I am writing a wrapper around the .NET RabbitMQ library and have a question that isn't answered in the documentation as far as I can see, and I can't think of a way to verify it experimentally.
The documentation here states the following:
In most cases, RabbitMQ will acknowledge messages to publishers in the
same order they were published (this applies for messages published on
a single channel). However, publisher acknowledgements are emitted
asynchronously and can confirm a single message or a group of
messages. The exact moment when a confirm is emitted depends on the
delivery mode of a message (persistent vs. transient) and the
properties of the queue(s) the message was routed to (see above).
Which is to say that different messages can be considered ready for
acknowledgement at different times. This means that acknowledgements
can arrive in a different order compared to their respective messages.
Applications should not depend on the order of acknowledgements when
possible.
So confirmations can be received out of order which I can cope with OK. However, how does this play with the Multiple flag that can be used?
Imagine the following scenario: I send messages 1,2,3,4 and 5. 1 fails and 2-5 succeed. The broker sends me a Nack for 1 and a single Ack for 5 with Multiple set to true. If order were guaranteed then I know I would get the Nack for 1 first and then I would know that the Ack was for 2-5. However, if they could be out of order, then I would assume, as I hadn't had a confirm for 1, that the Ack was for 1-5, and then when the Nack came in, it would be too late.
Are there guarantees somewhere where this scenario cannot happen?
This is the answer from the Google Group mentioned by Alex in the comments. Basically, this scenario can't happen.
Hi,
If you get a single nack for 1 then an ack for 5 with multiple=true this means 2-5 were all successful.
If 1 is delayed but 2-5 are not you should get 4 multiple=false acks with sequences 2-5 then a nack for 1.
It would be nicer if ack frames used ranges instead of the multiple flag but we are where we are.
Cheers
Karl
Related
One of the main characteristics of a message queue service, RabbitMQ included, is preserving message publication order. This is confirmed in the RabbitMQ documentation:
[QUOTE 1] Section 4.7 of the AMQP 0-9-1 core specification explains the
conditions under which ordering is guaranteed: messages published in
one channel, passing through one exchange and one queue and one
outgoing channel will be received in the same order that they were
sent. RabbitMQ offers stronger guarantees since release 2.7.0.
Let's assume in the following that there are no consumers active, to simplify things. We are publishing over one single channel.
So far, so good.
RabbitMQ also provides possibility to inform the publisher that a certain publication has been completely and correctly processed [*]. This is explained here. Basically, the broker will either send a basic.ack or basic.nack message. The documentation also says this:
[QUOTE 2] basic.ack for a persistent message routed to a durable queue will be
sent after persisting the message to disk.
In most cases, RabbitMQ will acknowledge messages to publishers in the
same order they were published (this applies for messages published on
a single channel). However, publisher acknowledgements are emitted
asynchronously and can confirm a single message or a group of
messages. The exact moment when a confirm is emitted depends on the
delivery mode of a message (persistent vs. transient) and the
properties of the queue(s) the message was routed to (see above).
Which is to say that different messages can be considered ready for
acknowledgement at different times. This means that acknowledgements
can arrive in a different order compared to their respective messages.
Applications should not depend on the order of acknowledgements when
possible.
At first glance, this makes sense: persisting a message takes much more time than just storing it in memory, so it's perfectly possibly that the acknowledgment of a later transient message will arrive before the acknowledgement of an earlier persistent message.
But, if we re-read the first quote regarding message order [QUOTE 1] here above, it gets confusing. I'll explain. Assume we are sending two messages to the same exchange: first a persistent and then a transient message. Since RabbitMQ claims to preserve message order, how can it send an acknowledgment of the second/transient message before it knows that the first/persistent message is indeed completely written to disk?
In other words, does the remark regarding illogical acknowledgement order [QUOTE 2] here above only apply in case the two messages are each routed to completely different target queue(s) (which might happen if they have different routing keys, for example)? In that case, we don't have to guarantee anything as done in [QUOTE 1].
[*] In most cases, this means 'queued'. However, if there are no routing rules applicable, it cannot be enqueued in a target queue. However, this is still a positive outcome regarding publication confirmation.
update
I read this answer on a similar question. This basically says that there are no guarantees whatsoever. Even the most naive implementation, where we delay the publication of message 2 to the point after we got an acknowledgment of message 1, might not result in the desired message order. Basically, [QUOTE 1] is not met.
Is this correct?
From this response on rabbitmq-users:
RabbitMQ knows message position in a queue regardless of whether it is transient or not.
My guess (I did not write that part of the docs) the ack ordering section primarily tries to communicate that if two messages are routed to two different queues, those queues will handle/replicate/persist them concurrently. Reasoning about ordering in more than one queue is pretty hard. A message can go into more than one queue as well.
Nonetheless, RabbitMQ queues know what position a message has in what queues. Once all routing/delivery acknowledgements are received by a channel that handled the publish, it is added to the list of acknowledgements to send out. Note that that
list may or may not be ordered the same way as the original publishes and worrying about that is not practical for many reasons, most importantly: the user typically primarily cares about the ordering in the queues.
NOTE: the RabbitMQ team monitors the rabbitmq-users mailing list and only sometimes answers questions on StackOverflow.
I have a Java application which publishes events to RabbitMQ. It has one very important characteristic: message order must be preserved at all times. The consumer can handle duplicates, but it cannot handle when message 2 is enqueued before message 1, so to say.
I have been reading a lot about RabbitMQ lately, and I feel there is only solution to do this: set the channel in confirm mode (https://www.rabbitmq.com/confirms.html - basically, it forces the broker to acknowledge the publication) and publish one by one. With one by one I mean that the message 2 is only published after RabbitMQ confirmed (via an asynchronous ACK response) that message 1 is actually well received and persisted.
I tried this in a conceptual implementation, and while this works fine, it's uber slow, without exaggerating. Which makes sense: after all, we are now limiting our message rate to 1 message at a time.
So this leads me to my question: are there other, more performant, ways to ensure that message ordering is always preserved (either in RabbitMQ or via different approaches)?
Although my concern is RabbitMQ, I believe this question might be applied to any kind of asynchronous message queue service.
RabbitMQ's clients enqueue in the same order that you sent. It's when subscribers go down, you get network splits or the subscriber NACKs messages that they can get re-ordered; and even then RMQ tries to keep them in the same approximate order by re-queueing at the same position, or as close to the same position.
You can do it like you suggest; take one message at a time, because if you take a message, but crash before you've ACKed it from the broker, it will pop up when your service comes back up, at the same position.
This assumes you only have a single service instance at any given time, consuming from the queue. Which in turn is a distributed systems problem on its own, if you have a scheduler like Kubernetes or Mesos, spawning your service instances.
Another solution would be to ensure ordering of processing in the receiving service, by "resequencing" the messages based on their logical timestamps/sequence numbers.
I've written a much more thorough guide as annotated code here https://github.com/haf/rmq-publisher-confirms-hopac/blob/master/src/Server/Shared/RabbitMQ.fs — with batching you can resequence. Furthermore, if your idempotence builds the consecutive sequence numbers into its logic, you can start taking batches and each event will be idempotent, despite being re-consumed.
I have a question about multi consumer concurrency.
I want to send works to rabbitmq that comes from web request to distributed queues.
I just want to be sure about order of works in multiple queues (FIFO).
Because this request comes from different users eech user requests/works must be ordered.
I have found this feature with different names on Azure ServiceBus and ActiveMQ message grouping.
Is there any way to do this in pretty RabbitMQ ?
I want to quaranty that customer's requests must be ordered each other.
Each customer may have multiple requests but those requests for that customer must be processed in order.
I desire to process quickly incoming requests with using multiple consumer on different nodes.
For example different customers 1 to 1000 send requests over 1 millions.
If I put this huge request in only one queue it takes a lot of time to consume. So I want to share this process load between n (5) node. For customer X 's requests must be in same sequence for processing
When working with event-based systems, and especially when using multiple producers and/or consumers, it is important to come to terms with the fact that there usually is no such thing as a guaranteed order of events. And to get a robust system, it is also wise to design the system so the message handlers are idempotent; they should tolerate to get the same message twice (or more).
There are way to many things that may (and actually should be allowed to) interfere with the order;
The producers may deliver the messages in a slightly different pace
One producer might miss an ack (due to a missed package) and will resend the message
One consumer may get and process a message, but the ack is lost on the way back, so the message is delivered twice (to another consumer).
Some other service that your handlers depend on might be down, so that you have to reject the message.
That being said, there is one pattern that servicebus-systems like NServicebus use to enforce the order messages are consumed. There are some requirements:
You will need a centralized storage (like a sql-server or document store) that allows for conditional updates; for instance you want to be able to store the sequence number of the last processed message (or how far you have come in the process), but only if the already stored sequence/progress is the right/expected one. Storing the user-id and the progress even for millions of customers should be a very easy operation for most databases.
You make sure the queue is configured with a dead-letter-queue/exchange for retries, and then set your original queue as a dead-letter-queue for that one again.
You set a TTL (for instance 30 seconds) on the retry/dead-letter-queue. This way the messages that appear on the dead-letter-queue will automatically be pushed back to your original queue after some timeout.
When processing your messages you check your storage/database if you are in the right state to handle the message (i.e. the needed previous steps are already done).
If you are ok to handle it you do and update the storage (conditionally!).
If not - you nack the message, so that it is thrown on the dead-letter queue. Basically you are saying "nah - I can't handle this message, there are probably some other message in the queue that should be handled first".
This way the happy-path is to process a great number of messages in the right order.
But if something happens and a you get a message out of band, you will throw it on the retry-queue (the dead-letter-queue) and Rabbit will make sure it will get back in the queue to be retried at a later stage. But only after a delay.
The beauty of this is that you are able to handle most of the situations that may interfere with processing the message (out of order messages, dependent services being down, your handler being shut down in the middle of handling the message) in exact the same way; by rejecting the message and letting your infrastructure (Rabbit) take care of it being retried after a while.
(Assuming the OP is asking about things like ActiveMQs "message grouping:)
This isn't currently built in to RabbitMQ AFAIK (it wasn't as of 2013 as per this answer) and I'm not aware of it now (though I haven't kept up lately).
However, RabbitMQ's model of exchanges and queues is very flexible - exchanges and queues can be easily created dynamically (this can be done in other messaging systems but, for example, if you read ActiveMQ documentation or Red Hat AMQ documentation you'll find all of the examples in the user guides are using pre-declared queues in configuration files loaded at system startup - except for RPC-like request/response communication).
Also it is very easy in RabbitMQ for a consumer (i.e., message consuming thread) to consume from multiple queues.
So you could build, on top of RabbitMQ, a system where you got your desired grouping semantics.
One way would be to create dynamic queues: The first time a customer order was seen or a new group of customer orders a queue would be created with a unique name for all messages for that group - that queue name would be communicated (via another queue) to a consumer who's sole purpose was to load-balance among other consumers that were responsible for handling customer order groups. I.e., the load-balancer would pull off of its queue a message saying "new group with queue name XYZ" and it would find in a pool of order group consumer a consumer which could take this load and pass it a message saying "start listening to XYZ".
Another way to do it is with pub/sub and topic routing - each customer order group would get a unique topic - and proceed as above.
RabbitMQ Consistent Hash Exchange Type
We are using RabbitMQ and we have found a plugin. It use Consistent Hashing algorithm to distribute messages in order to consistent keys.
For more information about Consistent Hashing ;
https://en.wikipedia.org/wiki/Consistent_hashing
https://www.youtube.com/watch?v=viaNG1zyx1g
You can find this plugin from rabbitmq web page
plugin : rabbitmq_consistent_hash_exchange
https://www.rabbitmq.com/plugins.html
I need to choose a new Queue broker for my new project.
This time I need a scalable queue that supports pub/sub, and keeping message ordering is a must.
I read Alexis comment: He writes:
"Indeed, we think RabbitMQ provides stronger ordering than Kafka"
I read the message ordering section in rabbitmq docs:
"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...With release 2.7.0 and later
it is still possible for individual consumers to observe messages out of
order if the queue has multiple subscribers. This is due to the actions of
other subscribers who may requeue messages. From the perspective of the queue
the messages are always held in the publication order."
If I need to handle messages by their order, I can only use rabbitMQ with an exclusive queue to each consumer?
Is RabbitMQ still considered a good solution for ordered message queuing?
Well, let's take a closer look at the scenario you are describing above. I think it's important to paste the documentation immediately prior to the snippet in your question to provide context:
Section 4.7 of the AMQP 0-9-1 core specification explains the
conditions under which ordering is guaranteed: messages published in
one channel, passing through one exchange and one queue and one
outgoing channel will be received in the same order that they were
sent. RabbitMQ offers stronger guarantees since release 2.7.0.
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. Any of
these scenarios caused messages to be requeued at the back of the
queue for RabbitMQ releases earlier than 2.7.0. From RabbitMQ release
2.7.0, messages are always held in the queue in publication order, even in the presence of requeueing or channel closure. (emphasis added)
So, it is clear that RabbitMQ, from 2.7.0 onward, is making a rather drastic improvement over the original AMQP specification with regard to message ordering.
With multiple (parallel) consumers, order of processing cannot be guaranteed.
The third paragraph (pasted in the question) goes on to give a disclaimer, which I will paraphrase: "if you have multiple processors in the queue, there is no longer a guarantee that messages will be processed in order." All they are saying here is that RabbitMQ cannot defy the laws of mathematics.
Consider a line of customers at a bank. This particular bank prides itself on helping customers in the order they came into the bank. Customers line up in a queue, and are served by the next of 3 available tellers.
This morning, it so happened that all three tellers became available at the same time, and the next 3 customers approached. Suddenly, the first of the three tellers became violently ill, and could not finish serving the first customer in the line. By the time this happened, teller 2 had finished with customer 2 and teller 3 had already begun to serve customer 3.
Now, one of two things can happen. (1) The first customer in line can go back to the head of the line or (2) the first customer can pre-empt the third customer, causing that teller to stop working on the third customer and start working on the first. This type of pre-emption logic is not supported by RabbitMQ, nor any other message broker that I'm aware of. In either case, the first customer actually does not end up getting helped first - the second customer does, being lucky enough to get a good, fast teller off the bat. The only way to guarantee customers are helped in order is to have one teller helping customers one at a time, which will cause major customer service issues for the bank.
It is not possible to ensure that messages get handled in order in every possible case, given that you have multiple consumers. It doesn't matter if you have multiple queues, multiple exclusive consumers, different brokers, etc. - there is no way to guarantee a priori that messages are answered in order with multiple consumers. But RabbitMQ will make a best-effort.
Message ordering is preserved in Kafka, but only within partitions rather than globally. If your data need both global ordering and partitions, this does make things difficult. However, if you just need to make sure that all of the same events for the same user, etc... end up in the same partition so that they are properly ordered, you may do so. The producer is in charge of the partition that they write to, so if you are able to logically partition your data this may be preferable.
I think there are two things in this question which are not similar, consumption order and processing order.
Message Queues can -to a degree- give you a guarantee that messages will get consumed in order, they can't, however, give you any guarantees on the order of their processing.
The main difference here is that there are some aspects of message processing which cannot be determined at consumption time, for example:
As mentioned a consumer can fail while processing, here the message's consumption order was correct, however, the consumer failed to process it correctly, which will make it go back to the queue. At this point the consumption order is intact, but the processing order is not.
If by "processing" we mean that the message is now discarded and finished processing completely, then consider the case when your processing time is not linear, in other words processing one message takes longer than the other. For example, if message 3 takes longer to process than usual, then messages 4 and 5 might get consumed and finish processing before message 3 does.
So even if you managed to get the message back to the front of the queue (which by the way violates the consumption order) you still cannot guarantee they will also be processed in order.
If you want to process the messages in order:
Have only 1 consumer instance at all times, or a main consumer and several stand-by consumers.
Or don't use a messaging queue and do the processing in a synchronous blocking method, which might sound bad but in many cases and business requirements it is completely valid and sometimes even mission critical.
There are proper ways to guarantuee the order of messages within RabbitMQ subscriptions.
If you use multiple consumers, they will process the message using a shared ExecutorService. See also ConnectionFactory.setSharedExecutor(...). You could set a Executors.newSingleThreadExecutor().
If you use one Consumer with a single queue, you can bind this queue using multiple bindingKeys (they may have wildcards). The messages will be placed into the queue in the same order that they were received by the message broker.
For example you have a single publisher that publishes messages where the order is important:
try (Connection connection2 = factory.newConnection();
Channel channel2 = connection.createChannel()) {
// publish messages alternating to two different topics
for (int i = 0; i < messageCount; i++) {
final String routingKey = i % 2 == 0 ? routingEven : routingOdd;
channel2.basicPublish(exchange, routingKey, null, ("Hello" + i).getBytes(UTF_8));
}
}
You now might want to receive messages from both topics in a queue in the same order that they were published:
// declare a queue for the consumer
final String queueName = channel.queueDeclare().getQueue();
// we bind to queue with the two different routingKeys
final String routingEven = "even";
final String routingOdd = "odd";
channel.queueBind(queueName, exchange, routingEven);
channel.queueBind(queueName, exchange, routingOdd);
channel.basicConsume(queueName, true, new DefaultConsumer(channel) { ... });
The Consumer will now receive the messages in the order that they were published, regardless of the fact that you used different topics.
There are some good 5-Minute Tutorials in the RabbitMQ documentation that might be helpful:
https://www.rabbitmq.com/tutorials/tutorial-five-java.html
I have a project that involves rabbitmq. The problem that I have is illustrated as follows:
So now, let me describe the scenario. I have n number of queues which subscribed to topic1.
Now my question is if I publish 3 messages in sequence, which are shown as 1, 2 and 3 into broker called Exchange, will rabbitmq Guarantee the sequence of those messages in all queues?
The only thing that I found was in rabbitmq documentation Message ordering guarantees which was taking about
Section 4.7 of the AMQP 0-9-1 core specification explains the conditions under which ordering is guaranteed: messages published in one channel, passing through one exchange and one queue and one outgoing channel will be received in the same order that they were sent. RabbitMQ offers stronger guarantees since release 2.7.0.
So can anyone help me out and point me to the right doc or example that shows whether it is guaranteed or not?
Thanks
As the other poster mentioned, your scenario should work fine assuming a simple/basic consumer setup. But here's some additional info that might explain why.
I wasn't sure quite what nuances might have been wrapped up in that section of the documentation either, until I looked up exactly what a Channel was. A connection to RabbitMQ can have multiple "mini-connections" within it called channels. Each of these channels are independent and thus you could send multiple messages to the broker via multiple channels.
So as long as the messages in your scenario are sent on a single channel (you'd have to explicitly try to use multiple channels), they'll arrive in the queue in the same order you sent them. As long as the messages are consumed via a single channel, they'd arrive on the consumer in the same order they arrived in the queue (also being the same order they were sent).
From: https://www.rabbitmq.com/tutorials/amqp-concepts.html
Some applications need multiple connections to an AMQP broker. However, it is undesirable to keep many TCP connections open at the same time because doing so consumes system resources and makes it more difficult to configure firewalls. AMQP 0-9-1 connections are multiplexed with channels that can be thought of as "lightweight connections that share a single TCP connection".
What you have quoted answers your question perfectly. The only question is what your consumer set up looks like. If you have each queue connected to its own channel and that consumer is running in its own thread, that thread will see each message in order as they were published.