Serf in a Docker image

This provides:

• The geodata/serf Docker image with serf installed.

• A distribution agnostic Makefile for installing Serf in a Docker image.

The docker and git repository tagging identifies the version of serf installed.

Using the geodata/serf image

The geodata/serf Docker image runs serf under runit using the serf service. Assuming runit is running in the container this means you can use runit commands to control the service e.g. sv stop serf to stop the service and sv start serf to start it again.

The serf service can be configured by adding JSON configuration files to the /etc/serf/config.d directory, either by extending the image or mounting a volume at that location. The format for the configuration files is described by the serf -config-dir option.

Using container linking

Serf agents can be clustered using linked containers. As an example linking two agents, open two terminals and in the first run:

docker run --name node1 geodata/serf

Next, ensure that you create a ./config.d/join.json file as follows:

{
    "start_join": ["node1"]
}

This is a serf configuration option that will tell the second agent to join the first when it starts, using the hostname node1 to identify the first agent. Start the second agend in your second terminal:

docker run --link node1:node1 -v $(pwd)/config.d:/etc/serf/config.d geodata/serf

You should see output from both terminals indicating a successful join.

Using mDNS multicasting

Serf agents can discover each other using multicasting. To test this, ensure that you create a ./config.d/discover.json file as follows:

{
    "discover": "my-cluster",
    "interface": "eth0"
}

Note that if you followed the example in the previous section, you must delete the ./config.d/join.json file as otherwise this will be read as part of the serf configuration (you can have as many JSON files as you want in the configuration directory).

Next, create as many serf agents as you want in the cluster by repeating this command (at least twice!).

docker run -v $(pwd)/config.d:/etc/serf/config.d geodata/serf

Customising images with the Makefile

You can use the Makefile in the repository to install serf when building your own Docker images. This is effectively a kind of mixin for images :).

Firstly clone the repository somewhere accessible to the build process. From within the repository root, typing make install in the repository will install the serf binary on the system. The only prerequisite is that the system be 64 bit and have the make, curl and funzip tools available. funzip is usually bundled as part of the zip package.

There is also a runit target which will install the necessary files for setting up a runit service. By default typing in make runit will install a service called serf. The name of the service can be customised by specifying the SERVICE make variable. E.g. running make runit -e SERVICE=foobar will install a serf agent under the foobar runit service. In order to customise the service you can add JSON serf configuration files to the /etc/${SERVICE}/config.d directory, as described in the previous section.

Multiple serf services under runit

Instead of running multiple containers, you may want to run multiple serf clusters from within a single container under runit. This can be done by installing more than one runit service using the make runit target as described in the previous section. Note, however, that by default both serf services will attempt to bind to the same ports, and one of them will fail. To prevent this the rpc_addr configuration key must be set in one of the service's JSON configuration files.

Note that the repository package, including the Makefile, is available in the geodata/serf image under /usr/local/src/serf: use this to extend the image with additional services if you need to.