writes`. This setting specifies the maximum number of events
that may be written to disk before forcing a checkpoint. The default is 1024. To
ensure maximum durability and avoid data loss in the persistent queue, you can
set `queue.checkpoint.writes: 1` to force a checkpoint after each event is
written. Keep in mind that disk writes have a resource cost. Setting this value
to `1` can severely impact performance. 

[[garbage-collection]]
===== Disk garbage collection

On disk, the queue is stored as a set of pages where each page is one file. Each page can be at most `queue.page_capacity` in size. Pages are deleted (garbage collected) after all events in that page have been ACKed. If an older page has at least one event that is not yet ACKed, that entire page will remain on disk until all events in that page are successfully processed. Each page containing unprocessed events will count against the `queue.max_bytes` byte size.

