I have trouble understanding the routing in RabbitMQ. Consider I have several producers (let call them clients) that produce messages to the queue. E.g., clients A, B, and C send messages to queue X1.
Let the consumer respond to all messages sending responses back to the queue. E.g., consumer gets message from queue X1, does something, and sends responses to the queue X1.
How can, client A determine where are in the queue X1 messages sent to it and where are messages sent to clients B or C?
I can't declare one queue per connection because of large number of connections expected (~10^6). So I'm in trouble here. Any suggestions? Thanks.
I think you need to look at the RPC tutorial. From your description it sounds like that is what you want to do. However that would probably require you to declare more queues than you want.
Approaching this a different way. I cannot understand why you would send a reply back to the producer not only by the same exchange but the same queue that the consumers are consuming from.
Would it not make sense to have producers P1,P2 and P3 send to exchange X1 with routing key "abc.aaa.xyz" / "abc.bbb.xyz" / "abc.ccc.xyz". Then have queues Q1, Q2 and Q3 bound to X1 with binding keys ".aaa." / ".bbb." / ".ccc." or just Q1 with binding key "abc.*.xyz" (I am unclear on exactly what you want so just making some suggestions). Which are consumed by Consumers C1, C2 and C3
When the Consumer has finished processing the message then it will send a message to X2, with routing key that identifies itself. The producers will consume from queues bound to X2.
The point I am trying to make is that you do not want more than one consumer reading from a queue. There is only one case in which you want that and that is a task queue. I am not clear on your use case so you may want a task queue. If you do then you should still not have your producers reading from the same task queue as your consumers. Aside from task queues you should have one consumer read from one queue. You may have many queues to one exchange and even many bindings from one queue to one exchange.
I hope this helps
Related
Is there is a way by which we can restrict RabbitMQ Queue to dispatch only a fixed number of messages from the Queue to the consumers?
I have 2 Queues Q1 and Q2 and 10 consumers.Every consumer can process the messages from Q1 and Q2.At any given time, only 2 consumers should process messages from Q2.All the 10 consumers can process message from Q1 simultaneously.
Is there any configuration in RabbitMQ which we can specify, so that RabbitMQ pushes only 2 messages from Q2 to any free consumer and push the next 2 only after they are acknowledged, even though other consumers are free and ready to consume.
More background on the issue:
Why only process 2 messages at a time ? :
Q2 messages are doing a web service call and the web service end point(third party) can only service 2 messages concurrently.
Cant we use concurrency ? :
If we use a ListenerContainer (Spring AMQP) the container is per consumer. We can restrict how many message one consumer can take at a time, but when we have 10 consumers, if there are messages in the Queue, each consumer will get its share.
Can we configure only 2 consumers listening to Q2 ? :
I understand we can achieve this by configuring only 2 consumers for Q2, but I am trying to avoid that. If for some reason these 2 consumers goes down, the processing of Q2 will be halted. If 10 consumers are configured, we can guarantee the processing will happen until the last consumer is down.
Looking to see if there is some config in RabbitMQ which we can make use of or any suggested solution.
Thanks in advance !
I'm pretty sure that consumer prefetch will accomplish what you want. But, Q2 can only have one consumer for this to work. There is no way to coordinate among multiple consumers - you would have to do that yourself, and could use RabbitMQ to do the coordination.
NOTE: the RabbitMQ team monitors the rabbitmq-users mailing list and only sometimes answers questions on StackOverflow.
I think you're getting wrapped up in the problem definition. What you really need is trivial, so let's break this down a bit.
Given two queues, Q1 and Q2
10 consumers
Every consumer can process the messages from Q1 and Q2.
At any given time, only 2 consumers should process messages from Q2.
All the 10 consumers can process message from Q1 simultaneously.
Comments on problem statement
First, queues are assumed to be indepenedent. An independent process P will have queue Q, thus Q1 serves process P1. This is a strict mathematical requirement - you cannot define two queues for a single process P.
Thus, the second constraint is mathematically incorrect, for the same reason that you could not write a valid function that accepts a parameter of type string and bool interchangeably. It must accept one or the other, as they are not compatible types, or it must accept a single common ancestor of the types without regard to the subtypes. This is a variant of the Liskov Substitution Principle.
Redefining the problem
There are a total of 12 consumers in the system:
Q1 has 10 consumers
Q2 has 2 consumers
[Important] Consumers are not shared between queues
Is there any configuration in RabbitMQ which we can specify, so that RabbitMQ pushes only 2 messages from Q2 to any free consumer and push the next 2 only after they are acknowledged, even though other consumers are free and ready to consume.
Based on the new definition of the problem, you have two options:
Use a Basic.Get - pull the next message from the queue as soon as the consumer finishes processing the last message.
Use consumer prefetch with limit 1. This will deliver the first and second messages for each consumer immediately, then deliver additional messages one at a time as the next message for that consumer is acknowledged. This is a bit more complicated, but might make sense if your latency margins are less than 10 milliseconds.
Note that by properly defining the problem space, we have eliminated the fundamental problem of trying to figure out how to ensure only two consumers are processing Q2 messages at any time.
try the new feature Single Active Consumer from version 3.8+.
Single active consumer allows to have only one consumer at a time consuming from a queue and to fail over to another registered consumer in case the active one is cancelled or dies.
Consuming with only one consumer is useful when messages must be consumed and processed in the same order they arrive in the queue.
Single active consumer can be enabled when declaring a queue, with the x-single-active-consumer argument set to true
https://www.rabbitmq.com/consumers.html#single-active-consumer
e.g. with the Java client:
I have the following scenario:
One Producer service
A dynamic amount of consumers services
Messages contain tasks with a specific product, so once consumer x handles a message of product y. In the future x should handle all messages of product y. Ideally the producer service should send all messages of product x on a queue which only consumer x reads from.
In order to divide workload evenly, there should be a way that once a new product needs to be managed, that next available consumer takes it.(I suppose a queue which all consumers are reading from)
My approach:
An exchange send new product jobs in a "newProduct" queue to which all the consumers are consuming from.
The consumer y that reads such a message notifies to the producer service (on a separate queue) that he is now in charge of product x.
The producer then sends all messages for product x to a queue proper to consumer y.
When a new consumer service z goes online, it notifies the producer service on a therefore specific queue that he is online such that the producer can create a binding in the exchange for z's proper queue.
Questions:
is my approach a good way to solve the problem, or am I missing rabbitmq solutions that would solve the problem in a less complicated way ?
How do I add a new queue during runtime to the exchange ?
An exchange send new product jobs in a "newProduct" queue to which all
the consumers are consuming from.
This looks good to me.
The consumer y that reads such a message notifies to the producer
service (on a separate queue) that he is now in charge of product x.
This is also fine, I guess if producer did not receive notification
that product X is taken care of it will need to do something. The
producer then sends all messages for product x to a queue proper to
consumer y.
I'd send all messages for product X with the same routing key, like product-X. Which is what you probably mean here. I'd avoid telling producer who exactly handles the product-X now. For better separation of concerns and simplicity producers should know as less as possible about consumers and their queues and vice versa.
When a new consumer service z goes online, it notifies the producer
service on a therefore specific queue that he is online such that the
producer can create a binding in the exchange for z's proper queue.
You could do it this way, but I'd do it differently:
When consumer goes online, it will create needed queues (or subscribe to existing queues) by itself.
I see it like this:
Consumer comes online and subscribes to newProduct queue.
When received a message to handle product Z:
Creates a new queue for itself with binding key product-Z
Notifies producer that product Z is now being handled
Producer starts to send messages with routing key product-Z and they end up in Consumer's queue.
Make sure your consumer has some High Availability, otherwise you may end up in the situation when your consumer started to handle some of the messages and then gone dead, while producer is continuing to send messages for now unhandled product.
I want to create a consumer that process messages from multiple variable number of sources, that are connected or disconnected dynamically.
What I need is that each consumer prioritize first N messages of each source. Then to run multiple consumers to improve the speed.
I have been reading docs for Work queues, Routing and Topics, and a lot of other docs without identifying how to implement this. Also I made some tests without luck.
Can someone point me how to do it or where to read about it?
--EDIT--
QueueA-----A3--A2--A1-┐
QueueB-----B3--B2--B1-┼------ Consumer
QueueC-----C3--C2--C1-┘
The desired effect is that each consumer gets first messages of each queue. For example: A1, B1, C1, A2, B2, C2, A3, B3, C3, and so on. If a new queue is created (QueueD), the consumer would start receiving messages from it in the same fashion.
Thanks in advance
What I need is that each consumer prioritize first N messages of each source. Then to run multiple consumers to improve the speed.
All message queues that I know of only provide ordering guarantees within the queue itself (Kafka provides ordering guarantee not at queue level but within the partitions within queues). However, here you are asking to serialize multiple queues. Which will not be possible in a distributed system context.
Why? because if you have more than one consumers to these queues, messages will be delivered to each connected consumers of a queue in a round robin fashion.
Assuming a prefetch_count=1 and with two connected consumers, say first set of messages delivered as follows:
A1, B1 & C1 delivered to consumer 1 (X)
A2, B2 & C2 delivered to consumer 2 (Y)
Now, in a distributed system, everything is async, and things could go wrong. For example:
If X acks A1, A3 will be delivered to X. But if Y acks A2 before X, A3 will be delivered to Y.
Who acks first is not within your control in a distributed system. Consider following scenarios:
X might had to wait for I/O or CPU bound task, while Y might got lucky that it doesn't had to wait. Then Y will advance through the messages in queue.
Or Y got killed (a partition) or n/w got slow, then X will continue consuming the queue.
I'll strongly advice you to re-think your requirements, and consider your expected guarantees in an async context (you wouldn't be considering a MoM otherwise, would you?).
PS: it is possible to implement what you are asking for with some consumer side logic (with a penalty on performance/throughput).
A single consumer has to connect to all queues
wait for messages from every queue before Ack'ing the messages.
Once a message from every queue is received, group them as a single message and publish to another queue (P).
Now many consumers could be subscribed to P to process the ordered group of messages.
I do not advise it, but hey, it is your system, who is going to stop you ;)
I know that achieving round-robin behaviour in a topic exchange can be tricky or impossible so my question in fact is if there is anything I can make out of RabbitMQ or look away to other message queues that support that.
Here's a detailed explanation of my application requirements:
There will be one producer, let's call it P
There (potentially) will be thousands of consumers, let's call them Cn
Each consumer can "subscribe" to 1 or more topic exchange and multiple consumers can be subscribed to the same topic
Every message published into the topic should be consumed by only ONE consumer
Use case #1
Assume:
Topics
foo.bar
foo.baz
Consumers
Consumer C1 is subscribed to topic #
Consumer C2 is subscribed to topic foo.*
Consumer C3 is subscribed to topic *.bar
Producer P publishes the following messages:
publish foo.qux: C1 and C2 can potentially consume this message but only one receives it
publish foo.bar: C1, C2 and C3 can potentially consume this message but only one receives it
Note
Unfortunately I can't have a separate queue for each "topic" therefore using the Direct Exchange doesn't work since the number of topic combinations can be huge (tens of thousands)
From what I've read, there is no out-of-the box solution with RabbitMQ. Does anybody know a workaround or there's another message queue solution that would support this, ex. Kafka, Kinesis etc.
Thank you
There appears to be a conflation of the role of the exchange, which is to route messages, and the queue, which is to provide a holding place for messages waiting to be processed. Funneling messages into one or more queues is the job of the exchange, while funneling messages from the queue into multiple consumers is the job of the queue. Round robin only comes into play for the latter.
Fundamentally, a topic exchange operates by duplicating messages, one for each queue matching the topic published with the message. Therefore, any expectation of round-robin behavior would be a mistake, as it goes against the very definition of the topic exchange.
All this does is to establish that, by definition, the scenario presented in the question does not make sense. That does not mean the desired behavior is impossible, but the terms and topology may need some clarifying adjustments.
Let's take a step back and look at the described lifetime for one message: It is produced by exactly one producer and consumed by one of many consumers. Ordinarily, that is the scenario addressed by a direct exchange. The complicating factor in this is that your consumers are selective about what types of messages they will consume (or, to put it another way, your producer is not consistent about what types of messages it produces).
Ordinarily in message-oriented processing, a single message type corresponds to a single consumer type. Therefore, each different type of message would get its own corresponding queue. However, based on the description given in this question, a single message type might correspond to multiple different consumer types. One issue I have is the following statement:
Unfortunately I can't have a separate queue for each "topic"
On its face, that statement makes no sense, because what it really says is that you have arbitrarily many (in fact, an unknown number of) message types; if that were the case, then how would you be able to write code to process them?
So, ignoring that statement for a bit, we are led to two possibilities with RabbitMQ out of the box:
Use a direct exchange and publish your messages using the type of message as a routing key. Then, have your various consumers subscribe to only the message types that they can process. This is the most common message processing pattern.
Use a topic exchange, as you have, and come up with some sort of external de-duplication logic (perhaps memcached), where messages are checked against it and discarded if another consumer has started to process it.
Now, neither of these deals explicitly with the round-robin requirement. Since it was not explained why or how this was important, it is assumed that it can be ignored. If not, further definition of the problem space is required.
I have a scenario in my RabbitMQ setup that I'm curious about how to solve. The diagram below illustrates it (exchanges and most queues removed for succinctness):
Scenario
Producer creates message A(1), it is received by the top consumer, which begins processing the message.
Producer creates message A(2), it is received by the bottom consumer (assuming both consumers are on a round-robin exchange).
The bottom consumer publishes message B(2), which is put into Message B consumer's queue
The poor slow top consumer finally finishes and emits its message B(1).
Problem
If we assume that B consumer cannot be made idempotent, how do we ensure the result of both B messages are applied in the correct order?
I had thought of using a timestamp that is applied to the initial publish of message A, and having the consumer maintain a timestamp of last change, rejecting any timestamps before that time, but that only works if each message causes the exact same kind of change and requires a lot of tracking.
Other ideas for how to approach this would be appreciated. Thanks!
I am not sure what is specific to RabbitMQ here, but the idea with timestamps sounds like a good start if you have a single producer.
The producer attaches a timestamp to the messages A, each message B take the same timestamp of its respective message A.
With your approach some messages would not be processed, eg, message B(1). If all messages should be processed by consumer B, but they should be processed in a deterministic order, then you can do a deterministic merge:
Consumer B is equipped with two queues, one queue for each consumer A. Consumer B always checks the top of both queues:
if both queues are non-empty, consumer B pops the message with the lowest timestamp.
if at least one queue is empty, the consumer B waits.
With this approach the order in which consumer B processes messages is given by the timestamps of the producer and no message is discarded. Assumptions are:
queues are FIFO
no process crashes
always the case that eventually each consumer A processes a message
consumer B can check the top of the queues in a non-blocking fashion