Amazon SQS

Amazon Simple Queue Service (SQS) is a highly scalable distributed message queuing service provided by Amazon.com. AWS SQS offers two types of message queues:

  • Standard

    • Nearly unlimited throughput
    • Best-effort ordering
    • At-least-once delivery
  • FIFO

    • Limited number of transactions per second (TPS). See Amazon SQS FIFO developer guide for more information on limits.
    • Order in which messages are sent/received is strictly preserved
    • Exactly-once delivery

Broadway can work seamlessly with both, Standard and FIFO queues.

Getting Started

In order to use Broadway with SQS, we need to:

  1. Create a SQS queue (or use an existing one)
  2. Configure our Elixir project to use Broadway
  3. Define your pipeline configuration
  4. Implement Broadway callbacks
  5. Run the Broadway pipeline
  6. Tuning the configuration (Optional)

Create a SQS queue

Amazon provides a comprehensive Step-by-step Guide on creating SQS queues. In case you don't have an AWS account and want to test Broadway locally, use can easily install ElasticMQ, which is a message queue system that offers a SQS-compatible query interface.

Configure the project

In this guide we're going to use BroadwaySQS, which is a Broadway SQS Connector provided by Plataformatec.

Starting a new project

If you plan to start a new project, just run:

mix new my_app --sup

The --sup flag instructs Elixir to generate an application with a supervision tree.

Setting up dependencies

Add :broadway_sqs to the list of dependencies in mix.exs along the HTTP client of your choice (defaults to :hackney):

def deps do
  [
    ...
    {:broadway_sqs, "~> 0.2.0"},
    {:hackney, "~> 1.9"},
  ]
end

Don't forget to check for the latest version of dependencies.

Define the pipeline configuration

Broadway is a process-based behaviour and to define a Broadway pipeline, we need to define three functions: start_link/1, handle_message/3 and handle_batch/4. We will cover start_link/1 in this section and the handle_ callbacks in the next one.

Similar to other process-based behaviour, start_link/1 simply delegates to Broadway.start_link/2, which should define the producers, processors, and batchers in the Broadway pipeline. Assuming we want to consume messages from a queue called my_queue, the minimal configuration would be:

defmodule MyBroadway do
  use Broadway

  alias Broadway.Message

  def start_link(_opts) do
    Broadway.start_link(__MODULE__,
      name: __MODULE__,
      producers: [
        default: [
          module: {BroadwaySQS.Producer, queue_name: "my_queue"}
        ]
      ],
      processors: [
        default: []
      ],
      batchers: [
        default: [
          batch_size: 10,
          batch_timeout: 2000
        ]
      ]
    )
  end

  ...callbacks...
end

The above configuration also assumes that you have the AWS credentials set up in your environment, for instance, by having the AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY environment variables set. If that's not the case, you will need to pass that information to the client so it can properly connect to the AWS servers. Here is how you can do it:

...
producers: [
  default: [
    module: {BroadwaySQS.Producer,
      queue_name: "my_queue",
      config: [
        access_key_id: "YOUR_AWS_ACCESS_KEY_ID",
        secret_access_key: "YOUR_AWS_SECRET_ACCESS_KEY"
      ]
    }
  ]
]
...

For a full list of options for BroadwaySQS.Producer, please see BroadwaySQS documentation.

For general information about setting up Broadway, see Broadway module docs as well as Broadway.start_link/2.

Note: Even though batching is optional in Broadway v0.2, we recommend all SQS pipelines to have at least a default batcher, with the default values defined above, unless you are expecting a very low rate of incoming messages. That's because batchers will also acknowledge messages in batches, which is the most cost and time efficient way of doing so on Amazon SQS.

Implement Broadway callbacks

In order to process incoming messages, we need to implement the required callbacks. For the sake of simplicity, we're considering that all messages received from the queue are just numbers:

defmodule MyBroadway do
  use Broadway

  alias Broadway.Message

  ...start_link...

  def handle_message(_, %Message{data: data} = message, _) do
    message
    |> Message.update_data(fn data -> data * data end)
  end

  def handle_batch(_, messages, _, _) do
    list = messages |> Enum.map(fn e -> e.data end)
    IO.inspect(list, label: "Got batch of finished jobs from processors, sending ACKs to SQS as a batch.")
    messages
  end
end

We are not doing anything fancy here, but it should be enough for our purpose. First we update the message's data individually inside handle_message/3 and then we print each batch inside handle_batch/4.

For more information, see Broadway.handle_message/3 and Broadway.handle_batch/4.

Run the Broadway pipeline

To run your Broadway pipeline, you just need to add as a child in a supervision tree. Most applications have a supervision tree defined at lib/my_app/application.ex. You can add Broadway as a child to a supervisor as follows:

children = [
  {MyBroadway, []}
]

Supervisor.start_link(children, strategy: :one_for_one)

Now the Broadway pipeline should be started when your application starts. Also, if your Broadway has any dependency (for example, it needs to talk to the database), make sure that Broadway is listed after its dependencies in the supervision tree.

Tuning the configuration

Some of the configuration options available for Broadway come already with a "reasonable" default value. However those values might not suit your requirements. Depending on the number of messages you get, how much processing they need and how much IO work is going to take place, you might need completely different values to optimize the flow of your pipeline. The stages option available for every set of producers, processors and batchers, among with batch_size and batch_timeout can give you a great deal of flexibility. The stages option controls the concurrency level in each layer of the pipeline. Here's an example on how you could tune them according to your needs.

defmodule MyBroadway do
  use Broadway

  def start_link(_opts) do
    Broadway.start_link(__MODULE__,
      name: __MODULE__,
      producers: [
        default: [
          ...
          stages: 60,
        ]
      ],
      processors: [
        default: [
          stages: 100,
        ]
      ],
      batchers: [
        default: [
          batch_size: 10,
          stages: 80,
        ]
      ]
    )
  end

  ...callbacks...
end

In order to get a good set of configurations for your pipeline, it's important to respect the limitations of the servers you're running, as well as the limitations of the services you're providing/consuming data to/from.