amqp-dispatcher 0.4.0

Consumers

Consumers are a class with 2 required methods: consume and shutdown. amqp-dispatcher will not monkey patch the environment, you will have to do that yourself.

• consume: consume is called once for each message being handled. It should take 2 parameters, a proxy for AMQP operations (amqp) and the message (msg).

• shutdown - shutdown is called before the instance of the consumer is removed. It takes a single argument exception which may be None. If your consumer raises an exception while consuming the shutdown method will be called. Once shutdown is finished a new instance of your consumer will be created to replace the one that raised the exception. If you would like to rate limit instance replacement you can call gevent.sleep(X) to sleep for X seconds after a failure.

Example consumer:

class Consumer(object):

    def __init__(self):
        self.init_msg = "I've been initiliazed"

    def consume(self, amqp, msg):
        print 'Consuming message', msg.body
        gevent.sleep(1)
        val = random.random()
        if val > .8:
            print 'publishing'
            amqp.publish('test_exchange', 'test_routing_key', {}, 'New body!')
        if val < .5:
            raise ValueError()
        print 'Done sleeping'
        amqp.ack()

    def shutdown(self, exception=None):
        print 'Shut down'

Configuration

amqp-dispatcher will read environment variable for connection information and a YAML file for worker configuration.

You can validate the yaml file configuration with the following command:

amqp-dispatcher --validate --config amqp-dispatcher-config.yml

This will validate that the startup_handler and consumers exist and can be imported. Note that if there is any logic contained outside of those functions, said logic will be executed.

Environment Variables

• RABBITMQ_URL: Connection string of the form amqp://USER:PASS@HOST:PORT/VHOST

Startup Configuration

If you need to perform custom actions (configure your logging, create initial objects) you can add a startup handler.

This is configured in the config yml with the startup_handler option.

startup_handler: amqpdispatcher.example_startup:startup

Queue configuration

Queues can be created on the fly by amqp dispatcher, and may bind existing exchanges on the fly as well.

There are a few obvious constraints:

• To create a non-passive queue (typical behavior) the current user must have configure=queue permission

• To bind to an exchange, the current user must have read permission on the binding exchange

Queue configuration is as follows:

• queue: (required) name of the queue

• durable: (optional) queue created in “durable” mode (default = True)

• auto_delete: (optional) queue created in “auto_delete” mode (default = False), meaning it will be deleted automatically once all consumers disconnect from it (e.g. on restart)

• exclusive: (optional) queue created in “exclusive” mode (default = False) meaning it will only be accessible by this process

• x_dead_letter_exchange: (optional) name of dead letter exchange

• x_dead_letter_routing_key: (optional) dead letter routing key

• x_max_length: (optional) maximum length of ready messages. (default = INFINITE)

• x_expires: (optional) How long a queue can be unused for before it is automatically deleted (milliseconds) (default=INFINITE)

• x_message_ttl: (optional) How long a message published to a queue can live before it is discarded (milliseconds) (default=INFINITE)

Bindings

bindings should contain a list of exchange/routing_key pairs and defines the binding for the queue (there can be multiple)

A complete configuration example would look like:

queues:
  - queue: notify_mat_job
    durable: true
    auto_delete: false
    passive: true
    exclusive: false
    x_dead_letter_exchange: null
    x_dead_letter_routing_key: null
    x_max_length: null
    x_expires: null
    x_message_ttl: null
    bindings:
      - exchange: notify
        routing_key: transaction.*
      - exchange: notify
        routing_key: click.*

  - queue: notify_apsalar_job
    bindings:
      - exchange: notify
        routing_key: transaction.*
      - exchange: notify
        routing_key: click.*

Worker configuration

Workers are autoloaded when AMQP Dispatcher starts. This means your worker must be importable from the environment.

A complete configuration example would look like:

consumers:
  - consumer: workers.module:Consumer
    consumer_count: 1
    queue: test_queue
    prefetch_count: 2
  - consumer: workers.module_2:Consumer
    consumer_count: 2
    queue: test_queue_2
    prefetch_count: 10

prefetch_count is the AMQP prefetch_count when consuming. The consumer_count is the number of instances of your consumer to handle messages from that queue. Connection pools are highly recommended. MySQL will require the MySQL Connector instead of mysqldb in order for gevent to switch properly.

Pools can be created and attached to the consumer class during the __init__. Example with SQLAlchemy

class Consumer(object):

    session_maker = None

    def __init__(self):
        self.session = None

        if Consumer._engine is None:
            print 'Creating session maker'
            Consumer._engine = create_engine(...)
            Consumer.sessionmaker = sessionmaker(bind=Consumer._engine)

And then a session created during the consume method.

def consume(self, proxy, msg):
    session = self.sessionmaker()
    # Do something with the session
    session.close()

Logging

Logging is performed on the logger amqp-dispatcher. The RabbitMQ connection provided by Haigha will log on amqp-dispatcher.haigha.