[rabbitmq-discuss] Headers Exchange
david at rabbitmq.com
Thu Jan 27 11:55:42 GMT 2011
"Jason J. W. Williams" <jasonjwwilliams at gmail.com> writes:
> Is the headers exchange still supported?
> Seems to be from the RabbitMQ
> AMQP compatibility chart but it's hard to find any concrete
> information on using it. What is the advantage if any over topic?
Short and fairly useless answer: Headers exchanges route based on the
headers of the message. Topic exchanges route based on the routing key
usd to publish the mesage.
- Headers exchanges can express an "any field matches" or an "all fields
match" condition. Topic exchanges can only express an "all words
match" (e.g. a binding key of "*.x.*.y.*" will only match routing keys
where the second word is "x" and the fourth word is "y"; OTOH multiple
bindings provide a way to express disjunctions - you could bind a
queue twice to a topic exchange, once with binding key "*.x.*.*.*" and
once with "*.*.*.y.*").
- Topic exchanges can do hierarchical matching via the "#" operator
(e.g. a binding key of "a.b.#" matches any routing keys beginning with
The headers exchange is quite limited in terms of the queries it can
express: It can only do equality and presence matching on fields. It
can only express an "and" and "or" combination over a set of field
conditions, not more elaborate boolean expressions (and no negation).
So while the header exchang might appear to offer a lot, in practice its
uses are restricted to a fairly small niche. A more general variant
would be a lot more interesting.
Staff Engineer, RabbitMQ
SpringSource, a division of VMware
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