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.
Related
I'm not sure how to resiliently handle RabbitMQ messages in the event of an intermittent outage.
I subscribe in a windows service, read the message, then store it my database. If I can't process the record because of the data I publish it to a dead letter queue for a human to address and reprocess.
I am not sure what to do if I have some intermittent technical issue that will fix itself (database reboot, network outage, drive space, etc). I don't want hundreds of messages showing up on dead letter that just needed to wait for a for a glitch but now would be waiting on a human.
Currently, I re-queue the event and retry it once, but it retries so fast the issue is not usually resolved. I thought of retrying forever but I don't want a real issue to get stuck in an infinite loop.
Is a broad topic but from the server side you could persist your messages and make your queues durable, this means that in the eventuality the server gets restarted they won't be lost, check more here How to persist messages during RabbitMQ broker restart?
For the consumer (client) it will depend on how you configure your client, from the docs:
In the event of network failure (or a node crashing), messages can be duplicated, and consumers must be prepared to handle them. If possible, the simplest way to handle this is to ensure that your consumers handle messages in an idempotent way rather than explicitly deal with deduplication.
If a message is delivered to a consumer and then requeued (because it was not acknowledged before the consumer connection dropped, for example) then RabbitMQ will set the redelivered flag on it when it is delivered again (whether to the same consumer or a different one). This is a hint that a consumer may have seen this message before (although that's not guaranteed, the message may have made it out of the broker but not into a consumer before the connection dropped). Conversely if the redelivered flag is not set then it is guaranteed that the message has not been seen before. Therefore if a consumer finds it more expensive to deduplicate messages or process them in an idempotent manner, it can do this only for messages with the redelivered flag set.
Check more here: https://www.rabbitmq.com/reliability.html#consumer
Let's say that a consumer for a queue has been disconnected for some time during which many number of messages are produced.
How long does RabbitMQ keep the messages for the disconnected consumer without durable mode?
(Will it discard the queue right after the consumer is disconnected? or will it keep the queue until the memory allows?)
Does the durable mode will give a functionality for a consumer to consume any message which is published until now? (i.e. random access to the queue, fetching messages out-of-order, or consuming from the beginning of the queue)
There are some TTL extensions.
TTL can be set for a given queue by setting the x-message-ttl argument
to queue.declare, or by setting the message-ttl policy.
No it doesn't. The messages are kept in queue until they are acknowledged, regardless of durability. (unless of course the server dies, then the messages are gone if not previously marked as durable).
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
I'm in a phase of learning RabbitMQ/AMQP from the RabbitMQ documentation. Something that is not clear to me that I wanted to ask those who have hands-on experience.
I want to have multiple consumers listening to the same queue in order to balance the work load. What I need is pretty much close to the "Work Queues" example in the RabbitMQ tutorial.
I want the consumer to acknowledge message explicitly after it finishes handling it to preserve the message and delegate it to another consumer in case of crash. Handling a message may take a while.
My question is whether AMQP postpones next message processing until the previous message is ack'ed? If so how do I achieve load balancing between multiple workers and guarantee no messages get lost?
No, the other consumers don't get blocked. Other messages will get delivered even if they have unacknowledged but delivered predecessors. If a channel closes while holding unacknowledged messages, those messages get returned to the queue.
See RabbitMQ Broker Semantics
Messages can be returned to the queue using AMQP methods that feature a requeue parameter (basic.recover, basic.reject and basic.nack), or due to a channel closing while holding unacknowledged messages.
EDIT In response to your comment:
Time to dive a little deeper into the AMQP specification then perhaps:
3.1.4 Message Queues
A message queue is a named FIFO buffer that holds message on behalf of a set of consumer applications.
Applications can freely create, share, use, and destroy message queues, within the limits of their authority.
Note that in the presence of multiple readers from a queue, or client transactions, or use of priority fields,
or use of message selectors, or implementation-specific delivery optimisations the queue MAY NOT
exhibit true FIFO characteristics. The only way to guarantee FIFO is to have just one consumer connected
to a queue. The queue may be described as “weak-FIFO” in these cases. [...]
3.1.8 Acknowledgements
An acknowledgement is a formal signal from the client application to a message queue that it has
successfully processed a message.[...]
So acknowledgement confirms processing, not receipt. The broker will hold on to the message until it's gotten acknowleged, so that it can redeliver them. But it is free to deliver more messages to consumers even if the prededing messages have not yet been acknowledged. The consumers will not be blocked.
It seems the longer I keep my rabbitmq server running, the more trouble I have with unacknowledged messages. I would love to requeue them. In fact there seems to be an amqp command to do this, but it only applies to the channel that your connection is using. I built a little pika script to at least try it out, but I am either missing something or it cannot be done this way (how about with rabbitmqctl?)
import pika
credentials = pika.PlainCredentials('***', '***')
parameters = pika.ConnectionParameters(host='localhost',port=5672,\
credentials=credentials, virtual_host='***')
def handle_delivery(body):
"""Called when we receive a message from RabbitMQ"""
print body
def on_connected(connection):
"""Called when we are fully connected to RabbitMQ"""
connection.channel(on_channel_open)
def on_channel_open(new_channel):
"""Called when our channel has opened"""
global channel
channel = new_channel
channel.basic_recover(callback=handle_delivery,requeue=True)
try:
connection = pika.SelectConnection(parameters=parameters,\
on_open_callback=on_connected)
# Loop so we can communicate with RabbitMQ
connection.ioloop.start()
except KeyboardInterrupt:
# Gracefully close the connection
connection.close()
# Loop until we're fully closed, will stop on its own
connection.ioloop.start()
Unacknowledged messages are those which have been delivered across the network to a consumer but have not yet been ack'ed or rejected -- but that consumer hasn't yet closed the channel or connection over which it originally received them. Therefore the broker can't figure out if the consumer is just taking a long time to process those messages or if it has forgotten about them. So, it leaves them in an unacknowledged state until either the consumer dies or they get ack'ed or rejected.
Since those messages could still be validly processed in the future by the still-alive consumer that originally consumed them, you can't (to my knowledge) insert another consumer into the mix and try to make external decisions about them. You need to fix your consumers to make decisions about each message as they get processed rather than leaving old messages unacknowledged.
If messages are unacked there are only two ways to get them back into the queue:
basic.nack
This command will cause the message to be placed back into the queue and redelivered.
Disconnect from the broker
This action will force all unacked messages from this channel to be put back into the queue.
NOTE: basic.recover will try to republish unacked messages on the same channel (to the same consumer), which is sometimes the desired behaviour.
RabbitMQ spec for basic.recover and basic.nack
The real question is: Why are the messages unacknowledged?
Possible scenarios to cause unacked messages:
Consumer fetching too many messages, then not processing and acking them quickly enough.
Solution: Prefetch as few messages as appropriate.
Buggy client library (I have this issue currently with pika 0.9.13. If the queue has a lot of messages, a certain number of messages will get stuck unacked, even hours later.
Solution: I have to restart the consumer several times until all unacked messages are gone from the queue.
All the unacknowledged messages will go to ready state once all the workers/consumers are stopped.
Ensure all workers are stopped by confirming with a grep on ps aux output, and stopping/killing them if found.
If you are managing workers using supervisor, which shows as worker is stopped, you may want to check for zombies. Supervisor reports the worker to be stopped but still you will find zombie processes running when grepped on ps aux output. Killing the zombie processes will bring messages back to ready state.