ha-handler/lib/ha_handler/control.ex

defmodule HAHandler.Control do
	@moduledoc """
	This module handles the decision-logic and actions to be
	taken regarding the current state of the infrastructure.
	"""

	@haproxy_pgsql_backend "pgsql"

	use GenServer

	require Logger

	alias HAHandler.{PGSQL, HAProxy}

	# How much do we wait (ms) between each check/decision-making round?
	@refresh 15_000

	def start_link(opts) do
		GenServer.start_link(__MODULE__, opts, name: __MODULE__)
	end

	@impl true
	def init(_opts) do
		state = []

		# Let's skip the initial startup round so that other components are all up
		# and running.
		Process.send_after self(), :sync, @refresh

		{:ok, state}
	end

	@impl true
	def handle_info(:sync, state) do
		Logger.debug("Executing control logic.")

		# Fetch PGSQL state, make sure HAProxy routes to the master
		# process.
		pgsql_state = PGSQL.get_instances()
		|> Enum.map(fn {hostname, pid} = instance->
			haproxy_server = HAHandler.pgsql_instances()
										|> Enum.filter(fn opts -> Keyword.get(opts, :hostname) == hostname end)
										|> Enum.at(0)
										|> Keyword.get(:haproxy_server)

			%{
				haproxy_server: haproxy_server,
				pgsql_watcher_pid: pid,
				pgsql_operation_mode: PGSQL.get_operation_mode(instance)
			}
		end)
		haproxy_state = HAProxy.get_stats()
									|> Map.get("Server", [])
									|> Enum.filter(fn mapping -> mapping["pxname"] == @haproxy_pgsql_backend end)
									|> Enum.map(fn mapping -> %{mapping["svname"] => mapping["status"]} end)
									|> Enum.reduce(&Map.merge/2)

		for pgsql_instance <- pgsql_state do
			haproxy_state = Map.get(haproxy_state, pgsql_instance.haproxy_server)

			case {pgsql_instance.pgsql_operation_mode, haproxy_state} do
				{:primary, "UP"} ->
					:noop
				{:primary, "MAINT"} ->
					Logger.info("Enabling routing PGSQL to (now) primary #{pgsql_instance.haproxy_server}.")
					HAProxy.set_server(@haproxy_pgsql_backend, pgsql_instance.haproxy_server, "state", "ready")
				{:secondary, "UP"} ->
					Logger.info("Disabling routing PGSQL to (now) secondary #{pgsql_instance.haproxy_server}.")
					HAProxy.set_server(@haproxy_pgsql_backend, pgsql_instance.haproxy_server, "state", "maint")
				{:secondary, "MAINT"} ->
					:noop
				unknown ->
					Logger.warning("Unhandled PGSQL/HAProxy state: #{inspect(unknown)}")
			end
		end

		# Schedule next round.
		Process.send_after self(), :sync, @refresh

		{:noreply, state}
	end
end