A Docker swarm service for automatically updating your services whenever their base image is refreshed.

docker service create --name shepherd \
                      --replicas 1 \
                      --constraint "node.role==manager" \
                      --mount type=bind,source=/var/run/docker.sock,target=/var/run/docker.sock,ro \
                      mazzolino/shepherd

Shepherd will try to update your services every 5 minutes by default. You can adjust this value using the SLEEP_TIME variable.

You can prevent services from being updated bei appending them to the BLACKLIST_SERVICES variable. This should be a space-separated list of service names.

Example:

docker service create --name shepherd \
                    --constraint "node.role==manager" \
                    --env SLEEP_TIME="5m" \
                    --env BLACKLIST_SERVICES="shepherd my-other-service" \
                    --mount type=bind,source=/var/run/docker.sock,target=/var/run/docker.sock,ro \
                    mazzolino/shepherd

Shepherd just triggers updates by updating the image specification for each service, removing the current digest.

Most of the work is thankfully done by Docker which resolves the image tag, checks the registry for a newer version and updates running container tasks as needed.

Also, Docker handles all the work of applying rolling updates. So at least with replicated services, there should be no noticeable downtime.