[rabbitmq-discuss] Unofficial guide to AMQP 1.0 PR3

[Michael Bridgen]

All,

I'd like to second Alexis's request for eyes on the new AMQP 1.0 spec 
("PR3").

To help out, here is a brief guide to the spec, with reference to the 
AMQP 0-8 implemented in RabbitMQ.  I've left out a lot of details, so I 
encourage everyone to read the spec in depth, and simply use this guide 
as a map of the territory to get started.

I'll *emphasise* anything that's a 1.0 term, and leave 0-8 terms in roman.

Book I Messaging Model

The core AMQP 1.0 spec abstracts, to a large extent, away from the 
purely client-broker model of previous specs.  Since we're referring to 
0-8, take *Container* or "peer" (and other synonyms) to mean Broker or 
Application + Client library, but bear in mind that it's aimed at 
something more general.

A *Node* is something that's addressed in a *Container*.  For a broker 
that would be an exchange or queue.  For an application, that's a 
consumer or (oddly, in 0-8) a publisher.

*Links* are a generalisation of a publication or consumption 
relationship between *Nodes*.  So on the consuming side, i.e., with 
basic.consume, this has a queue name (addressing a queue) at one end and 
a consumer tag at the other (addressing, probably, some kind of buffer 
from which the application will retrieve messages).

In 0-8, there's no declaration of a publication relation, one just 
mentions the target in the basic.publish method (or not, in the case of 
the default exchange).

In PR3, publication is treated as symmetric to consumption.

*Credit* is like a combination of channel.flow and basic.qos.  I'll come 
back to it later.

For *Sessions* think channels.  One difference is that things aren't 
necessarily scoped to *session* lifetimes; in particular, *Links* can be 
resumed after a connection drop.

Book II Data types

This represents something more ambitious than the codec in 0-8; it is a 
data type system as well.  But for the purpose of this guide, it's the 
codec.

Book III Transport

Connections and framing are much the same as they are in 0-8.

This section talks about *channels*, but doesn't mean 0-8 channels, 
instead it's more like a "session handle".  *Sessions* are opened by 
saying "I'm going start a session called xyz, I'll be speaking on 
channel 4" and hearing back "ok, I'll talk about xyz on channel 7" -- so 
each party now can use a number to refer to the *session*.

*Transfer* is used in the place of both basic.publish (which clients 
use) and basic.deliver (which clients receive), because they both happen 
over a *link*.

Unlike 0-8, publishing can be acked (though this may not be required) -- 
a *link endpoint* (i.e., the state of the *link* kept in the broker and 
in the client) maintains a replay buffer, and can resend messages that 
haven't been acked.  Similarly, the receiving *link endpoint* can keep a 
deduplication filter.  *Disposition* is used to report the state of 
messages, e.g., "consider this acknowledged".

Resuming *links* is something like what basic.recover tries to achieve, 
resending messages that aren't known to have been processed.

Flow control acts like a more fine-grained channel.flow for publishers, 
and like basic.qos for consumers; in each case, the receiving side 
issues *credit*, which the sending side uses up by transferring 
messages.  To do something like a basic.get, a consumer issues a single 
unit of *credit*, and waits for the single message to come in.

Book IV Messaging

In 0-8, exchanges route messages to all matching bindings, while queues 
deliver messages to only one consumer.  In PR3, *non-destructive links* 
convey messages while leaving them available for other links (like 
exchanges do), and *destructive links* acquire messages and so make them 
unavailable for other links (like queues do).  In principle these can be 
mixed.

Both kinds of *links* can have *filters*, which are like exchange types 
in addition to binding keys, in that they each specify an algorithm and 
the particulars; a bit like if bindings could be "topic:ticker.nyse.*".

Book V Transactions

In PR3, transactions are decoupled from *sessions*, which is unlike 0-8 
in which the transactional behaviour is determined per channel.

Transactions can also cover publication, message acknowledgement (or 
rejection), /and/ deliveries, unlike 0-8 in which publication and 
acknowledgement only are covered.

Book VI Security

TLS is negotiated after a special protocol header is sent over the 
normal AMQP port 5672, instead of being established first on a different 
port as in RabbitMQ.

SASL does much the same thing, using a special protocol header then 
negotiation, and replaces in part the 
connection.{start,start-ok,secure,secure-ok} methods in 0-8.