Freddy

OTP behaviours for creating AMQP publishers and consumers.

The project is in active development stage, expect breaking changes between minor versions up to 1.0.

Installation

Add freddy to your list of dependencies in mix.exs:

def deps do
  [{:freddy, "~> 0.15.0"}]
end

Stable connection

Neither official RabbitMQ client, nor its Elixir wrapper amqp provide an out-of-box way to create a stable monitored connection to RabbitMQ server, which will be gracefully reestablished after server restart or intermittent network failures.

Freddy attempts to provide such abstraction, which is called Freddy.Connection. It is a standard OTP-compliant process that can be easily integrated into OTP supervision tree using standard capabilities.

All Freddy behaviors (publishers and consumers) require Freddy.Connection.

The connection process can be started like this:

{:ok, conn} = Freddy.Connection.start_link(config)

Check out Freddy.Connection.start_link/2 for available options.

Add this process to an OTP application supervision tree:

defmodule MyApp do
  use Application

  def start(_type, _args) do
    import Supervisor.Spec

    children = [
      worker(Freddy.Connection, [[], [name: Freddy.Connection]])
    ]

    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

We recommend to start Freddy.Connection and all your publishers and consumers in the OTP supervision tree. Ideally connection and dependent processes should be grouped in one supervisor with restart strategy :rest_for_one.

Connection to multiple hosts

It is possible to leverage H/A RabbitMQ setup by providing multiple connection options when starting a Freddy.Connection process. Don't forget to specify connection_timeout, or your process may stuck in infinite wait loop.

host1 = [host: "10.0.100.1", connection_timeout: 1000]
host2 = [host: "10.0.100.2", connection_timeout: 1000]
host3 = [host: "10.0.100.3", connection_timeout: 1000]
{:ok, conn} = Freddy.Connection.start_link([host1, host2, host3])

Freddy.Connection will establish connection to one of the specified hosts, prioritizing them by order of appearing in the list. If it can't establish connection to the first host, it will immediately attempt to establish connection to second, and so on. If none of the hosts are responding, Freddy.Connection will wait a second and attempt to connect to all hosts again.

Publishers

Freddy provides a behaviour module Freddy.Publisher to implement your own stateful publishers.

Check out the behaviour documentation for information about all available callbacks.

By default publisher processes encode message payload to JSON before sending the message to RabbitMQ server, it is responsibility of consumers to decode message back. This behaviour can be changed by redefining the default implementation of Freddy.Publisher.encode_message/4 callback.

Example

Below is an example of how to implement a publishing process that queues up messages when RabbitMQ connection is disrupted (instead of silently dropping them):

defmodule ReliableBroadcaster do
  use Freddy.Publisher

  @exchange %Freddy.Core.Exchange{name: "notifications", type: :fanout, opts: [durable: true]}
  @config [exchange: @exchange]

  def start_link(connection, opts \\ []) do
    Freddy.Publisher.start_link(__MODULE__, connection, @config, nil, opts)
  end

  @impl true
  def init(_) do
    state = %{connected: false, queue: :queue.new()}
    {:ok, state}
  end

  @impl true
  # This function is called after an exchange has been declared
  def handle_connected(meta, %{queue: queue} = state) do
    new_state = %{state | connected: true, queue: drain_queue(queue, meta)}
    {:noreply, new_state}
  end

  @impl true
  # This function is called right after disconnect
  def handle_disconnected(_reason, state) do
    {:noreply, %{state | connected: false}}
  end

  @impl true
  # Catch messages before publication and queue them up if connection is not available
  def before_publication(
    payload, routing_key, opts, %{connected: connected?, queue: queue} = state
  ) do
    if not connected? do
      message = {payload, routing_key, opts}
      {:ignore, %{state | queue: :queue.in(message, queue)}}
    else
      {:ok, state}
    end
  end

  defp drain_queue(queue, meta) do
    case :queue.out(queue) do
      {{:value, {payload, routing_key, opts}}, new_queue} ->
        Freddy.Publisher.publish(meta, payload, routing_key, opts)
        drain_queue(new_queue, meta)

      {:empty, empty_queue} ->
        empty_queue
    end
  end
end

Consumers

Stateful consumer processes are implemented with Freddy.Consumer behaviour module.

Check out the behaviour documentation for information about all available callbacks.

Consumer process typically works as follows:

1. After initialization consumer opens an AMQP channel
2. An exchange and a queue are declared using the opened channel
3. The declared queue is bound to the exchange (see RabbitMQ routing tutorial),
4. The consumer process starts consumption from the queue
5. Broker confirms that consumer process is registered on the server
6. Messages from the queue are delivered to the consumer process
7. When consumer has successfully processed a message, it acknowledges the message on server, and the server removes the message from the queue.

Message format

By default consumer processes assume that incoming messages payload are encoded into JSON and decode them before starting processing. This behaviour can be changed by redefining the default implementation of Freddy.Consumer.decode_message/3 callback.

Example

This is an example of a process that creates an exclusive queue with server-generated name, binds this queue to fanout exchange "notifications" and processes each message in a separate asynchronous task.

Please note that as we process messages asynchronously and we don't consume messages with :no_ack option, we must explicitly acknowledge or reject processed messages using Freddy.Consumer.ack/2 or Freddy.Consumer.reject/2.

The chosen approach is very naive and we do not recommend to use this code in production, it is here only for educational purposes.

defmodule NotificationsProcessor do
  use Freddy.Consumer

  @config [
    exchange: [name: "notifications", type: :fanout],
    queue: [opts: [exclusive: true, auto_delete: true]],
    qos: [prefetch_count: 10],
    routing_keys: ["#"]
  ]

  def start_link(conn, handler_mfa) do
    Freddy.Consumer.start_link(__MODULE__, conn, @config, handler_mfa)
  end

  @impl true
  def init(handler) do
    {:ok, handler}
  end

  @impl true
  def handle_message(payload, %{routing_key: key} = meta, {m, f, a} = handler) do
    Task.start_link(fn ->
      try do
        apply(m, f, [payload, key | a])

        Freddy.Consumer.ack(meta)
      rescue _error ->
        # we might want to log error here too
        Freddy.Consumer.reject(meta, requeue: true)
      end
    end)

    {:noreply, state}
  end
end

Remote Procedure Call (RPC)

Client

RPC Client in a nutshell is a combination of consumer and publisher. A process publishes RPC requests into default or any other exchange and expects a server to publish response message to a special anonymous queue from which an RPC client process consumes.

Each request contains a name of reply queue and a correlation ID - an identifier which allows RPC client to understand for which request the response has arrived. When server sends a reply, it publishes a message into default exchange with a routing key equal to the name of the client reply queue and copies correlation ID from the request to the response message.

A diagram below illustrates an RPC request-response lifecycle:

  +------------+             +----------------+
  |   Client   |------------>|  Pub exchange  |
  +------------+             +----------------+
         ^                            |
         |                            |
         |                            v
  +-------------+            +----------------+
  | Reply queue |            |  Server queue  |
  +-------------+            +----------------+
         ^                            |
         |                            |
         |                            v
+----------------+           +----------------+
|Default exchange|<----------|   RPC Server   |
+----------------+           +----------------+

Freddy.RPC.Client is also implemented as behaviour module, leaving you an opportunity to customize your application logic through set of callback functions.

Check out Freddy.RPC.Client documentation for information about available callbacks.

Example

This is an example of RPC client that publishes requests to the default exchange, logs unsuccessful requests and emits response time to a StatsD server.

defmodule RPC.Client do
  use Freddy.RPC.Client

  require Logger

  alias Freddy.RPC.Request

  @server_queue "RemoteService"

  def start_link(conn, opts \\ []) do
    Freddy.RPC.Client.start_link(__MODULE__, conn, [], nil, opts)
  end

  def request(client, payload) do
    Freddy.RPC.Client.request(client, @server_queue, payload)
  end

  @impl true
  def on_timeout(request, state) do
    Logger.warn("Request to server #{request.routing_key} timed out after #{Request.duration(request)} ms")
    {:reply, {:error, :timeout}, state}
  end

  @impl true
  def on_return(request, state) do
    Logger.warn("Request to server #{request.routing_key} couldn't be routed")
    {:reply, {:error, :no_route}, state}
  end

  @impl true
  def on_response(response, request, state) do
    send_metrics(request)
    {:reply, response, state}
  end

  defp send_metrics(request) do
    MyApp.Statix.histogram("rpc.request", Request.duration(request), tags: ["server:#{request.routing_key}"])
  end
end

Testing

We recommend you to structure your code in such way that your test environment will not use real RPC client.

For example, you can create a client behavior and use Mox to mock all calls to a client. This way you can test how your code communicates with RPC client. Please refer to Mox documentation for more information about mocks and explicit contracts.

If you need to test RPC client itself, you can hook into request lifecycle and use before_request/2 callback to return required responses. It is recommended to use fake connection in test environment:

defmodule MockClient do
  use Freddy.RPC.Client

  def start_link(conn) do
    Freddy.RPC.Client.start_link(__MODULE__, conn, [], [])
  end

  def flush(client) do
    Freddy.RPC.Client.call(client, :flush)
  end

  def before_request(request, sink) do
    {:reply, :ok, [request | sink]}
  end

  def handle_call(:flush, sink) do
    {:reply, Enum.reverse(sink), []}
  end
end

And use it in tests:

test "sends an RPC request" do
  {:ok, conn} = Freddy.Connection.start_link(adapter: :sandbox)
  {:ok, client} = MockClient.start_link(conn)
  MyLib.call(client: client)

  assert [%{routing_key: "server"}] = MockClient.flush(client)
end

Server

Similarly to RPC client, RPC server is also a combination of consumer and publisher, but on the server side, the consumer is used to accept RPC request messages, whilst the publisher is used to send back response messages.

Check out Freddy.RPC.Server documentation for information about available callbacks.

Message format

By default server processes assume that incoming messages payload are encoded into JSON and decode them before processing. This behaviour can be changed by redefining the default implementation of Freddy.RPC.Server.decode_request/3 callback.

Acknowledgement mode

By default RPC server starts in automatic acknowledgement mode. It means that all incoming requests will be acknowledged automatically by RabbitMQ server once delivered to a client (RPC server process).

If your logic requires manual acknowledgements, you should start server with configuration option [consumer: [no_ack: false]] and acknowledge messages manually using Freddy.RPC.Server.ack/2 function.

Example

Below is an example of simple synchronous echo RPC server, which can process only one request at a time:

defmodule RPC.Server do
  use Freddy.RPC.Server

  def start_link(conn) do
    config = [
      queue: [name: "EchoServer"]
    ]

    Freddy.RPC.Server.start_link(__MODULE__, conn, config, [])
  end

  def handle_request(payload, _meta, state) do
    {:reply, payload, state}
  end
end

A slightly more complicated example of asynchronous RPC server, which processes every request in a separate process with manual acknowledgement of processed requests:

defmodule RPC.Server do
  use Freddy.RPC.Server

  import Freddy.RPC.Server, only: [ack: 1, reply: 2]

  def start_link(conn, handler) when is_function(handler, 1) do
    config = [
      queue: [name: "AsyncServer"],
      qos: [prefetch_count: 100], # this is protection from DoS
      consumer: [no_ack: false] # this enables manual acknowledgements
    ]

    Freddy.RPC.Server.start_link(__MODULE__, conn, config, handler)
  end

  @impl true
  def init(handler) do
    {:ok, task_sup} = Task.Supervisor.start_link()

    {:ok, {task_sup, handler}}
  end

  @impl true
  def handle_request(request, meta, {task_sup, handler}} = state) do
    Task.Supervisor.start_child(task_sup, fn ->
      result = handler.(request)
      ack(meta)
      reply(meta, result)
    end)

    {:noreply, state}
  end
end