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update docs for lifecycle hooks

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Niklas Wigertz 2019-08-26 16:44:10 +02:00
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docs/lifecycle-hooks.md
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## Executing commands before and after updating
> **DO NOTE**: Both commands are shell commands executed with `sh`, and therefore require the
> **DO NOTE**: These are shell commands executed with `sh`, and therefore require the
> container to provide the `sh` executable.
It is possible to execute a *pre-update* command and a *post-update* command
**inside** every container updated by watchtower. The *pre-update* command is
executed before stopping the container, and the *post-update* command is
executed after restarting the container.
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 before checking the container for updates.
- 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 last after a updated container is started or no update was needed.
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, with
`com.centurylinklabs.watchtower.lifecycle.pre-update-command` for the *pre-update*
command and `com.centurylinklabs.watchtower.lifecycle.post-update` for the
*post-update* command.
The commands are specified using docker container labels, the following are currently available:
These labels can be declared as instructions in a Dockerfile:
- `com.centurylinklabs.watchtower.lifecycle.pre-check` - _pre-check_
- `com.centurylinklabs.watchtower.lifecycle.pre-update-command` - _pre-update_
- `com.centurylinklabs.watchtower.lifecycle.post-update` - _post-update_
- `com.centurylinklabs.watchtower.lifecycle.post-check` - _post-check_
These labels can be declared as instructions in a Dockerfile (with some example .sh files):
```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"
```
Or be specified as part of the `docker run` command line:
```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" \
```
### Execution failure