How to Run a Command Once in Docker Compose for Data-Dependent Processes?

Running a Command Once in Docker Compose

Introduction

Docker compose allows you to manage multiple containers as a single unit. However, it can be challenging to execute commands only once, especially when dealing with data-dependent processes like setting up databases.

Understanding Data Volume Containers

In your compose file, you've created a "mongodata" data volume container to store the MongoDB data. This container is essentially a paused container that simply mounts the assigned volume.

Avoiding Repeated Command Execution

Entrypoint Script for One-Time Initialization:

To prevent the "-setup" command from executing multiple times, you can employ an entrypoint script within the "my_project" container. This script will check if the database has been initialized and perform the setup only if necessary.

Example:

#!/bin/sh

# Check if database is initialized
if test -f /app/initialized; then
  # Database already initialized, skip setup
  echo "Database already initialized"
else
  # Initialize database
  ./my-project -setup
  touch /app/initialized
fi

# Start the application
exec /go/bin/my_project

Utilizing Volume Management

Docker 1.9 and later provide native volume management capabilities, eliminating the need for "data-only" containers. You can simplify your compose file as follows:

services:
  mongo:
    image: mongo:latest
    volumes:
      - mongodata:/data/db
    ports:
      - "28001:27017"
    command: --smallfiles --rest --auth

Note on Entrypoint Script

In the entrypoint script, it's essential to handle potential delays in database availability. The script should retry connection attempts if the database is not yet accessible.

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