Watchtower

A process for watching your Docker containers and automatically restarting them whenever their base image is refreshed.

Overview

Watchtower is an application that will monitor your running Docker containers and watch for changes to the images that those containers were originally started from. If watchtower detects that an image has changed, it will automatically restart the container using the new image.

With watchtower you can update the running version of your containerized app simply by pushing a new image to the Docker Hub or your own image registry. Watchtower will pull down your new image, gracefully shut down your existing container and restart it with the same options that were used when it was deployed initially.

For example, let's say you were running watchtower along with an instance of centurylink/wetty-cli image:

$ docker ps
CONTAINER ID   IMAGE                   STATUS          PORTS                    NAMES
967848166a45   centurylink/wetty-cli   Up 10 minutes   0.0.0.0:8080->3000/tcp   wetty
6cc4d2a9d1a5   centurylink/watchtower  Up 15 minutes                            watchtower

Every few mintutes watchtower will pull the latest centurylink/wetty-cli image and compare it to the one that was used to run the "wetty" container. If it sees that the image has changed it will stop/remove the "wetty" container and then restart it using the new image and the same docker run options that were used to start the container initially (in this case, that would include the -p 8080:3000 port mapping).

Usage

Watchtower is itself packaged as a Docker container so installation is a simple as pulling the centurylink/watchtower image.

Since the watchtower code needs to interact with the Docker API in order to monitor the running containers, you need to mount /var/run/docker.sock into the container with the -v flag when you run it.

Run the watchtower container with the following command:

docker run -d \
  --name watchtower \
  -v /var/run/docker.sock:/var/run/docker.sock \
  centurylink/watchtower

Options

Any of the options described below can be passed to the watchtower process by setting them after the image name in the docker run string:

docker run --rm centurylink/watchtower --help

• --host, -h - Docker daemon socket to connect to. Defaults to "unix:///var/run/docker.sock" but can be pointed at a remote Docker host by specifying a TCP endpoint as "tcp://hostname:port". The host value can also be provided by setting the DOCKER_HOST environment variable.
• --interval, -i - Poll interval (in seconds). This value controls how frequently watchtower will poll for new images. Defaults to 300 seconds (5 minutes).
• --no-pull - Do not pull new images. When this flag is specified, watchtower will not attempt to pull new images from the registry. Instead it will only monitor the local image cache for changes. Use this option if you are building new images directly on the Docker host without pushing them to a registry.
• --tls - Use TLS when connecting to the Docker socket but do NOT verify the server's certificate. If you are connecting a TCP Docker socket protected by TLS you'll need to use either this flag or the --tlsverify flag (described below). The --tlsverify flag is preferred as it will cause the server's certificate to be verified before a connection is made.
• --tlsverify - Use TLS when connecting to the Docker socket and verify the server's certificate. If you are connecting a TCP Docker socket protected by TLS you'll need to use either this flag or the --tls flag (describe above).
• --tlscacert - Trust only certificates signed by this CA. Used in conjunction with the --tlsverify flag to identify the CA certificate which should be used to verify the identity of the server. The value for this flag can be either the fully-qualified path to the .pem file containing the CA certificate or a string containing the CA certificate itself. Defaults to "/etc/ssl/docker/ca.pem".
• --tlscert - Client certificate for TLS authentication. Used in conjunction with the --tls or --tlsverify flags to identify the certificate to use for client authentication. The value for this flag can be either the fully-qualified path to the .pem file containing the client certificate or a string containing the certificate itself. Defaults to "/etc/ssl/docker/cert.pem".
• --tlskey - Client key for TLS authentication. Used in conjunction with the --tls or --tlsverify flags to identify the key to use for client authentication. The value for this flag can be either the fully-qualified path to the .pem file containing the client key or a string containing the key itself. Defaults to "/etc/ssl/docker/key.pem".
• --debug - Enable debug mode. When this option is specified you'll see more verbose logging in the watchtower log file.
• --help - Show documentation about the supported flags.

Stopping Containers

When watchtower detects that a running container needs to be updated it will stop the container by sending it a SIGTERM signal. If your container should be shutdown with a different signal you can communicate this to watchtower by setting a label named com.centurylinklabs.watchtower.stop-signal with the value of the desired signal.

This label can be coded directly into your image by using the LABEL instruction in your Dockerfile:

LABEL com.centurylinklabs.watchtower.stop-signal="SIGHUP"

Or, it can be specified as part of the docker run command line:

docker run -d --label=com.centurylinklabs.watchtower.stop-signal=SIGHUP someimage

Remote Hosts

By default, watchtower is set-up to monitor the local Docker daemon (the same daemon running the watchtower container itself). However, it is possible to configure watchtower to monitor a remote Docker endpoint. When starting the watchtower container you can specify a remote Docker endpoint with either the --host flag or the DOCKER_HOST environment variable:

docker run -d \
  --name watchtower \
  centurylink/watchtower --host "tcp://10.0.1.2:2375"

or

docker run -d \
  --name watchtower \
  -e DOCKER_HOST="tcp://10.0.1.2:2375" \
  centurylink/watchtower

Note in both of the examples above that it is unnecessary to mount the /var/run/docker.sock into the watchtower container.

Secure Connections

Watchtower is also capable of connecting to Docker endpoints which are protected by SSL/TLS. If you've used docker-machine to provision your remote Docker host, you simply need to volume mount the certificates generated by docker-machine into the watchtower container and specify either the --tls or --tlsverify flags.

The docker-machine certificates for a particular host can be located by executing the docker-machine env command for the desired host (note the values for the DOCKER_HOST and DOCKER_CERT_PATH environment variables that are returned from this command). The directory containing the certificates for the remote host needs to be mounted into the watchtower container at /etc/ssl/docker.

With the certificates mounted into the watchtower container you simply need to specify either the --tls or --tlsverify flags to enable the TLS connection to the remote host:

docker run -d \
  --name watchtower \
  -e DOCKER_HOST=$DOCKER_HOST \
  -v $DOCKER_CERT_PATH:/etc/ssl/docker \
  centurylink/watchtower --tlsverify

Updating Watchtower

If watchtower is monitoring the same Docker daemon under which the watchtower container itself is running (i.e. if you volume-mounted /var/run/docker.sock into the watchtower container) then it has the ability to update itself. If a new version of the centurylink/watchtower image is pushed to the Docker Hub, your watchtower will pull down the new image and restart itself automatically.