wait-for

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: "2"
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
  >&2 echo "Postgres is unavailable - sleeping"
  sleep 1
done

>&2 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"]