rickhanlonii / amqp-dispatcher

A daemon to run AMQP consumers

AMQP Dispatcher

A daemon to run AMQP consumers

Running

RABBITMQ_HOST=rabbitmq.example.com amqp-dispatcher --config amqp-dispatcher-config.yml

The environment variable RABBITMQ\_HOSTS can also be used which will cause attempt to connect to a host in a random order. The list should be comma separated.

RABBITMQ_HOSTS=rabbitmq1.example.com,rabbitmq2.example.com

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

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.

Environment Variables

RABBITMQ_HOST - Host to connect to

RABBITMQ_USER - Username to connect with

RABBITMQ_PASS - Password to connect with

RABBITMQ_VHOST - Virtual host to use (default is "/")

At the moment guest:guest are used to connect, just because I have gotten around to changing it.

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

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.