phoenixframework / dns_cluster

Simple DNS clustering for distributed Elixir nodes

DNSCluster

Simple DNS clustering for distributed Elixir nodes.

Installation

The package can be installed by adding dns_cluster to your list of dependencies in mix.exs:

def deps do
  [
    {:dns_cluster, "~> 0.1.1"}
  ]
end

Next, you can configure and start the cluster by adding it to your supervision tree in your application.ex:

children = [
  {Phoenix.PubSub, ...},
  {DNSCluster, query: Application.get_env(:my_app, :dns_cluster_query) || :ignore},
  MyAppWeb.Endpoint
]

If you are deploying with Elixir releases, the release must be set to support longnames and the node must be named. These can be set in your rel/env.sh.eex file:

#!/bin/sh
export RELEASE_DISTRIBUTION=name
export RELEASE_NODE="myapp@fully-qualified-host-or-ip"

By default, nodes from the same release will have the same cookie. If you want different applications or releases to connect to each other, then you must set the RELEASE_COOKIE, either in your deployment platform or inside rel/env.sh.eex:

#!/bin/sh
...
export RELEASE_COOKIE="my-app-cookie"

dns_cluster's Issues

Search list for host-name lookup is not used when resolving the hostname

DNSCluster.Resolver.lookup/2 uses :inet_res.lookup/2 to resolve the given hostname to a list of IP addresses. This function does not take into account the search list for host-name lookup. The search list is used often in Kubernetes clusters where you reference a service by name, not by the full domain. (E.g. my-service instead of my-service.my-namespace.svc.cluster.local). my-namespace.svc.cluster.local is part of the search list in /etc/resolv.conf and can change from cluster to cluster.

I think this could be enhanced by using :inet_res.getbyname/2. From its documentation:

This function uses resolver option search that is a list of domain names. If the name to resolve contains no dots, it is prepended to each domain name in the search list, and they are tried in order. If the name contains dots, it is first tried as an absolute name and if that fails, the search list is used. If the name has a trailing dot, it is supposed to be an absolute name and the search list is not used.