A RabbitMQ producer for Broadway.

Features

• Automatically acknowledges/rejects messages.
• Handles connection outages using backoff for retries.

Options

• :queue - Required. The name of the queue.
• :connection - Optional. Defines an AMQP URI or a set of options used by the RabbitMQ client to open the connection with the RabbitMQ broker. See AMQP.Connection.open/1 for the full list of options.
• :qos - Optional. Defines a set of prefetch options used by the RabbitMQ client. See AMQP.Basic.qos/2 for the full list of options. Pay attention that the :global option is not supported by Broadway since each producer holds only one channel per connection.
• :buffer_size - Optional, but required if :prefetch_count under :qos is set to 0. Defines the size of the buffer to store events without demand. Can be :infinity to signal no limit on the buffer size. This is used to configure the GenStage producer, see the GenStage docs for more details. Defaults to :prefetch_count * 5.
• :buffer_keep - Optional. Used in the GenStage producer configuration. Defines whether the :first or :last entries should be kept on the buffer in case the buffer size is exceeded. Defaults to :last.
• :requeue - Optional. Defines a strategy for requeuing failed messages. Possible values are: :always - always requeue, :never - never requeue, :once - requeue it once when the message was first delivered. Reject it without requeueing, if it's been redelivered. Default is :always.
• :backoff_min - The minimum backoff interval (default: 1_000)
• :backoff_max - The maximum backoff interval (default: 30_000)
• :backoff_type - The backoff strategy, :stop for no backoff and to stop, :exp for exponential, :rand for random and :rand_exp for random exponential (default: :rand_exp)
• :metadata - The list of AMQP metadata fields to copy (default: [])

Note: choose the requeue strategy carefully. If you set the value to :never or :once, make sure you handle failed messages properly, either by logging them somewhere or redirecting them to a dead-letter queue for future inspection. By sticking with :always, pay attention that requeued messages by default will be instantly redelivered, this may result in very high unnecessary workload. One way to handle this is by using Dead Letter Exchanges and TTL and Expiration.

Example

Broadway.start_link(MyBroadway,
  name: MyBroadway,
  producers: [
    default: [
      module:
        {BroadwayRabbitMQ.Producer,
        queue: "my_queue",
        requeue: :once,
        connection: [
          username: "user",
          password: "password",
          host: "192.168.0.10"
        ],
        qos: [
          prefetch_count: 50
        ]},
      stages: 5
    ]
  ],
  processors: [
    default: []
  ]
)

Back-pressure and :prefetch_count

Unlike the RabbitMQ client that has a default :prefetch_count = 0, which disables back-pressure, BroadwayRabbitMQ overwrite the default value to 50 enabling the back-pressure mechanism. You can still define it as 0, however, if you do this, make sure the machine has enough resources to handle the number of messages coming from the broker, and set :buffer_size to an appropriate value.

This is important because the BroadwayRabbitMQ producer does not work as a poller like BroadwaySQS. Instead, it maintains an active connection with a subscribed consumer that receives messages continuously as they arrive in the queue. This is more efficient than using the basic.get method, however, it removes the ability of the GenStage producer to control the demand. Therefore we need to use the :prefetch_count option to impose back-pressure at the channel level.

Connection loss and backoff

In case the connection cannot be opened or if a stablished connection is lost, the producer will try to reconnect using an exponential random backoff strategy. The strategy can be configured using the :backoff_type option.

Unsupported options

Currently, Broadway does not accept options for Basic.consume/4 which is called internally by the producer with default values. That means options like :no_ack are not supported. If you have a scenario where you need to customize those options, please open an issue, so we can consider adding this feature.