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defmodule Peerage.Via.Dns do
@behaviour Peerage.Provider
require Logger
@moduledoc """
Use Dns-based service discovery to find other Nodes.
### Example
config :peerage, via: Peerage.Via.Dns,
dns_name: "localhost",
app_name: "myapp"
Will look up the ip(s) for 'localhost', and then try to
connect to `myapp@$IP`.
### Kubernetes
Kubernetes supports this out of the box for 'headless
services' -- if you have a service named `myapp`, doing
`nslookup myapp` in a deployed container will return a
list of IP addresses for that service.
More context and resources for using DNS for this:
- This project's README
- [SkyDNS announcement](https://blog.gopheracademy.com/skydns/)
- [Kubernetes DNS for services](http://kubernetes.io/docs/admin/dns/)
"""
def poll, do: lookup() |> to_names([])
defp lookup do
hostname() |> String.to_charlist |> :inet_res.lookup(:in, :a)
end
# turn list of ips into list of node names
defp to_names([ip | rest], acc) when is_list(acc) do
Logger.debug " -> Peerage.Via.Dns resolved '#{hostname()}' to #{ to_s(ip) } "
to_names rest, [:"#{ app_name() }@#{ to_s(ip) }"] ++ acc
end
defp to_names([], lst), do: lst
defp to_names(err,[]), do: Logger.error(["dns err",err]); []
# helpers
defp app_name do
Application.get_env(:peerage, :app_name, "nonode")
end
defp hostname do
Application.get_env(:peerage, :dns_name, "localhost")
end
defp to_s(_ip = {a,b,c,d}), do: "#{a}.#{b}.#{c}.#{d}"
end