How to Execute a One-Time Command in Docker Compose?

Executing a One-Time Command in Docker Compose

In the context of Docker compose, you seek a solution to execute a specific command (./my-project -setup) only once, regardless of container restarts or deployments. This need arises for situations where initialization or data setup is necessary.

To address this issue, consider implementing an entrypoint script within your container. This script will handle the task of checking whether the database has already been initialized. If it hasn't, the script will perform the required initialization steps.

It's crucial to note that container startup order is not guaranteed, so it's possible for the application container to start before the database container. To account for this, the script should incorporate logic for handling such occurrences.

An example of this approach can be found in the official WordPress image (https://github.com/docker-library/wordpress/blob/df190dc9c5752fd09317d836bd2bdcd09ee379a5/apache/docker-entrypoint.sh#L146-L171). The script attempts to connect to the database and checks if initialization is needed, retrying if the database is not yet accessible.

Optimizing Volume Management

In your initial Docker compose file, you used a "data-only" container to attach your volume. However, modern versions of Docker (1.9 and above) provide volume management capabilities. This eliminates the need for separate data-only containers.

You can simplify your Docker compose file by removing the data-only container and modifying the mongo service as follows:

This configuration will create a volume named mongodata if it doesn't already exist, or it will reuse an existing volume with that name. You can manage volumes using the commands docker volume ls for listing and docker volume rm for removing.

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