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