AMQP

Simple Elixir wrapper for the Erlang RabbitMQ client.

The API is based on Langohr, a Clojure client for RabbitMQ.

Usage

Add AMQP as a dependency in your mix.exs file.

def deps do
  [{:amqp, "~> 0.3.1"}]
end

You should also update your application list to include :amqp:

def application do
  [applications: [:amqp]]
end

After you are done, run mix deps.get in your shell to fetch and compile AMQP. Start an interactive Elixir shell with iex -S mix.

iex> {:ok, conn} = AMQP.Connection.open
{:ok, %AMQP.Connection{pid: #PID<0.165.0>}}
iex> {:ok, chan} = AMQP.Channel.open(conn)
{:ok, %AMQP.Channel{conn: %AMQP.Connection{pid: #PID<0.165.0>}, pid: #PID<0.177.0>}
iex> AMQP.Queue.declare chan, "test_queue"
{:ok, %{consumer_count: 0, message_count: 0, queue: "test_queue"}}
iex> AMQP.Exchange.declare chan, "test_exchange"
:ok
iex> AMQP.Queue.bind chan, "test_queue", "test_exchange"
:ok
iex> AMQP.Basic.publish chan, "test_exchange", "", "Hello, World!"
:ok
iex> {:ok, payload, meta} = AMQP.Basic.get chan, "test_queue"
iex> payload
"Hello, World!"
iex> AMQP.Queue.subscribe chan, "test_queue", fn(payload, _meta) -> IO.puts("Received: #{payload}") end
{:ok, "amq.ctag-5L8U-n0HU5doEsNTQpaXWg"}
iex> AMQP.Basic.publish chan, "test_exchange", "", "Hello, World!"
:ok
Received: Hello, World!

Setup a consumer GenServer

defmodule Consumer do
  use GenServer
  use AMQP

  def start_link do
    GenServer.start_link(__MODULE__, [], [])
  end

  @exchange    "gen_server_test_exchange"
  @queue       "gen_server_test_queue"
  @queue_error "#{@queue}_error"

  def init(_opts) do
    {:ok, conn} = Connection.open("amqp://guest:guest@localhost")
    {:ok, chan} = Channel.open(conn)
    # Limit unacknowledged messages to 10
    Basic.qos(chan, prefetch_count: 10)
    Queue.declare(chan, @queue_error, durable: true)
    # Messages that cannot be delivered to any consumer in the main queue will be routed to the error queue
    Queue.declare(chan, @queue, durable: true,
                                arguments: [{"x-dead-letter-exchange", :longstr, ""},
                                            {"x-dead-letter-routing-key", :longstr, @queue_error}])
    Exchange.fanout(chan, @exchange, durable: true)
    Queue.bind(chan, @queue, @exchange)
    # Register the GenServer process as a consumer
    {:ok, _consumer_tag} = Basic.consume(chan, @queue)
    {:ok, chan}
  end

  # Confirmation sent by the broker after registering this process as a consumer
  def handle_info({:basic_consume_ok, %{consumer_tag: consumer_tag}}, chan) do
    {:noreply, chan}
  end

  # Sent by the broker when the consumer is unexpectedly cancelled (such as after a queue deletion)
  def handle_info({:basic_cancel, %{consumer_tag: consumer_tag}}, chan) do
    {:stop, :normal, chan}
  end

  # Confirmation sent by the broker to the consumer process after a Basic.cancel
  def handle_info({:basic_cancel_ok, %{consumer_tag: consumer_tag}}, chan) do
    {:noreply, chan}
  end

  def handle_info({:basic_deliver, payload, %{delivery_tag: tag, redelivered: redelivered}}, chan) do
    spawn fn -> consume(chan, tag, redelivered, payload) end
    {:noreply, chan}
  end

  defp consume(channel, tag, redelivered, payload) do
    number = String.to_integer(payload)
    if number <= 10 do
      Basic.ack channel, tag
      IO.puts "Consumed a #{number}."
    else
      Basic.reject channel, tag, requeue: false
      IO.puts "#{number} is too big and was rejected."
    end

  rescue
    # Requeue unless it's a redelivered message.
    # This means we will retry consuming a message once in case of exception
    # before we give up and have it moved to the error queue
    #
    # You might also want to catch :exit signal in production code.
    # Make sure you call ack, nack or reject otherwise comsumer will stop
    # receiving messages.
    exception ->
      Basic.reject channel, tag, requeue: not redelivered
      IO.puts "Error converting #{payload} to integer"
  end
end

iex> Consumer.start_link
{:ok, #PID<0.261.0>}
iex> {:ok, conn} = AMQP.Connection.open
{:ok, %AMQP.Connection{pid: #PID<0.165.0>}}
iex> {:ok, chan} = AMQP.Channel.open(conn)
{:ok, %AMQP.Channel{conn: %AMQP.Connection{pid: #PID<0.165.0>}, pid: #PID<0.177.0>}
iex> AMQP.Basic.publish chan, "gen_server_test_exchange", "", "5"
:ok
Consumed a 5.
iex> AMQP.Basic.publish chan, "gen_server_test_exchange", "", "42"
:ok
42 is too big and was rejected.
iex> AMQP.Basic.publish chan, "gen_server_test_exchange", "", "Hello, World!"
:ok
Error converting Hello, World! to integer
Error converting Hello, World! to integer

Stable RabbitMQ Connection

While the above example works, it does nothing to handle RabbitMQ connection outages. In case of an outage your Genserver will remain stale and won’t receive any messages from the broker as the connection is never restarted.

Luckily, implementing a reconnection logic is quite straight forward. Since the connection record holds the pid of the connection itself, we can monitor it and get a notification when it goes down.

Example implementation (only changes from the last example):

# 1. Extract your connect logic into a private method rabbitmq_connect

def init(_opts) do
  rabbitmq_connect
end

defp rabbitmq_connect do
    case Connection.open("amqp://guest:guest@localhost") do
      {:ok, conn} ->
        # Get notifications when the connection goes down
        Process.monitor(conn.pid)
        # Everything else remains the same
        {:ok, chan} = Channel.open(conn)
        Basic.qos(chan, prefetch_count: 10)
        Queue.declare(chan, @queue_error, durable: true)
        Queue.declare(chan, @queue, durable: true,
                                    arguments: [{"x-dead-letter-exchange", :longstr, ""},
                                                {"x-dead-letter-routing-key", :longstr, @queue_error}])
        Exchange.fanout(chan, @exchange, durable: true)
        Queue.bind(chan, @queue, @exchange)
        {:ok, _consumer_tag} = Basic.consume(chan, @queue)
        {:ok, chan}
      {:error, _} ->
        # Reconnection loop
        :timer.sleep(10000)
        rabbitmq_connect
    end
end

# 2. Implement a callback to handle DOWN notifications from the system
#    This callback should try to reconnect to the server

def handle_info({:DOWN, _, :process, _pid, _reason}, _) do
  {:ok, chan} = rabbitmq_connect
  {:noreply, chan}
end

Now, when the connection drops, or if the server is down when your application starts, it will try to reconnect indefinitely until it succeeds.

Types of arguments and headers

The parameter arguments in Queue.declare, Exchange.declare, Basic.consume and the parameter headers in Basic.publish are a list of tuples in the form {name, type, value}, where name is a binary containing the argument/header name, type is an atom describing the AMQP field type and value a term compatible with the AMQP field type.

The valid AMQP field types are:

Valid argument names in Queue.declare include:

“x-expires”
“x-message-ttl”
“x-dead-letter-routing-key”
“x-dead-letter-exchange”
“x-max-length”
“x-max-length-bytes”

Valid argument names in Basic.consume include:

“x-priority”
“x-cancel-on-ha-failover”

Valid argument names in Exchange.declare include:

“alternate-exchange”

Troubleshooting

Consumer stops receiving messages

Most popular cause is your code not sending acknowledgement(ack, nack or reject) after receiving a message. You want to investigate if…

an exception was raised and how it would be handled
:exit signal was thrown and how it would be handled
a message processing took long time.

If you use GenServer in consumer, try storing number of messages the server is currently processing to the GenServer state. If the number equals prefetch_count, those messages were left without acknowledgements and that’s why consumer have stopped receiving more messages.

Old version of Elixir or OTP

OTP 17 and 18 are supported only on version 0.1.x. Please understand that we won’t make further changes to 0.1 except for major security issues.

amqp

v0.3.1