[rabbitmq-discuss] Headers Exchange

[Jason J. W. Williams]

Hi David,

Thank you for the explanation. Are there any docs on how to construct the bind string? 

Also, is the performance better than topic?

Thank you in advance. 

-J

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On Jan 27, 2011, at 4:55, David Wragg <david at rabbitmq.com> wrote:

> "Jason J. W. Williams" <jasonjwwilliams at gmail.com> writes:
>> Is the headers exchange still supported?
> 
> Yes.
> 
>> 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.
> 
> Longer answer:
> 
> - 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
>  "a.b").
> 
> 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.
> 
> David
> 
> -- 
> David Wragg
> Staff Engineer, RabbitMQ
> SpringSource, a division of VMware