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Lars Kellogg-Stedman 869ecba652 journald log driver: use CONTAINER_ID field for container id

This patch modifies the journald log driver to store the container ID in
a field named CONTAINER_ID, rather than (ab)using the MESSAGE_ID field.
Additionally, this adds the CONTAINER_ID_FULL field containing the
complete container ID and CONTAINER_NAME, containing the container name.

When using the journald log driver, this permits you to see log messages
from a particular container like this:

    # journalctl CONTAINER_ID=a9238443e193

Example output from "journalctl -o verbose" includes the following:

    CONTAINER_ID=27aae7361e67
    CONTAINER_ID_FULL=27aae7361e67e2b4d3864280acd2b80e78daf8ec73786d8b68f3afeeaabbd4c4
    CONTAINER_NAME=web

Closes: #12864
Signed-off-by: Lars Kellogg-Stedman <lars@redhat.com>

2015-04-30 10:42:27 -04:00

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Journald logging driver

The journald logging driver sends container logs to the systemd journal. Log entries can be retrieved using the journalctl command or through use of the journal API.

In addition to the text of the log message itself, the journald log driver stores the following metadata in the journal with each message:

Field	Description
`CONTAINER_ID`	The container ID truncated to 12 characters.
`CONTAINER_ID_FULL`	The full 64-character container ID.
`CONTAINER_NAME`	The container name at the time it was started. If you use `docker rename` to rename a container, the new name is not reflected in the journal entries.

Usage

You can configure the default logging driver by passing the --log-driver option to the Docker daemon:

docker --log-driver=journald

You can set the logging driver for a specific container by using the --log-driver option to docker run:

docker run --log-driver=journald ...

Note regarding container names

The value logged in the CONTAINER_NAME field is the container name that was set at startup. If you use docker rename to rename a container, the new name will not be reflected in the journal entries. Journal entries will continue to use the original name.

Retrieving log messages with journalctl

You can use the journalctl command to retrieve log messages. You can apply filter expressions to limit the retrieved messages to a specific container. For example, to retrieve all log messages from a container referenced by name:

# journalctl CONTAINER_NAME=webserver

You can make use of additional filters to further limit the messages retrieved. For example, to see just those messages generated since the system last booted:

# journalctl -b CONTAINER_NAME=webserver

Or to retrieve log messages in JSON format with complete metadata:

# journalctl -o json CONTAINER_NAME=webserver

Retrieving log messages with the journal API

This example uses the systemd Python module to retrieve container logs:

import systemd.journal

reader = systemd.journal.Reader()
reader.add_match('CONTAINER_NAME=web')

for msg in reader:
  print '{CONTAINER_ID_FULL}: {MESSAGE}'.format(**msg)

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