IO and Event Looping¶
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As AMQP is a two-way RPC protocol where the client can send requests to the server and the server can send requests to a client, Pika implements or extends IO loops in each of its asynchronous connection adapters. These IO loops are blocking methods which loop and listen for events. Each asynchronous adapter follows the same standard for invoking the IO loop. The IO loop is created when the connection adapter is created. To start an IO loop for any given adapter, call the
connection.ioloop.start()
method.Game 2.0
If you are using an external IO loop such as Tornado’s
IOLoop
you invoke it normally and then add the Pika Tornado adapter to it.Example:
Continuation-Passing Style¶
Interfacing with Pika asynchronously is done by passing in callback methods you would like to have invoked when a certain event completes. For example, if you are going to declare a queue, you pass in a method that will be called when the RabbitMQ server returns a Queue.DeclareOk response.
In our example below we use the following five easy steps:
- We start by creating our connection object, then starting our event loop.
- When we are connected, the on_connected method is called. In that method we create a channel.
- When the channel is created, the on_channel_open method is called. In that method we declare a queue.
- When the queue is declared successfully, on_queue_declared is called. In that method we call
channel.basic_consume
telling it to call the handle_delivery for each message RabbitMQ delivers to us. - When RabbitMQ has a message to send us, it calls the handle_delivery method passing the AMQP Method frame, Header frame, and Body.
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Note
Step #1 is on line #28 and Step #2 is on line #6. This is so that Python knows about the functions we’ll call in Steps #2 through #5. Autodesk flame 2018 3 mac crack serial free download.
Example:
Credentials¶
The
pika.credentials
module provides the mechanism by which you pass the username and password to the ConnectionParameters
class when it is created.Example:
Connection Parameters¶
There are two types of connection parameter classes in Pika to allow you to pass the connection information into a connection adapter,
ConnectionParameters
and URLParameters
. Both classes share the same default connection values.TCP Backpressure¶
As of RabbitMQ 2.0, client side Channel.Flow has been removed [1]. Instead, the RabbitMQ broker uses TCP Backpressure to slow your client if it is delivering messages too fast. If you pass in backpressure_detection into your connection parameters, Pika attempts to help you handle this situation by providing a mechanism by which you may be notified if Pika has noticed too many frames have yet to be delivered. By registering a callback function with the
add_backpressure_callback
method of any connection adapter, your function will be called when Pika sees that a backlog of 10 times the average frame size you have been sending has been exceeded. You may tweak the notification multiplier value by calling the set_backpressure_multiplier
method passing any integer value.Example:
Footnotes
[1] | “more effective flow control mechanism that does not require cooperation from clients and reacts quickly to prevent the broker from exhausting memory - see http://lists.rabbitmq.com/pipermail/rabbitmq-announce/attachments/20100825/2c672695/attachment.txt |