Compose init container demo

• Compose init container demo
  • Foreword
  • Disclaimer
  • Example basics
  • Prerequisite for running the demo
  • How to run the demo

Foreword

This example was crafted as an answer to a StackOverflow question about using init containers with docker-compose.

According to This PR the feature is available since docker-compose 1.29

Meanwhile, at time of this writing, it seems that it has not made its full way to documentation

Disclaimer

Please note this example is far from perfect and that the final target (i.e. getting the db initialized) can be acheived without using an init container (the mysql image can perfectly do that out of the box). Meanwhile it was crafted to illustrate the concept. The interesting part is that you can run many init task in the dedicated container and wait for different services to be fully ready before you spin your final application containers.

Example basics

What the example basically does is:

1. spin up a mysql container.
2. spin up an init container which will wait for db to be available (retries in executed bash script), create a db, a table and add some entries (from executed bash script), then exits.
3. spin up an application container (php/mysqli/apache) which will only start once the init container has fully done its job and exits with a RC=0.

Prerequisite for running the demo

• A running docker installation with internet access
• Docker compose v1.29 or higher installed
• Free port 9999 on your local machine (else change the local port in docker-compose.yml)

How to run the demo

git clone git@github.com:zeitounator/compose-init-demo.git
cd compose-init-demo
docker-compose up -d

On first run this will build a local image named compose-ini-container_my_app based on php:apache (this is just an extension of the base image to add the mysqli driver used for demo)

You should then see the db container start, followed by the init-db container. At this point the process will wait for the db container to fully end its job. This should take several seconds as the initprojet.sh script contains a loop to wait for the mysql connection to be available prior to creating a scaffold db with some data.

Once the init container exits successfully, you should finally see the actual application container my_app start. You can point your browser to http://localhost:9999/ to see the test page which will show the 3 rows that were created by the init container

To clean-up the demo, simply run

docker-compose down -v