I know that an exchange can bind multi queues in rabbitmq, does it support binding a single queue to multi exchanges?
YES, it can.
A queue can have any number of bindings to different exchanges, even multiple bindings to the same exchange with different parameters.
You do this by invoking the bind method multiple times, see:
https://www.rabbitmq.com/amqp-0-9-1-reference.html#queue.bind
Related
I have a use-case where the producers publish some events and the consumers are initially bound to the producers' exchanges. Over time, certain conditions change and the binding or the routing keys (for consumers) needs to be updated, since their subscription interests have changed. Can I handle this dynamic routing scenario in RabbitMQ?
Yes, this is fairly straightforward.
RabbitMQ provides the Queue.Bind and Queue.Unbind methods. When your consumers wish to receive information about a particular topic, they would Bind the queue to that topic via a topic exchange. When they are no longer interested in a particular topic, the binding would be removed for that topic using Unbind.
RabbitMQ allows distribute messages over several queues based on routing_key matching. For exchange of type='topic' one can specify wildcard symbols which allows more flexible filtering (than in case of type='direct' exchange), they are:
* (star) can substitute for exactly one word.
# (hash) can substitute for zero or more words.
E.g., for queue with routing_key='A.B.*' the exchange is pushing all the messages with routing_key of the patterns 'A.B.A', 'A.B.1', 'A.B.XXX', etc.
In my use case I need two queues bound to the same exchange, one queue receives all messages with routing_key='A.B.A' and another queue receives all other messages which is not matching to 'A.B.A'. It is not a big deal to do it for the first queue but I cannot really find anything in tutorials which helps me with the second queue...
Please, would be nice to have an example in python + pika library.
Could you clarify your requirement of bound to the same exchange?
If it's "just" having the messages pushed through the same exchange instead, you could check out alternate exchange
Basically it's a configuration on your "main" exchange which states that any message that doesn't match any of the bindings to queues bound to it will be transferred to an alternate exchange (can be of different type) for processing.
A simple setup would be to have that alternate exchange of type fanout, routing all of those unbound messages to a given queue.
Why do we need routing key to route messages from exchange to queue? Can't we simply use the queue name to route the message? Also, in case of publishing to multiple queues, we can use multiple queue names. Can anyone point out the scenario where we actually need routing key and queue name won't be suffice?
There are several types of exchanges. The fanout exchange ignores the routing key and sends messages to all queues. But pretty much all other exchange types use the routing key to determine which queue, if any, will receive a message.
The tutorials on the RabbitMQ website describes several usecases where different exchange types are useful and where the routing key is relevant.
For instance, tutorial 5 demonstrates how to use a topic exchange to route log messages to different queues depending on the log level of each message.
If you want to target multiple queues, you need to bind them to a fanout exchange and use that exchange in your publisher.
You can't specify multiple queue names in your publisher. In AMQP, you do not publish a message to queues, you publish a message to an exchange. It's the exchange responsability to determine the relevant queues. It's possible that a message is routed to no queue at all and just dropped.
Decoupling queue names from applications is useful for flexibility.
You could establish multiple queues to consume the same message, but queues can't have the same name.
In some cases, message's originator doesn't know the names of queues. (like when you have randomly generated queue names when horizontally scaling a server)
An exchange may be routing messages for more than just one type of consumer. Then you would need some wildcards in your routing keys to route messages to concerned consumers.
I have exactly 2 types of messages that I want to be sent via RabbitMQ. So I have 2 options how I can do this:
sent a message to default empty-named exchange with routing_key corresponding to the queue name
use direct exchange's routing_key parameter corresponding to consumer's routing_key parameter in queue binding
So which option is preferable and why?
A default exchange is a direct exchange. RabbitMQ creates the default exchange by default, but it uses an empty string for the name. If you look at the RabbitMQ AMQP concepts page, under Default Exchange:
The default exchange is a direct exchange with no name (empty string)
pre-declared by the broker.
You can see this by running rabbitmqctl list_exchanges as well:
direct
Foo direct < Same thing as the above
amq.direct direct
amq.fanout fanout
...and so on
As far as I'm aware, there isn't any benefits of using one over the other. I would stick with the default exchange if you only need to route based on routing keys.
Let's say you direct-bind to an exchange broadcasting logs to routing keys of "info", "warn", and "error". Using the default exchange, you would need to create three different queues with those names to receive all logs; and adjustments to which log levels you receive would require changing your queue declarations. By using a named exchange, you can simply change your queue's bindings and continue processing things as normal.
In short, it provides one extra level of abstraction.
As I see it, the default direct exchange give the possibility for the consumers and the producers to not know about each other, by binding a queue (used by a consumer) to an exchange (used by a producer) implicitly using the queue's name.
I use the default direct-exchange for a specific case: the consumer and producers don't know about each other. In my case, each consumer have its proper queue. From the producer, I cannot know by advance which queues are going to be declared and used, as it depends on the consumers. So it is impossible to define the bindings between a custom direct-exchange and the queues on the producer side.
One way to solve it with a custom (user-defined) direct-exchange would be to define the binding-key on consumer side. But it would means to know about the producer from the consumer side as I need to know the exchange name used by the producer.
Therefore, automatically binding a queue by its name on the default direct-exchange makes it possible, in my case, to only declare a queue on consumer side and send message to it from producer by only knowing the queue's name.
Of course it implies to know the queue's name at runtime, when invoking the producer, as it would be required to know the binding-key of a custom direct-exchange (in my case, the queue's name is given by the application using the producer). But when configuring the broker, producers and consumers don't have to know about each other.
some description in rabbitmq web manager.
//default exchange 's Bindings
Default exchange
The default exchange is implicitly bound to every queue,
with a routing key equal to the queue name.
It is not possible to
explicitly bind to, or
unbind from the default exchange.
It also cannot be deleted.
Is it possible to bind a single queue to many topics using RabbitMQ STOMP client?
Each time a client sending SUBSCRIBE frame server creates a new queue for it, it makes usage of "prefetch-count" useless for me, because it applies to each subscription individually.
I am just looking for any way to get messages with many topics in the single queue via RabbitMQ Web-STOMP. Any ideas?
See Documentation:User generated queue names for Topic and Exchange destinations
The header x-queue-name specified queue name should binding to same queue if there exist, but will exist multiple subscription on client.
The different between AMQP and STOMP concept not compatible in some ways.