AMQP

Simple Elixir wrapper for the Erlang RabbitMQ 3/4 client (AMQP 0.9.1).

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

Upgrading guides

To upgrade from the old version, please read our upgrade guides:

• 0.x to 1.x
• 1.x to 2.x
• 2.x to 3.x
• 3.X to 4.x

OTP 27 support

If you want to use this library with OTP 27, please be aware that the RabbitMQ team has reported some performance regressions with OTP 27.

It might not be a big matter to the client side in most cases, but we recommend you monitor it carefully.

Usage

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

def deps do
  [
    {:amqp, "~> 4.0"}
  ]
end

Elixir will start amqp automatically with this if you use Elixir 1.6+.

If that's not the case (use Application.started_applications/0 to check), try adding :amqp to applications or extra_applications in your mix.exs. Or call Application.ensure_started(:amqp) at the start.

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)
    setup_queue(chan)

    # Limit unacknowledged messages to 10
    :ok = Basic.qos(chan, prefetch_count: 10)
    # 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
    # You might want to run payload consumption in separate Tasks in production
    consume(chan, tag, redelivered, payload)
    {:noreply, chan}
  end

  defp setup_queue(chan) do
    {:ok, _} = 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
    {:ok, _} = Queue.declare(chan, @queue,
                             durable: true,
                             arguments: [
                               {"x-dead-letter-exchange", :longstr, ""},
                               {"x-dead-letter-routing-key", :longstr, @queue_error}
                             ]
                            )
    :ok = Exchange.fanout(chan, @exchange, durable: true)
    :ok = Queue.bind(chan, @queue, @exchange)
  end

  defp consume(channel, tag, redelivered, payload) do
    number = String.to_integer(payload)
    if number <= 10 do
      :ok = Basic.ack channel, tag
      IO.puts "Consumed a #{number}."
    else
      :ok = 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 consumer will stop
    # receiving messages.
    exception ->
      :ok = 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

Configuration

Connections and channels

You can define a connection and channel in your config and AMQP will automatically...

• Open the connection and channel at the start of the application
• Automatically try to reconnect if they are disconnected

config :amqp,
  connections: [
    myconn: [url: "amqp://guest:guest@myhost:12345"],
  ],
  channels: [
    mychan: [connection: :myconn]
  ]

You can access the connection/channel via AMQP.Application.

iex> {:ok, chan} = AMQP.Application.get_channel(:mychan)
iex> :ok = AMQP.Basic.publish(chan, "", "", "Hello")

When a channel is down and reconnected, you have to make sure your consumer subscribes to a channel again.

See the documentation for AMQP.Application.get_connection/1 and AMQP.Application.get_channel/1 for more details.

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:

:longstr | :signedint | :decimal | :timestamp | :table | :byte | :double | :float | :long | :short | :bool | :binary | :void | :array

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 / FAQ

Is amqp 4.x compatible with RabbitMQ 3.x?

Yes, it is.

This library uses the official Erlang RabbitMQ client under the hood. As long as the client works with the old RabbitMQ version, our library will also support the old version.

Here is the comment from the RabbitMQ team.

Does the library support AMQP 1.0?

No, it doesn't. This library supports only AMQP 0.9.1 and we have no plan to support 1.0 at this moment.

RabbitMQ 4 now officially supports AMQP 1.0 along with 0.9.1. You might get some good benefits from using the protocol.

• https://www.rabbitmq.com/blog/2024/08/05/native-amqp
• https://www.rabbitmq.com/blog/2024/08/21/amqp-benchmarks
• https://www.rabbitmq.com/blog/2024/09/02/amqp-flow-control

Since AMQP 1.0 protocol design is significantly different from 0.9.1, we also think it is a good idea to start from scratch instead of building on top of this library.

Consumer stops receiving messages

It usually happens when your code doesn't send an acknowledgement(ack, nack or reject) after receiving a message.

If you use GenServer for your consumer, try storing the 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 the consumer has stopped receiving more messages.

Also, review the following points:

• when an exception was raised how it would be handled
• when :exit signal was thrown how it would be handled
• when a message processing took long time what could happen

Also, make sure that the consumer monitors the channel pid. When the channel is gone, you have to reopen it and subscribe to a new channel again.

The version compatibility

Check out this article to find out the compatibility with Elixir, OTP and RabbitMQ.

Heartbeats

If the connection is dropped automatically, consider enabling heartbeats.

You can set the heartbeat option when you open a connection.

For more details, read this article

Copyright and License

Copyright (c) 2014 Paulo Almeida

This library is MIT licensed. See the LICENSE for details.