[[docker]]
=== Running Logstash on Docker
Docker images for Logstash are available from the Elastic Docker
registry. The base image is https://hub.docker.com/_/centos/[centos:7].

A list of all published Docker images and tags is available at
https://www.docker.elastic.co[www.docker.elastic.co]. The source code is in
https://github.com/elastic/logstash/tree/{branch}[GitHub].

These images are free to use under the Elastic license. They contain open source 
and free commercial features and access to paid commercial features.  
{kibana-ref}/managing-licenses.html[Start a 30-day trial] to try out all of the 
paid commercial features. See the 
https://www.elastic.co/subscriptions[Subscriptions] page for information about 
Elastic license levels.

==== Pulling the image

Obtaining Logstash for Docker is as simple as issuing a +docker
pull+ command against the Elastic Docker registry.

ifeval::["{release-state}"=="unreleased"]

However, version {logstash_version} of Logstash has not yet been
released, so no Docker image is currently available for this version.

endif::[]

ifeval::["{release-state}"!="unreleased"]

["source","sh",subs="attributes"]
--------------------------------------------
docker pull {docker-image}
--------------------------------------------

Alternatively, you can download other Docker images that contain only features
available under the Apache 2.0 license. To download the images, go to 
https://www.docker.elastic.co[www.docker.elastic.co]. 

endif::[]

[[docker-config]]
=== Configuring Logstash for Docker

Logstash differentiates between two types of configuration:
<<config-setting-files,Settings and Pipeline Configuration>>.

==== Pipeline Configuration

It is essential to place your pipeline configuration where it can be
found by Logstash. By default, the container will look in
+/usr/share/logstash/pipeline/+ for pipeline configuration files.

In this example we use a bind-mounted volume to provide the
configuration via the +docker run+ command:

["source","sh",subs="attributes"]
--------------------------------------------
docker run --rm -it -v ~/pipeline/:/usr/share/logstash/pipeline/ {docker-image}
--------------------------------------------

Every file in the host directory +~/pipeline/+ will then be parsed
by Logstash as pipeline configuration.

If you don't provide configuration to Logstash, it will run with a
minimal config that listens for messages from the
<<plugins-inputs-beats,Beats input plugin>> and echoes any that are
received to `stdout`. In this case, the startup logs will be similar
to the following:

["source","text"]
--------------------------------------------
Sending Logstash logs to /usr/share/logstash/logs which is now configured via log4j2.properties.
[2016-10-26T05:11:34,992][INFO ][logstash.inputs.beats    ] Beats inputs: Starting input listener {:address=>"0.0.0.0:5044"}
[2016-10-26T05:11:35,068][INFO ][logstash.pipeline        ] Starting pipeline {"id"=>"main", "pipeline.workers"=>4, "pipeline.batch.size"=>125, "pipeline.batch.delay"=>5, "pipeline.max_inflight"=>500}
[2016-10-26T05:11:35,078][INFO ][org.logstash.beats.Server] Starting server on port: 5044
[2016-10-26T05:11:35,078][INFO ][logstash.pipeline        ] Pipeline main started
[2016-10-26T05:11:35,105][INFO ][logstash.agent           ] Successfully started Logstash API endpoint {:port=>9600}
--------------------------------------------

This is the default configuration for the image, defined in
+/usr/share/logstash/pipeline/logstash.conf+.  If this is the
behaviour that you are observing, ensure that your pipeline
configuration is being picked up correctly, and that you are replacing
either +logstash.conf+ or the entire +pipeline+ directory.

==== Settings

The image provides several methods for configuring settings. The conventional
approach is to provide a custom `logstash.yml` file, but it's
also possible to use environment variables to define settings.

[[docker-bind-mount-settings]]
===== Bind-mounted settings files

Settings files can also be provided through bind-mounts. Logstash
expects to find them at +/usr/share/logstash/config/+.

It's possible to provide an entire directory containing all needed
files:

["source","sh",subs="attributes"]
--------------------------------------------
docker run --rm -it -v ~/settings/:/usr/share/logstash/config/ {docker-image}
--------------------------------------------

Alternatively, a single file can be mounted:

["source","sh",subs="attributes"]
--------------------------------------------
docker run --rm -it -v ~/settings/logstash.yml:/usr/share/logstash/config/logstash.yml {docker-image}
--------------------------------------------

NOTE: Bind-mounted configuration files will retain the same permissions and
ownership within the container that they have on the host system. Be sure
to set permissions such that the files will be readable and, ideally, not
writeable by the container's +logstash+ user (UID 1000).

===== Custom Images

Bind-mounted configuration is not the only option, naturally. If you
prefer the _Immutable Infrastructure_ approach, you can prepare a
custom image containing your configuration by using a +Dockerfile+
like this one:

["source","dockerfile",subs="attributes"]
--------------------------------------------
FROM {docker-image}
RUN rm -f /usr/share/logstash/pipeline/logstash.conf
ADD pipeline/ /usr/share/logstash/pipeline/
ADD config/ /usr/share/logstash/config/
--------------------------------------------

Be sure to replace or delete `logstash.conf` in your custom image, so
that you don't retain the example config from the base image.

[[docker-env-config]]
===== Environment variable configuration

Under Docker, Logstash settings can be configured via environment
variables. When the container starts, a helper process checks the environment
for variables that can be mapped to Logstash settings. Settings that are found
in the environment are merged into `logstash.yml` as the container starts up.

For compatibility with container orchestration systems, these environment
variables are written in all capitals, with underscores as word
separators

Some example translations are shown here:

.Example Docker Environment Variables
[horizontal]
**Environment Variable**:: **Logstash Setting**
`PIPELINE_WORKERS`:: `pipeline.workers`
`LOG_LEVEL`:: `log.level`
`MONITORING_ENABLED`:: `monitoring.enabled`

In general, any setting listed in the <<logstash-settings-file, settings
documentation>> can be configured with this technique.

NOTE: Defining settings with environment variables causes `logstash.yml` to
be modified in place. This behaviour is likely undesirable if `logstash.yml` was
bind-mounted from the host system. Thus, it is not recommended to
combine the bind-mount technique with the environment variable technique. It
is best to choose a single method for defining Logstash settings.

==== Docker defaults
The following settings have different default values when using the Docker
images:

[horizontal]
`http.host`:: `0.0.0.0`
`monitoring.elasticsearch.hosts`:: `http://elasticsearch:9200`

NOTE: The setting `monitoring.elasticsearch.hosts` is not
defined in the `-oss` image.

These settings are defined in the default `logstash.yml`. They can be overridden
with a <<docker-bind-mount-settings,custom `logstash.yml`>> or via
<<docker-env-config,environment variables>>.

IMPORTANT: If replacing `logstash.yml` with a custom version, be sure to copy the
above defaults to the custom file if you want to retain them. If not, they will
be "masked" by the new file.

==== Logging Configuration

Under Docker, Logstash logs go to standard output by default. To
change this behaviour, use any of the techniques above to replace the
file at +/usr/share/logstash/config/log4j2.properties+.
