Peerage

Peerage helps your nodes find each other.

It supports DNS-based discovery, which means you can use it out of the box with Kubernetes (and probably also Weave, discoverd, Swarm, or other anything else with dns-based service discovery).

It also supports UDP-based discovery, so that nodes on the same network (like docker containers on the same host) can find each other.

It’s also easy to extend: adding a new Provider can be as simple as writing a single function.

Installation

Add peerage to your list of dependencies in mix.exs, and start its application:

    def application do
      [applications: [:peerage]]
    end
    def deps do
      [{:peerage, "~> 0.3.4"}]
    end

Usage

Peerage will attempt to Node.connect/1 to node names returned by a provider that you choose:

   config :peerage, via: Peerage.Via.Dns

There are several providers available, each with examples further down the page.

  • Peerage.Via.Self is a ‘hello, world’ provider that only connects to itself.
  • Peerage.Via.List uses a hardcoded list of node names. It’s good for development, or when you know production node names ahead of time. See example below.
  • Peerage.Via.Dns gets IP addresses from DNS. Works for production config on Kubernetes; probably works on Weave, Flynn, and Swarm. You can test it locally with one node by telling it your app’s dns name is localhost (this is also the default). See example below.
  • Peerage.Via.Udp uses peer-to-peer UDP multicast. Docker containers on a single host can use this out of the box (example below). Could be useful in a variety of situations where nodes are on the same network/overlay network, providing the network doesn’t restrict multicast.
  • Custom providers are simple (but there’s more detail below). TL;DR is:
      defmodule MyWayToFindHomies do
        def poll, do: [ :"node@somewhere" ]
      end
      config :peerage, via: MyWayToFindHomies

Usually, I use the List or Udp providers in dev config, so I can easily test multiple nodes on my machine, and the Dns provider for my production releases, which run in places (like Kubernetes) where nodes can be found by DNS discovery.

Config

Optional config for all apps

config :abc, via: SomeProvider,
  serves: false,     # true if provider runs supervised
  interval: 15,      # default 10
  log_results: true  # show connection results in debug mode?

See specific providers for their additional config.

Deferred Config

Supporting release config is easy now.

The :peerage app uses DeferredConfig, at app startup, so all config for the app (including any custom providers) automatically supports run-time config via {:system, "MY_ENV_VAR"} tuples (with optional defaults and conversion functions), as well as arbitrary functions via MFA tuples. See DeferredConfig for more details, but the upshot is that everything you want to be runtime configurable should be.

Peerage.Via.List

config :peerage, via: Peerage.Via.List, node_list: [
  :"myapp1@127.0.0.1",
  :"myapp2@127.0.0.1"
]
$ iex --name myapp1@127.0.0.1 -S mix   # one shell
$ iex --name myapp2@127.0.0.1 -S mix   # other shell

I wrap this with a script for launching dev shell and prod release shells, so that I just call bin/dev 1 or bin/prod 1 to test locally in mix, or a release with env vars set to work on my machine.

Peerage.Via.Dns

DNS-based discovery is where it’s at.

Minimal dns example, one node: after installing, add the following to your config:

    config :peerage, via: Peerage.Via.Dns,
      dns_name: "localhost"
      app_name: "myapp"

And then run iex like this:

$ iex --name myapp@127.0.0.1 -S mix

It’ll use dns to get the IP addresses for ‘localhost’, and try to connect to them. In this case, there’ll only be one result, but it’s working; it got that ip by looking up localhost!

Longer example:

Say you have an app called :myapp. You’re deploying it to Kubernetes as a headless service with name: myapp and clusterIP: None. Or to Convox with the Weave AMI, with an app name of myapp. You launch your application so that its node will be called myapp@${NODE_IP}, with the ip of the container being provided by an overlay network.

  • If you’re using releases, you’ll get that IP in an env variable (or just set it yourself on container startup), splice that address into rel/vm.args with a line like -name ${NODE_NAME}@${NODE_IP}, and run your release with REPLACE_OS_VARS=true. (See exrm or distillery docs if necessary).

Your config/prod.exs might look like this:

    config :peerage, via: Peerage.Via.Dns
      dns_name: "myapp", # or k8s FQDN: "myapp.ns.svc.clust.local"
      app_name: "myapp"  

Now your app will look up the name “myapp” from within your container. In Kubernetes, hostname -i myapp will (as of 1.3 or somesuch) return the list of IP addresses of each of the pods that make up the service; the same is true on a system with Weave for container networking.

  • If the network name needs to be specified more (perhaps the dns discovery supports encoding version numbers, regions, etc into the name, and you want to only connect to the same version), change dns_name.

In Kubernetes, you can test all of this with minikube.

Peerage.Via.Udp

  config :peerage, via: Peerage.Via.Udp, serves: true
    port: 45900

Will both broadcast and receive node names via UDP on port 45900, and keep track of ones it’s seen so that Peerage connect to them. It’s a GenServer, so we let Peerage know it needs to be run and supervised with serves: true.

Custom Providers

For simple cases, where you’re polling a source of truth (some API, etc): just provide a poll method that returns a list of node-name atoms:

defmodule MyWayToFindHomies do
  def poll, do: [ :"name@host" ] # poll source of truth
end
config :peerage, via: MyWayToFindHomies

For more complex, peer-to-peer cases — say, UDP multicast, where each node is broadcasting itself on the network — you’ll probably want to use a GenServer, listening for incoming broadcast messages, and building up a list of ‘seen’ nodes over time.

If your Provider is an OTP process, just add serves: true to your config:

config :peerage, via: MyWayToFindHomies, serves: true

For a full-fledged example, see Peerage.Via.Udp.