I have a (RabbitMQ) exchange with different queues, each queue containing messages in which the order is important.
I want to select and process the earliest declared queue and progress in the order of creation. Like the exchange functioning as another queue.
How do I achieve this in AMQP?
Solved by creating a new queue for storing the unique identifiers of other queues. Since it's FIFO, the items are placed to the queue in order. For cases when we push the currently processed queue, we simply push the same identifier again - so if not all messages got processed, their processing get queued for later.
Related
I have been exploring EventStoreDB and trying to understand more about the ordering of messages on the consumer side. Read about persistent subscriptions and also the Pinned consumer strategy here.
I have a scenario wherein inventory updates get pushed to eventstore and different streams get created by the different unique inventoryIds in the inventory event.
We have multiple consumers with the same consumerGroup name to read these inventory events. We are using Pinned Persistent Subscription with ResolveLinkTos enabled.
My question:
Will every message from a particular stream always go to the same consumer instance of the consumerGroup?
If the answer to the above question is yes, will every message from that particular stream reach the particular consumer instance in the same order as the events were ingested?
The documentation has a warning that ordered message processing using persistent subscriptions is not guaranteed. Any strategy delivers messages with the best-effort level of ordering guarantees, if applicable.
There are a few reasons for this, some of those are:
Spreading out messages across consumer groups lead to a non-linearised checkpoint commit. It means that some messages can be processed before other messages.
Persistent subscriptions attempt to buffer messages, but when a timeout happens on the client side, the whole buffer is redelivered, which can eventually break the processing order
Built-in retry policies essentially can break the message order at any time
Most event log-based brokers, if not all, don't even attempt to guarantee ordered message delivery across multiple consumers. I often hear "but Kafka does it", ignoring the fact that Kafka delivers messages from one partition to at most one consumer in a group. There's no load balancing of one partition between multiple consumers due to exactly the same issue. That being said, EventStoreDB is still not a broker, but a database for events.
So, here are the answers:
Will every message from a particular stream always go to the same consumer instance of the consumer group?
No. It might work most of the time, but it will eventually break.
will every message from that particular stream reach the particular consumer instance in the same order as the events were ingested?
Most of the time, yes, but again, if a message is being retried, you might get the next message before the previous one is Acked.
Overall, load-balancing ordered processing of messages, which aren't pre-partitioned on the server is not an easy task. At most, you get messages re-delivered if the checkpoint fails to persist at some point, and the consumers restart.
I'm trying to configure a queue that is aware of the events that are being processed.
Questions
Does this make sense? :)
Is it possible to configure/customize ActiveMQ?
Are there any other library that can be "easily" configured to handle such cases? Kafka?
Problem
The queue contains events. Each event is associated with an object. A consumer takes the event from the queue and performs a task. Each event should be taken only by exactly one consumer.
Constraints
Events for the same object cannot be processed concurrently.
But events for different objects should be processed in parallel.
Example
The queue is
ObjectA-Event1
ObjectA-Event2
ObjectB-Event1
ObjectC-Event1
The Consumer1 should receive ObjectA-Event1 from the queue. The Consumer2 should receive ObjectB-Event1 from the queue and not the ObjectA-Event2. The ObjectA-Event2 should be available for consumers only when the first consumer completes the task for the ObjectA-Event1.
It looks to me like you should use message groups. Messages for each object should be in the same group so that they are received by the same consumer and processed serially. Messages in different groups are free to be processed by different consumers.
I have implemented the example from the RabbitMQ website:
RabbitMQ Example
I have expanded it to have an application with a button to send a message.
Now I started two consumer on two different computers.
When I send the message the first message is sent to computer1, then the second message is sent to computer2, the thrid to computer1 and so on.
Why is this, and how can I change the behavior to send each message to each consumer?
Why is this
As noted by Yazan, messages are consumed from a single queue in a round-robin manner. The behavior your are seeing is by design, making it easy to scale up the number of consumers for a given queue.
how can I change the behavior to send each message to each consumer?
To have each consumer receive the same message, you need to create a queue for each consumer and deliver the same message to each queue.
The easiest way to do this is to use a fanout exchange. This will send every message to every queue that is bound to the exchange, completely ignoring the routing key.
If you need more control over the routing, you can use a topic or direct exchange and manage the routing keys.
Whatever type of exchange you choose, though, you will need to have a queue per consumer and have each message routed to each queue.
you can't it's controlled by the server check Round-robin dispatching section
It decides which consumer turn is. i'm not sure if there is a set of algorithms you can pick from, but at the end server will control this (i think round robin algorithm is default)
unless you want to use routing keys and exchanges
I would see this more as a design question. Ideally, producers should create the exchanges and the consumers create the queues and each consumer can create its own queue and hook it up to an exchange. This makes sure every consumer gets its message with its private queue.
What youre doing is essentially 'worker queues' model which is used to distribute tasks among worker nodes. Since each task needs to be performed only once, the message is sent to only one node. If you want to send a message to all the nodes, you need a different model called 'pub-sub' where each message is broadcasted to all the subscribers. The following link shows a simple pub-sub tutorial
https://www.rabbitmq.com/tutorials/tutorial-three-python.html
My system has a constrain for specific consumer that messages should be handled in order, one after the other. To implement that we set the concurrency to 1.
Now we want to scale out and add more instance of this consumer.
To keep the order I want to use distributed lock manager like 'RedLock'. It can tell each consumer if it is OK to fetch the next message.
I work with RabbitMq and my question is if there is kind of observer event that comes before getting messages from the queue. In other words I need a way to enable/disable the operation of polling messages from the queue.
I have a MDB which listens to WebSphere MQ. It does not picks up the messages in the order that has been received by the Queue. How can i make it read it in that order? Is it possible? Should i not use a MDB.
In general, WMQ delivers messages in the order that they were received. However, several things can impact that...
If the queue is set to priority instead of FIFO delivery and messages arrive in different priorities, they will be delivered "out of order".
Distinguish between order produced and order delivered. If the messages are produced on a remote QMgr and there are multiple paths to the local QMgr, messages may arrive out of order.
Difference in persistence - if messages are produced on a remote QMgr and are of different persistences, the non-persistent messages may arrive faster than the persistent ones, especially with channel NPMSPEED(FAST) set.
Multiple readers/writers - Any dependency on sequence implies a single producer sending to a single consumer over a single path. Any redundancy in producers, consumers or paths between them can result in messages delivered out of sequence.
Syncpoint - To preserve sequence, ALL messages must be written and consumed under syncpoint or else ALL must be written and consumed outside of syncpoint.
Selectors - These specifically are intended to deliver messages out of order with respect to the context of all messages in the queue.
Message groups - Retrieval of grouped messages typically waits until the entire group is present. If groups are interleaved, messages are delivered out of sequence.
DLQ - if the target queue fills, messages may be delivered to the DLQ. As the target queue is drained, messages start going back there. With a queue near capacity, messages can alternate between the target queue and DLQ.
So when an MDB is receiving messages out of order any of these things, or even several of them in combination, may be at cause. Either eliminate the dependency on message sequence (best choice) or else go back over the design and reconcile all the factors that may lead to out-of-sequence processing.
To add to T.Rob's list, MDBs use the application server WorkManager to schedule message delivery, so message order is also dependent on the order in which the WorkManager starts Work items. This is outside the control of WMQ. If you limit the MDB ServerSessionPool depth to one, then this limit is removed as there will only ever be one in-flight Work instance, but at the cost of reducing maximum throughput.
If you're running in WebSphere application server, then non-ASF mode with ListenerPorts can preserve message order subject to some transactional/backout caveats. There's a support technote here:
http://www-01.ibm.com/support/docview.wss?uid=swg21446463