# Using lifecycle hooks with Compose




## Services lifecycle hooks

When Docker Compose runs a container, it uses two elements, 
[ENTRYPOINT and COMMAND](/engine/containers/run/#default-command-and-options), 
to manage what happens when the container starts and stops.

However, it can sometimes be easier to handle these tasks separately with lifecycle hooks - 
commands that run right after the container starts or just before it stops.

Lifecycle hooks are particularly useful because they can have special privileges 
(like running as the root user), even when the container itself runs with lower privileges 
for security. This means that certain tasks requiring higher permissions can be done without 
compromising the overall security of the container.

### Post-start hooks

Post-start hooks are commands that run after the container has started, but there's no 
set time for when exactly they will execute. The hook execution timing is not assured during 
the execution of the container's `entrypoint`.

In the example provided:

- The hook is used to change the ownership of a volume to a non-root user (because volumes 
are created with root ownership by default).
- After the container starts, the `chown` command changes the ownership of the `/data` directory to user `1001`.

```yaml
services:
  app:
    image: backend
    user: 1001
    volumes:
      - data:/data    
    post_start:
      - command: chown -R 1001:1001 /data
        user: root

volumes:
  data: {} # a Docker volume is created with root ownership
```

### Pre-stop hooks

Pre-stop hooks are commands that run before the container is stopped by a specific 
command (like `docker compose down` or stopping it manually with `Ctrl+C`). 
These hooks won't run if the container stops by itself or gets killed suddenly.

In the following example, before the container stops, the `./data_flush.sh` script is 
run to perform any necessary cleanup.

```yaml
services:
  app:
    image: backend
    pre_stop:
      - command: ./data_flush.sh
```

## Reference information

- [`post_start`](/reference/compose-file/services/#post_start)
- [`pre_stop`](/reference/compose-file/services/#pre_stop)