watchtower/docs/lifecycle-hooks.md

Executing commands before and after updating

DO NOTE: These are shell commands executed with sh, and therefore require the container to provide the sh executable.

It is possible to execute pre/post-check and pre/post-update commands inside every container updated by watchtower.

• The pre-check command is executed for each container prior to every update cycle.
• The pre-update command is executed before stopping the container when an update is about to start.
• The post-update command is executed after restarting the updated container
• The post-check command is executed for each container post every update cycle.

This feature is disabled by default. To enable it, you need to set the option --enable-lifecycle-hooks on the command line, or set the environment variable WATCHTOWER_LIFECYCLE_HOOKS to true.

Specifying update commands

The commands are specified using docker container labels, the following are currently available:

Type	Docker Container Label
Pre Check	com.centurylinklabs.watchtower.lifecycle.pre-check
Pre Update	com.centurylinklabs.watchtower.lifecycle.pre-update
Post Update	com.centurylinklabs.watchtower.lifecycle.post-update
Post Check	com.centurylinklabs.watchtower.lifecycle.post-check

These labels can be declared as instructions in a Dockerfile (with some example .sh files):

LABEL com.centurylinklabs.watchtower.lifecycle.pre-check="/sync.sh"
LABEL com.centurylinklabs.watchtower.lifecycle.pre-update="/dump-data.sh"
LABEL com.centurylinklabs.watchtower.lifecycle.post-update="/restore-data.sh"
LABEL com.centurylinklabs.watchtower.lifecycle.post-check="/send-heartbeat.sh"

Or be specified as part of the docker run command line:

docker run -d \
  --label=com.centurylinklabs.watchtower.lifecycle.pre-check="/sync.sh" \
  --label=com.centurylinklabs.watchtower.lifecycle.pre-update="/dump-data.sh" \
  --label=com.centurylinklabs.watchtower.lifecycle.post-update="/restore-data.sh" \
  someimage
  --label=com.centurylinklabs.watchtower.lifecycle.post-check="/send-heartbeat.sh" \

Execution failure

The failure of a command to execute, identified by an exit code different than 0, will not prevent watchtower from updating the container. Only an error log statement containing the exit code will be reported.