Implements a libcluster strategy for node distribution based on Consul DNS. By default it uses :inet_res to query the nameservers, though it can be configured to use any module conforming to the Caravan.DnsClient behaviour.

Prerequisites

First things first, is that you'll need to have Consul setup and configured as a nameserver either in your hosts file or via an erl_inetrc file You'll need to create a service that will return SRV records with the hostname and distribution port. The Consul documentation has directions on what needs to be setup and how to test with dig.

Let's look at an example:

'likes-service-3434@prod.socialmedia.consul`

Above, likes-service is an app name. It will correspond with the :node_sname config option. The port is the last integer to the left of the '@'. You'll need this because our nodes will be using Caravan.Epmd.Client and Caravan.Epmd.Dist_dist to use the port number of the node name instead of being assigned a port randomly by epmd.

Also note that the hostname of cluster nodes returned by Consul must be the same as that in the nodes -name parameter

Configuration

Here's a sample configuration

config :libcluster,
  topologies: [
    caravan: [
      # The selected clustering strategy. Required.
      strategy: Caravan.Cluster.DnsStrategy,
      config: [
        #service name that returns the distribution port in a SRV record
        query: "likes-service-dist.service.consul",
        #forms the base of the node name. App name is a good one.
        node_sname: "profile-service",
        #The poll interval for the Consul service in milliseconds. Defaults to 5s
        poll_interval: 5_000
        #The module of the DNS client to use.
        dns_client: Caravan.DnsClient
      ],
    ]
  ]