[rabbitmq-discuss] RabbitMQ 2.8.0 released

Mon Mar 19 12:36:14 GMT 2012

On 19/03/12 12:24, Brendan Hay wrote:
> Would it be possible to describe in more (technical/erlangy) detail how
> the new flow-control mechanism works?

There's a few words at:

http://www.rabbitmq.com/memory.html

under "Per-Connection Flow Control", although that might not count as 
"technical/erlangy". In terms of implementation:

* A process that forwards AMQP messages on inside the broker is issued 
credit for each process it sends messages to.

* The receiving process will issue more credit as it processes these 
messages.

* If a process which receives messages from inside the broker runs out 
of credit, it will in turn stop issuing credit until it gets more.

* If a process which receives messages from the network runs out of 
credit, it will stop reading from its socket until it gets more.

* See the 'credit_flow' module (and its call sites) for more details.

Since all the processes in the chain need to opt in to this new 
mechanism for it to work, only the most common paths do flow control: 
messages published via AMQP and STOMP. Messages published via other 
direct client users (e.g. JSON-RPC or management) and messages which 
arise spontaneously in the broker (dead lettering, log and trace 
messages) are not flow controlled. Indeed it's hard to see how they 
could be.

Cheers, Simon

-- 
Simon MacMullen
RabbitMQ, VMware