Ears

Ears is a small, simple library for writing RabbitMQ consumers.

CodeQL

Installation

Add this line to your application's Gemfile:

gem 'ears'

And then execute:

$ bundle install

Or install it yourself as:

$ gem install ears

Usage

Basic usage

First, you should configure Ears.

require 'ears'

Ears.configure do |config|
  config.rabbitmq_url = 'amqp://user:password@myrmq:5672'
  config.connection_name = 'My Consumer'
  config.recover_from_connection_close = false # optional configuration, defaults to true if not set
  config.recovery_attempts = 3 # optional configuration, defaults to 10, Bunny::Session would have been nil
end

Note: connection_name is a mandatory setting!

Next, define your exchanges, queues, and consumers by calling Ears.setup.

Ears.setup do
  # define a durable topic exchange
  my_exchange = exchange('my_exchange', :topic, durable: true)

  # define a queue
  my_queue = queue('my_queue', durable: true)

  # bind your queue to the exchange
  my_queue.bind(my_exchange, routing_key: 'my.routing.key')

  # define a consumer that handles messages for that queue
  consumer(my_queue, MyConsumer)
end

Finally, you need to implement MyConsumer by subclassing Ears::Consumer.

class MyConsumer < Ears::Consumer
  def work(delivery_info, metadata, payload)
    message = JSON.parse(payload)
    do_stuff(message)

    ack
  end
end

And, do not forget to run it. Be prepared that unhandled errors will be reraised. So, take care of cleanup work.

begin
  Ears.run!
ensure
  # all your cleanup work goes here...
end

At the end of the #work method, you must always return ack, reject, or requeue to signal what should be done with the message.

Middlewares

Ears supports middlewares that you can use for recurring tasks that you don't always want to reimplement. It comes with some built-in middlewares:

  • Ears::JSON for automatically parsing JSON payloads
  • Ears::Appsignal for automatically wrapping #work in an Appsignal transaction

You can use a middleware by just calling use with the middleware you want to register in your consumer.

require 'ears/middlewares/json'

class MyConsumer < Ears::Consumer
  # register the JSON middleware and don't symbolize keys (this can be omitted, the default is true)
  # and nack the message on parsing error. This defaults to Proc.new { :reject }.
  use Ears::Middlewares::JSON,
      on_error: Proc.new { :nack },
      symbolize_keys: false

  def work(delivery_info, metadata, payload)
    return ack unless payload['data'].nil? # this now just works
  end
end

If you want to implement your own middleware, just subclass Ears::Middleware and implement #call (and if you need it #initialize).

class MyMiddleware < Ears::Middleware
  def initialize(opts = {})
    @my_option = opts.fetch(:my_option, nil)
  end

  def call(delivery_info, metadata, payload, app)
    do_stuff

    # always call the next middleware in the chain or your consumer will never be called
    app.call(delivery_info, metadata, payload)
  end
end

Multiple threads

If you need to handle a lot of messages, you might want to have multiple instances of the same consumer all working on a dedicated thread. This is supported out of the box. You just have to define how many consumers you want when calling consumer in Ears.setup.

Ears.setup do
  my_exchange = exchange('my_exchange', :topic, durable: true)
  my_queue = queue('my_queue', durable: true)
  my_queue.bind(my_exchange, routing_key: 'my.routing.key')

  # this will instantiate MyConsumer 10 times and run every instance on a dedicated thread
  consumer(my_queue, MyConsumer, 10)
end

It may also be interesting for you to increase the prefetch amount. The default prefetch amount is 1, but if you have a lot of very small, fast to process messages, a higher prefetch is a good idea. Just set it when defining your consumer.

Ears.setup do
  my_exchange = exchange('my_exchange', :topic, durable: true)
  my_queue = queue('my_queue', durable: true)
  my_queue.bind(my_exchange, routing_key: 'my.routing.key')

  # this will instantiate one consumer but with a prefetch value of 10
  consumer(my_queue, MyConsumer, 1, prefetch: 10)
end

Setting arbitrary exchange/queue parameters

If you need some custom arguments on your exchange or queue, you can just pass these to queue or exchange inside Ears.setup. These are then just passed on to Bunny::Queue and Bunny::Exchange.

Ears.setup do
  my_queue =
    queue('my_queue', durable: true, arguments: { 'x-message-ttl' => 10_000 })
end

Implementing a retrying queue

Sometimes you want to automatically retry processing a message, in case it just failed due to temporary problems. In that case, you can set the retry_queue and retry_delay parameters when creating the queue.

my_queue =
  queue('my_queue', durable: true, retry_queue: true, retry_delay: 5000)

This will automatically create a queue named my_queue.retry and use the arguments x-dead-letter-exchange and x-dead-letter-routing-key to route rejected messages to it. When routed to the retry queue, messages will wait there for the number of milliseconds specified in retry_delay, after which they will be redelivered to the original queue. Note that this will not automatically catch unhandled errors. You still have to catch any errors yourself and reject your message manually for the retry mechanism to work.

This will happen indefinitely, so if you want to bail out of this cycle at some point, it is best to use the error_queue option to create an error queue and then use the MaxRetries middleware to route messages to this error queue after a certain amount of retries.

Implementing an error queue

You can set the error_queue parameter to automatically create an error queue.

my_queue =
  queue(
    'my_queue',
    durable: true,
    retry_queue: true,
    retry_delay: 5000,
    error_queue: true,
  )

This will automatically create a queue named my_queue.error. It does not have any special properties, the helper's main purpose is to enforce naming conventions. In your consumer, you should then use the MaxRetries middleware to route messages to the error queue after a certain amount of retries.

class MyConsumer < Ears::Consumer
  use Ears::Middlewares::MaxRetries, retries: 3, error_queue: 'my_queue.error'

  def work(delivery_info, metadata, payload)
    # ...
  end
end

This will automatically route messages to my_queue.error after they have been re-tried three times. This prevents you from infinitely retrying a faulty message.

Stopping any running consumers

When you are running Ears in a non-blocking way (e.g. in a Thread), it might be cumbersome to remove the running consumers without restarting the whole app.

For this use case, there is a stop! method:

Ears.stop!

It will close and reset the current Bunny connection, leading to all consumers being shut down. Also, it will reset the channel.

Documentation

If you need more in-depth information, look at our API documentation.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/ivx/ears. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the code of conduct.

License

The gem is available as open-source under the terms of the MIT License.

Code of Conduct

Everyone interacting in the Ears project's codebases, issue trackers, chat rooms, and mailing lists is expected to follow the code of conduct.