page	https://idle.run/ready-wait
title	Ready-Wait For Docker Containers
tags	docker
date	2015-11-24

Docker supports linking together containers and will ensure that containers only start if the containers they depend on are started. It doesn't have anything (yet) for app-specific logic which indicates whether the dependency is actually ready.

Here is a simple Python script which can be run inside of a container to delay starting the app code until preconditions are met.

Script

The following is the script which will run and wait until the preconditions defined in the yaml definition below are met

ready_wait.py

Configuration

The script above expects a configuration file at /etc/ready_wait.yaml like the following

hostname1:
  http:
    port: 80
    uri: /status
    status: 200
    contains: OK

hostname2:
  tcp:
    port: 80

Docker Usage

Modify the Docker container CMD to wait for ready_wait to complete before running the normal app entrypoint for your container. For example:

FROM ubuntu
ADD ready_wait.py /usr/bin/ready_wait
ADD ready_wait.yaml /etc/ready_wait.yaml
...
CMD ready_wait && /entrypoint.sh