Defines a Receiver Plugin.

A Receiver plugin will run in an independent process and will run the metrics_received/3 function every time it receives a metric from the agents.

POABackend app reads the Receivers configuration from the config.exs file when bootstrap and will create a process per each one of them. That configuration is referenced by :receivers key.

config :poa_backend,
   :receivers,
   [
     {name, module, args}
   ]

for example

config :poa_backend,
   :receivers,
   [
     {:my_receiver, POABackend.Receivers.MyReceiver, [host: "localhost", port: 1234]}
   ]

name, module and args must be defined in the configuration file.

• name: Name for the new created process. Must be unique
• module: Module which implements the Receiver behaviour
• args: Initial args which will be passed to the init_receiver/1 function

The Receiver’s mechanism is built on top of GenStage. Receivers are Consumers (sinks) and they must be subscribed to one or more Producers. The Producers are the Metric types (i.e. ethereum_metrics) and are defined in the config file too:

config :poa_backend,
       :metrics,
       [
         :ethereum_metrics
       ]

In order to work properly we have to define in the configuration file the relation between the Receiver and the Metric types it wants to receive.

config :poa_backend,
     :subscriptions,
     [
       {receiver_name, [metric_type1, metric_type2]}
     ]

for example

config :poa_backend,
       :subscriptions,
       [
         {:my_receiver, [:ethereum_metrics]}
       ]

Implementing A Receiver Plugin

In order to implement your Receiver Plugin you must implement 3 functions.

• init_receiver/1: Called only once when the process starts
• metrics_received/3: This function is called eveytime the Producer (metric type) receives a message.
• handle_message/1: This is called when the Receiver process receives an Erlang message
• handle_inactive/2: This function is called when one client has been disconnected or is not active for a period of time.
• terminate/1: Called just before stopping the process

This is a simple example of custom Receiver Plugin

defmodule POABackend.Receivers.MyReceiver do
  use POABackend.Receiver

  def init_receiver(_args) do
    {:ok, :no_state}
  end

  def metrics_received(metrics, from, state) do
    for metric <- metrics do
      IO.puts "metric received"
    end
    {:ok, state}
  end

  def handle_message(_message, state) do
    {:ok, state}
  end

  def handle_inactive(agent_id, state) do
    {:ok, state}
  end

  def terminate(_state) do
    :ok
  end

end