We make extensive use of Docker and Docker Compose at Tasktop for building, testing and deploying our products. One of the pain points that we run in to is services that fail to start when other services are not available - for example a web app that fails to start unless the database is already up and running. Apparently we’re not the only ones, so here’s a bit of bash hackery to work around the problem:

until nc -z -v -w5 $SERVICE_HOST $SERVICE_PORT;
    echo "waiting for service to become available on $SERVICE_HOST:$SERVICE_PORT"
    sleep 1

This hack uses netcat (nc) to wait for a socket accepting connections on the specified host and port. The -z flag causes netcat to detect the listening socket without actually sending any data.

To make this work in a Docker container, you’ll have to ensure that netcat is available:

RUN apt-get update && \
    apt-get install -y \
        netcat && \
    apt-get clean && \
    rm -rf /var/lib/apt/lists/*

There are a few down sides to this approach, such as:

  • the script will not terminate if the service isn’t available - there should be some kind of hard failure if the dependent service doesn’t show up after awhile
  • this hack ends up adding complexity to each Dockerfile that uses it
  • some dependent services are still not “ready” even though they listen on a port - those need a more sophisticated approach, something better than sleep 30

Despite the shortcomings, the approach works well enough to get us over the hump of service dependencies. Somehow, Docker has helped to reinvigorate shell scripting.