When do I need to respond with a nack on my RabbitMQ listener?
So far I have never witness an error in the message over the network.
I guess the network layer should take care of it, no?
If I get an error message from the publisher a nack will just leave it in the queue, wont it?
Thanks.
A NACK could be sent in case the consumer was (maybe temporarily) not able to process the delivery. The messages will be redelivered by the server to any available consumer (potentially including the original) and processing could be tried again.
It is different from REJECT in the sense that rejected deliveries are not resend to the rejecting consumer (redelivery happens on a different channel).
Related
There are some of the messages got stuck in UnAck state in RabbitMQ. Is there any way to move them to ready state without restarting the consumer application or without restarting the RabbitMQ server?
Unacked state literally means messages are being consumed and awaiting for Acknowledgement i.e. status update. If your messages are stuck in this state, it mostly likely means your consumers have not provided appropriate acknowledgements for those message.
You can provide acknowledgements in the following ways.
ack the message. This signals to RabbitMQ that the message has been successfully processed/consumed and can be pop from the queue. See https://www.rabbitmq.com/amqp-0-9-1-quickref.html#basic.ack
reject or nack the message. This signals that the message was not processed correctly and should be either "dead-lettered" or "re-queue", depending on the message/queue configuration. See https://www.rabbitmq.com/amqp-0-9-1-quickref.html#basic.reject
Alternatively, you can also set a TTL for your messages, in which case, they will be automatically reject if their time in the queue exceed their TTL. See here https://www.rabbitmq.com/ttl.html.
Is it possible to use Dead Letter Exchanges to handle both retrying a task and archiving tasks that failed more than retry_max number of times?
Most articles I've read set it up so that when a task fails, it is nacked so the task is sent to a retry-queue via a DLX. retry-queue has a TTL and is configured to send the dead message back to worker-queue.
What I would like to do is to send the message to a different queue, e.g.: dead-queue, when the maximum number of retries fails for manual intervention later. Is this possible using nack?
The diagram in the second answer here suggests that I can somehow nack the message when the max retries is exceeded and send it to a "Fail Exchange" but I'm not sure how I can nack the message and have it delivered to a different exchange.
I have a basic setup where a consumer is listening to a "/queue/work" for items. The consumer is meant to only consume one item at a time.
to do so i NACK any other item received concurrently and also UNSUBSCRIBE from the queue while the item is getting processed.
if i now SUBSCRIBE to the same queue again, the NACKed messages are not redelivered to the client - unless i drop the whole connection and reconnect the session - which is not exactly what i want to do since it impacts other subscriptions too :(
is there another way to implement this "take one item - ignore while busy" pattern ?
According to the STOMP Protocol Spec, the server does not re-deliver the NACKed message to the client that sent the NACK.
When you re-subscribe, try using a different id header value that is not used by any other subscriber.
To answer my own question - the way to implement it without ever having to NACK a message which produces the redelivery problem is to use a combination of transactions and prefetch settings:
if setting the "prefetch-count": 1 in the STOMP header - the server will only allow one message "in flight" on the channel before sending the next. This means that the client has to ACK/NACK the message before a new one is sent.
So instead of sending an ACK only if the "job" is completed - we instead start a transaction when we receive the message - ACK the message immediately - and COMMIT the transaction when the job is done. This way "failed" jobs are redelivered correctly.
Basic nack provides facility to return negative acknowledgement for one or multiple messages.
Basic reject has facility to return negative acknowledgement for only one message.
Do we have any use case where we definitely need basic reject?
With rabbitmq you can always nack over reject. Logically there is no explicit need to use reject, since nack can do anything reject can. As you said, both of these work for one message, but nack also for multiple.
Also nack is introduced by rabbitmq (so it's an extension), and is not part of amqp 0.9.1 specs.
The answer by #cantSleepNow is correct, I would also like to add one more difference which is in their default behaviour.
By default, nack will put the message back in the queue for later handling. You can change the setting to not re-queue with nack.
With reject, by default, the message is not re-queued by RabbitMQ but will drop the message from the queue entirely.
I'm trying to implement a work queue architecture using RabbitMQ. I have a single sender application and multiple consumers.
I use manual ack on the consumers, so in case of failure in handling a request, it will be re-queued for another consumer to handle.
I was wondering what would happen if all the consumers return nack on a specific request. Is there a way to recognize this behavior and mark the request as 'dead' so it's rerouted to the dead letter exchange? In such a case, I'd like to have a separate consumer open on the queue bound to the dead letter exchange and receive all the messages that failed to be handled by any consumer (for logging purposes or executing this request's task locally, without distributed consumers).
Another question I had. When requeueing the request upon receiving NACK from a consumer, will it try to send this request to other consumers or will it try to send to the first available, even if it's the one that already nacked the request?
Thanks
no there is no such a feature in RabbitMQ. You may handle exceptions, and for specific exception send message to the dead queue or if know maximum time that message must live, configure TTL on queue.
if you nack message, it will go to the next AVAILABLE consumer