I have the following problem.
My program sends messages directly to the Queue (without exchange). I need to monitor incoming of new messages and send them to other Queue without removing them from source queue.
I don't have access to program code, so I'm not able to publish messages to exchange first.
Is it possible to solve this problem using the management web interface of RabbitMQ?
I tried to use shovel plugin, but it removes all messages from source queue after ack.
First to clear up few things:
My program sends messages directly to the Queue (without exchange) This is not true, at the very least (and most likely in this case) nameless exchange is used.
removes all messages from source queue after ack
this is by design and therefore perfectly fine.
You should never keep messages in the queue, queue is made to be consumed. As Derick Bailey says here
RabbitMQ is not a database. RabbitMQ is a message broker and queueing system.
on the same link you will find your answer. I cannot give a concrete one since you didn't provide motivation, but whatever it is keeping messages in the queue is never good!
Maybe you want to log/store your message first and then process it with the consequence of processing being some 3rd action or whatever...
Related
Requirement
A system undergoes some state change, and multiple other parts of the system has to know this(lets call them observers) so that they can perform some actions based on the current state, the actions of the observers are important, if some of the observers are not online(not listening currently due to some trouble, but will be back soon), the message should not be discarded till all the observers gets the message.
Trying to accomplish this with pub/sub model, here are my findings, (please correct if this understanding is wrong) -
The publisher creates an event on specific topic, and multiple subscribers can consume the same message. This model either provides no delivery guarantee(in redis), or delivery is guaranteed once(with messaging queues), ie. when one of the consumer acknowledges a message, the message is discarded(rabbitmq).
Example
A new Person Profile entity gets created in DB
Now,
A background verification service has to know this to trigger the verification process.
Subscriptions service has to know this to add default subscriptions to the user.
Now both the tasks are important, unrelated and can run in parallel.
Now In Queue model, if subscription service is down for some reason, a BG verification process acknowledges the message, the message will be removed from the queue, or if it is fire and forget like most of pub/sub, the delivery is anyhow not guaranteed for both the services.
One more point is both the tasks are unrelated and need not be triggered one after other.
In short, my need is to make sure all the consumers gets the same message and they should be able to acknowledge them individually, the message should be evicted only after all the consumers acknowledged it either of the above approaches doesn't do this.
Anything I am missing here ? How should I approach this problem ?
This scenario is explicitly supported by RabbitMQ's model, which separates "exchanges" from "queues":
A publisher always sends a message to an "exchange", which is just a stateless routing address; it doesn't need to know what queue(s) the message should end up in
A consumer always reads messages from a "queue", which contains its own copy of messages, regardless of where they originated
Multiple consumers can subscribe to the same queue, and each message will be delivered to exactly one consumer
Crucially, an exchange can route the same message to multiple queues, and each will receive a copy of the message
The key thing to understand here is that while we talk about consumers "subscribing" to a queue, the "subscription" part of a "pub-sub" setup is actually the routing from the exchange to the queue.
So a RabbitMQ pub-sub system might look like this:
A new Person Profile entity gets created in DB
This event is published as a message to an "events" topic exchange with a routing key of "entity.profile.created"
The exchange routes copies of the message to multiple queues:
A "verification_service" queue has been bound to this exchange to receive a copy of all messages matching "entity.profile.#"
A "subscription_setup_service" queue has been bound to this exchange to receive a copy of all messages matching "entity.profile.created"
The consuming scripts don't know anything about this routing, they just know that messages will appear in the queue for events that are relevant to them:
The verification service picks up the copy of the message on the "verification_service" queue, processes, and acknowledges it
The subscription setup service picks up the copy of the message on the "subscription_setup_service" queue, processes, and acknowledges it
If there are multiple consuming scripts looking at the same queue, they'll share the messages on that queue between them, but still completely independent of any other queue.
Here's a screenshot from this interactive visualisation tool that shows this scenario:
As you mentioned it is not something that you can control with Redis Pub/Sub data structure.
But you can do it easily with Redis Streams.
Streams will allow you to post messages using the XADD command and then control which consumers are dealing with the message and acknowledge that message has been processed.
You can look at these sample application that provides (in Java) example about:
posting and consuming messages
create multiple consumer groups
manage exceptions
Links:
Getting Started with Redis Streams and Java
Redis Streams in Action ( Project that shows how to use ADD/ACK/PENDING/CLAIM and build an error proof streaming application with Redis Streams and SpringData )
If I use publisher confirms, I can be (reasonably) sure that a message sent to an exchange on the RabbitMQ server, and which received ACK from the RabbitMQ server is not lost even if the RabbitMQ server crashes (power outage for example).
However what happens when a message arrives at a dead letter exchange after a manual rejection in the consumer? (channel.basicReject, I use Spring AMQP.)
Can I still be sure that in the case in which the original message is dequeued from the queue to which the consumer is listening, and the RabbitMQ server subsequently crashes, I will eventually find the message, after the RabbitMQ server is restarted, in the queues which are bound to the dead letter exchange (if normally the message would have arrived there)?
If the answer is negative, is there a way to ensure that this is the case?
As #GaryRussell suggested, I posted a similar question on rabbitmq-users Google group.
Here is the answer I got from Daniil Fedotov
"Hi,
There is no delivery guarantees in place. Dead lettering does not check if the message was enqueued or saved to disk.
Dead-lettering does not use publisher confirms or any other confirm mechanisms.
It's not that easy to implement reliable dead-lettering from one queue to another and there are plans to address this issue eventually, but it may take a while.
If you want to safely reject messages from the consumer without a risk of losing them - you can publish them from the consumer application manually to the dead-letter queue, wait for the confirmation and then reject."
I have got several messages on error queue which has name TestQueue_errors.
One of the messages on error queue is important and should be moved back to service queue TestQueue so it can be processed again. The other messages on error queue are broken and should stay on error queue.
I have tried to do that with shovel plugin but it seems it is able only to move all messages from one queue to another. Is there a way I could achieve that, to move single message from one queue to another?
As far as I know Rabbit Management does not allow to do it. The only thing you can do is to publish this message again.
Maybe there are some tools which give possibility to achieve it but it is not a standard behaviour.
Here are actions which you are able to perform on the queue (from RabbitMQ Management page):
Move all messages from one queue to another
Get all messages without requeue option (they would not be in the queue anymore)
Get first N messages without requeue option and then move the rest of messages to another queue
I have thousands of unacked messages in my dev environment which I can't restart.
Is there a way to remove (purge) all messages even if they are unacknowledged?
Close the channel that the unacked messages reside on, which will nack them back into the queue, then call purge.
You have to make consumer ack them (or nack) and only after that they will be removed. Alternatively you can shutdown consumers and purge the queue completely.
If you are looking for some way to purge all unacked messages - there are no such feature nor in AMQP protocol neither in RabbitMQ.
It looks like your consumer is the cause of the problem, so you have to adjust it (rewrite) to release message immediately after it processed or failed.
Once there are no "ready" messages in the queue, delete it and recreate.
YOU WILL LOSE THE QUEUE CONTENTS with this method.
You need to put messages back into the queue before you can purge them:
close the channel
close the connection (the script doesn't work for me)
As an alternative, this doesn't require to wait:
delete and recreate the queue
restart the server
You need to call basic.recover to force all unacked messages to be re-enqueued to a channel that failed. Be aware of the errata concerning this function specifying that only the requeue mode is supported by RabbitMQ.
For software developer use below code.
channel.purgeQueue(queue-name);
if we use this code the Queue will be clear and same queue will exist.
One way this can happen is if the consumer is stuck recycling the same messages due to a processing error. In this case, the RabbitMQ queue management interface may show the messages as Unacked, but really they are being read from the queue and processed (to the point of the failure) then requeued (to enable a retry) at a rapid pace -- maybe thousands of times per second.
During this loop, the messages exist briefly in the Ready state, but are immediately removed again by you application -- and the cycle begins again. As an example, this auto-requeue behavior is the default for Spring AMQP.
Since the messages are never left in the Ready state, the Management Interface's Get Message(s) button is unlikely work. What can work, if you have queue access, is to run a separate custom consumer instance, perhaps locally, but with the specific intent of removing and not requeuing the messages in question.
By RabbitMQ's Fair Dispatch mechanism, your additional consumer will likely receive the messages in question and have the opportunity to perform your custom handling.
You might even write a custom utility to do this, with logic to filter, analyze, or deadletter the messages of interest.
If you want to clear the contents of the queue, then you can use the AMQP method queue.purge: There is queue purge in AMQP: http://www.rabbitmq.com/amqp-0-9-1-reference.html#queue.purge
You could do similar using the management plugin.
I'm using ActiveMQ and I would like to know how to solve this specific case.
When the consumer is down, the producer sends a message to the queue. The message will remain in the queue until the consumer is running to consume it.
Now imagine I shutdown the producer, the message will STILL remain in the queue. Now i run the consumer and it will try to consume that message, but won't be able to reply back to the producer since its down.
I would like to solve this problem by cleaning the messages if the producer is out.
The ActiveMQ Broker cleans the Queue after stopping. I would like to do the same for the messages of a respective producer.
Thanks.
Based on what I understand now from your question and additional comments I propose to add a Message Property to your messages to identify the Producer, and write a small utility that uses a Message Selector to read all messages matching the Producer from the queue. You can run that utility straight after the Producer is stopped (or crashes), and that should quite accurately do what you want to achieve.
EDIT: although primarily focused on EE, the Sun/Oracle JavaEE Tutorial contains a very good chapter on general JMS programming that starts off with standalone producers and consumers. The accompanying source code bundle can be downloaded here, the ready to comoile samples in that bundle should get you started very quickly.
You can solve it a couple of ways. One is to set a TTL on the message so it goes away. The other is to connect via JMX and purge the Queue or remove the specific message using a selector statement or with the Message's specific MessageId value.
See this article for some hints.