A Custom Handler is responsible of handling data sent from Agents (i.e. REST over HTTP, WebSockets…) “speaking” the POA Protocol.

The main responsability is getting calls from Agents, transform the data into a POABackend.Protocol.Message Struct and sending it to the receivers.

Writing your custom Handler

You must to use the POABackend.CustomHandler module. That will requires you to implement the function child_spec/1 which will be called from the POABackend.CustomHandler.Supervisor and it must returns the child spec for the process you are going to spawn.

defmodule MyHandler do
  use POABackend.CustomHandler

  def child_spec(options) do
    Plug.Adapters.Cowboy.child_spec(scheme: options[:scheme], plug: POABackend.CustomHandler.Rest.Router, options: [port: options[:port]])
  end

end

In this example we are initializing our CustomHandler for REST requests using the Cowboy Plug and defining the endpoints in the POABackend.CustomHandler.Rest.Router module.

Configuring the handlers in the config file

So far we have created a Custom Handler but we didn’t tell poa_backend to start it. In order to do it we have to define the new Handler in the config file.

config :poa_backend, 
 :custom_handlers,
 [
   {:rest_custom_handler, POABackend.CustomHandler.Rest, [port: 4002]}
 ]

Inside the :custom_handlers list we define the handlers we want to start. Each Handler is defined in a triple tuple where the first argument is the id for that handler, the second one is the Elixir module which implements the CustomHandler behaviour and the third one is a list for arguments which will be passed to the child_spec/1 function as a parameter

Helpful functions

This module also define some helpful functions:

• send_to_receivers/1: This function will publish the incomming message to the appropiate metric type (Data Type). A Custom Handler must call it when it wants to dispatch a message.
• publish_inactive/1: Will publish an inactive message to all the metrics in the system. A Custom Handler must call it when detects if a client is disconnected or/and inactive