How to add a start step to a container#

A portal often has work to do before its server starts, such as clearing expired sessions or loading reference data. You add a start step to run that work on every start of the web container, after the database is migrated and the static files are collected.

Before you start#

You have a production image for your portal, built from the portal Dockerfile.

Steps#

Write the step as an executable file. It can be a shell script or a Python script with a #!/usr/bin/env python first line. This one removes expired sessions.
```
#!/bin/sh
set -eu
python manage.py clearsessions
```
Save it in your portal’s repository as docker-entrypoint.d/10-clear-sessions. The steps run in the order of their file names, so prefix the names with numbers to choose the order.
Add one line to your portal’s Dockerfile, after the COPY that puts your code in /app.
```
COPY --chmod=755 docker-entrypoint.d/ /docker-entrypoint.d/
```
--chmod=755 makes the file executable.

Rebuild the image and start the container.

docker build --build-arg FAIRDM_IMAGE_VERSION=1.0.0 -t my-portal .
docker run --rm --publish 8000:8000 --env-file /etc/my-portal/portal.env my-portal

The result#

The step runs on every start of the web command, after migrate and collectstatic and before gunicorn. It runs as the user fairdm in /app with the container’s environment, and it can use python manage.py because the database has been migrated. Write it so it is safe to run again, because it runs on every start.

A step that exits with a non-zero status stops the start. The container exits with that status and the server does not start, as it does when migrate fails. A file that is not executable is skipped, and the container’s log names it.

skipping /docker-entrypoint.d/10-clear-sessions: not executable

The worker command and any other command run no start steps.

To try a step without rebuilding the image, mount a directory of steps when you run it.

docker run ... --volume "$PWD/docker-entrypoint.d:/docker-entrypoint.d:ro" my-portal

The container runs as user id 1000, so the mounted directory and its files must be readable by that user. A directory it cannot read counts as empty, and no step runs. To read the steps from another directory, set FAIRDM_START_DIR. All the rules for steps are in Container images.