Rabbit v0.1.0 Rabbit.Consumer behaviour View Source

A RabbitMQ consumer process.

This wraps around the standard AMQP.Channel. It provides the following benefits:

  • Durability during connection and channel failures through use of expotential backoff.
  • Ability to create module-based consumers that permit easy runtime setup through the init/1 and after_connect/2 callbacks.
  • Automatic acknowledgements based on the return value of the handle_message/1 callback.
  • Ability to handle exceptions through the handle_error/1 callback.
  • Each message is executed within its own supervised task.
  • Automatic payload decoding based on available serializers and message content type.

Example

# This is a connection
defmodule MyConnection do
  use Rabbit.Connection
end

# This is a consumer
defmodule MyConsumer do
  use Rabbit.Consumer

  # Callbacks

  # Perform any runtime configuration
  def init(opts) do
    {:ok, opts}
  end

  # Perform exchange or queue setup
  def after_connect(channel, queue) do
    AMQP.Queue.declare(channel, queue)

    :ok
  end

  # Handle consumed messages
  def handle_message(message) do
    {:ack, message}
  end

  # Handle errors that occur within handle_message/1
  def handle_error(message) do
    {:nack, message}
  end
end

# Start the connection
MyConnection.start_link()

# Start the consumer
MyConsumer.start_link(MyConnection, queue: "my_queue", prefetch_count: 20)

Serializers

When a message is consumed, its content type is compared to the list of available serializers. If a serializer matches the content type, the message will be automatically decoded.

You can find out more about serializers at Rabbit.Serializer.

Link to this section Summary

Callbacks

A callback executed after the channel is open, but before consumption.

A callback executed to handle message exceptions.

A callback executed to handle message consumption.

A callback executed when the consumer starts.

Starts a RabbitMQ consumer process.

Stops a RabbitMQ consumer process.

Link to this section Types

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action_options() View Source
action_options() :: [multiple: boolean(), requeue: boolean()]

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delivery_tag() View Source
delivery_tag() :: non_neg_integer()

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option() View Source
option() ::
  {:queue, String.t()}
  | {:prefetch_count, non_neg_integer()}
  | {:prefetch_size, non_neg_integer()}
  | {:consumer_tag, String.t()}
  | {:no_local, boolean()}
  | {:no_ack, boolean()}
  | {:exclusive, boolean()}
  | {:no_wait, boolean()}
  | {:arguments, Keyword.t()}

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options() View Source
options() :: [option()]

Link to this section Callbacks

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after_connect(channel, queue) View Source (optional)
after_connect(channel :: AMQP.Channel.t(), queue :: String.t()) :: :ok

A callback executed after the channel is open, but before consumption.

The callback is called with an AMQP.Channel, as well as the queue that will be consumed. At the most basic, you will most likely want to declare the queue to ensure its available. This will be entirely application dependent though.

def after_connect(channel, queue) do
  AMQP.Queue.declare(channel, queue)

  :ok
end

The callback must return an :ok atom - otherise it will be marked as failed, and the consumer will attempt to go through the connection setup process again.

Link to this callback

handle_error(message) View Source
handle_error(message :: Rabbit.Message.t()) ::
  {:ack, Rabbit.Message.t()}
  | {:ack, Rabbit.Message.t(), action_options()}
  | {:nack, Rabbit.Message.t()}
  | {:nack, Rabbit.Message.t(), action_options()}
  | {:reject, Rabbit.Message.t()}
  | {:reject, Rabbit.Message.t(), action_options()}
  | any()

A callback executed to handle message exceptions.

If the original handle_message/1 callback raises an error, this callback will be called with the message - but with the :error_reason and :error_stack fields filled.

You may choose to return the same values as handle_message/1.

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handle_message(message) View Source
handle_message(message :: Rabbit.Message.t()) ::
  {:ack, Rabbit.Message.t()}
  | {:ack, Rabbit.Message.t(), action_options()}
  | {:nack, Rabbit.Message.t()}
  | {:nack, Rabbit.Message.t(), action_options()}
  | {:reject, Rabbit.Message.t()}
  | {:reject, Rabbit.Message.t(), action_options()}
  | any()

A callback executed to handle message consumption.

The callback is called with a Rabbit.Message struct. You may find more information about the message structure within its own documentation. The message may be automatically decoded based on the content type and available serializers.

You may choose to ack, nack, or reject the message based on the return value.

  • {:ack, message} - will awknowledge the message.
  • {:ack, message, options} - will awknowledge the message with options.
  • {:nack, message} - will negative awknowledge the message.
  • {:nack, message, options} - will negative awknowledge the message with options.
  • {:reject, message} - will reject the message.
  • {:reject, message, options} - will reject the message with options.

If you dont return one of these values - nothing will be done.

Link to this callback

init(options) View Source (optional)
init(options()) :: {:ok, options()} | :ignore

A callback executed when the consumer starts.

The options passed represent the options that will be used by the consumer. This must return {:ok, keyword} or :ignore.

Link to this callback

start_link(connection, options) View Source
start_link(connection :: Rabbit.Connection.t(), options()) ::
  GenServer.on_start()

Starts a RabbitMQ consumer process.

Options

  • :queue - The queue to consume messages from.
  • :prefetch_count - The basic unit of concurrency for a given consumer - defaults to 0.
  • :prefetch_size - The prefetch window size in octets - defaults to 0, meaning no specific limit.
  • :consumer_tag - The identifier of the consumer. If empty, one will be generared by the server.
  • :no_local - A boolean representing whether messages should not be sent to the same connection that published them - defaults to false.
  • :no_ack - A boolean representing whether acknowledgements are not required for messages - defaults to false.
  • :exclusive - A boolean representing whether only this consumer can access the queue - defaults to false.
  • :no_wait - A boolean representing whether the server should not respond to methods - defaults to false.
  • :arguments - A set of arguments for the consume.

Stops a RabbitMQ consumer process.