Controlling startup order in Compose

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You can control the order of service startup with the depends_on option. Compose always starts containers in dependency order, where dependencies are determined by depends_on, links, volumes_from, and network_mode: "service:...".

However, Compose will not wait until a container is “ready” (whatever that means for your particular application) - only until it’s running. There’s a good reason for this.

The problem of waiting for a database (for example) to be ready is really just a subset of a much larger problem of distributed systems. In production, your database could become unavailable or move hosts at any time. Your application needs to be resilient to these types of failures.

To handle this, your application should attempt to re-establish a connection to the database after a failure. If the application retries the connection, it should eventually be able to connect to the database.

The best solution is to perform this check in your application code, both at startup and whenever a connection is lost for any reason. However, if you don’t need this level of resilience, you can work around the problem with a wrapper script:

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 and will 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; e.g., it doesn’t verify when a specific service is really ready. If you add more arguments to the command, you’ll need to 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 '\l'; 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"]

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