Start containers automatically

As of Docker 1.2, restart policies are the built-in Docker mechanism for restarting containers when they exit. If set, restart policies will be used when the Docker daemon starts up, as typically happens after a system boot. Restart policies will ensure that linked containers are started in the correct order.

If restart policies don’t suit your needs (i.e., you have non-Docker processes that depend on Docker containers), you can use a process manager like upstart, systemd or supervisor instead.

Using a process manager

Docker does not set any restart policies by default, but be aware that they will conflict with most process managers. So don’t set restart policies if you are using a process manager.

When you have finished setting up your image and are happy with your running container, you can then attach a process manager to manage it. When you run docker start -a, Docker will automatically attach to the running container, or start it if needed and forward all signals so that the process manager can detect when a container stops and correctly restart it.

Here are a few sample scripts for systemd and upstart to integrate with Docker.

Examples

The examples below show configuration files for two popular process managers, upstart and systemd. In these examples, we’ll assume that we have already created a container to run Redis with --name=redis_server. These files define a new service that will be started after the docker daemon service has started.

upstart

description "Redis container"
author "Me"
start on filesystem and started docker
stop on runlevel [!2345]
respawn
script
  /usr/bin/docker start -a redis_server
end script

systemd

[Unit]
Description=Redis container
Requires=docker.service
After=docker.service

[Service]
Restart=always
ExecStart=/usr/bin/docker start -a redis_server
ExecStop=/usr/bin/docker stop -t 2 redis_server

[Install]
WantedBy=default.target

If you intend to use this as a system service, put the above contents in a file in the /etc/systemd/system directory, e.g. /etc/systemd/system/docker-redis_server.service.

If you need to pass options to the redis container (such as --env), then you’ll need to use docker run rather than docker start. This will create a new container every time the service is started, which will be stopped and removed when the service is stopped. Make sure you don’t use “-d” for “detached mode”. The command run from “ExecStart” needs to run in the foreground.

[Service]
...
ExecStart=/usr/bin/docker run --env foo=bar --name redis_server redis
ExecStop=/usr/bin/docker stop -t 2 redis_server
ExecStopPost=/usr/bin/docker rm -f redis_server
...

To start using the service, reload systemd and start the service:

systemctl daemon-reload
systemctl start docker-redis_server.service

To enable the service at system startup, execute:

systemctl enable docker-redis_server.service

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