Use a tool such as wait-for-it, dockerize, or sh-compatible wait-for. These are small wrapper scripts which you can include in your application’s image to poll a given host and port until it’s accepting TCP connections.

For example, to use wait-for-it.sh or wait-for to wrap your service’s command:

version: ""

services:

  web:

    build: .

    ports:

      - "80:8000"

    depends_on:

      - "db"

    command: ["./wait-for-it.sh", "db:5432", "--", "python", "app.py"]

  db:

    image: postgres

Tip: There are limitations to this first solution. For example, it doesn’t verify when a specific service is really ready. If you add more arguments to the command, use the bash shift command with a loop, as shown in the next example.

Alternatively, write your own wrapper script to perform a more application-specific health check. For example, you might want to wait until Postgres is definitely ready to accept commands:

#!/bin/bash

# wait-for-postgres.sh

set -e

host="$1"

shift

cmd="$@"

until psql -h "$host" -U "postgres" -c '\q'; do

  >& echo "Postgres is unavailable - sleeping"

  sleep

done

>& echo "Postgres is up - executing command"

exec $cmd

You can use this as a wrapper script as in the previous example, by setting:

command: ["./wait-for-postgres.sh", "db", "python", "app.py"]