Order of containers in Docker Compose

kinow @ Mar 14, 2017 21:22:03 ()

In Docker Compose you are able to control the startup order of the containers via the depends_on statement. This is documented in Controlling startup order in Compose.

If you have a simple setup, with Tomcat and Postgres, sometimes Postgres will start first, but Compose will initialize Tomcat before Postgres has fully booted. When that happens, you may receive 401, 404, or other application errors.

You can fix it by combining depends_on with a healthcheck. For example:

# File: docker-compose.yml
version: '2.1'
services:
  db:
    container_name: twpg
    build:
      context: .
      dockerfile: Dockerfile.postgres
    restart: always
    ports:
      - "5432:5432"
    healthcheck:
      test: "pg_isready -h localhost -p 5432 -q -U postgres"
      interval: 10s
      timeout: 5s
      retries: 5

  web:
    container_name: twtc
    build:
      context: .
      dockerfile: Dockerfile.tomcat
    restart: always
    depends_on:
      db:
        condition: service_healthy
    links:
      - db
    ports:
      - "80:8080"

In the example docker-compose.yml, there are two containers, db and web. web is running a Tomcat, and db is running Postgres. Web depends on db (see depends_on), and uses a condition service_healthy. Which indicates it depends that that container is healthy.

The healthcheck entry under the db container settings define how to check whether Postgres is running or not. In this case, we are using pg_isready, which is available in the vanilla Postgres 9 container.

It will try 5 times, with a 10 seconds interval, and will time out after 5 seconds. You may have to tune these parameters for your application.

This code snippet is from a pull request submitted to Foxoncz/docker-thingworx.

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