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Executing commands before and after updating
!!! 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) or be specified as part of
the docker run command line:
=== "Dockerfile"
docker 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"
=== "docker run"
bash 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" \
Timeouts
The timeout for all lifecycle commands is 60 seconds. After that, a timeout will occur, forcing Watchtower to continue the update loop.
Pre-update timeouts
For the pre-update lifecycle command, it is possible to override this timeout to
allow the script to finish before forcefully killing it. This is done by adding the
label com.centurylinklabs.watchtower.lifecycle.pre-update-timeout followed by
the timeout expressed in minutes.
If the label value is explicitly set to 0, the timeout will be disabled.
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.