Can there be a scenario wherein one can use the synchronous Pub-Sub model to build a system? Generally, they are used to achieve asynchronous behavior wherein the producer publishes messages on the queue and gets an acknowledgment and once the consumer consumes the message and processes it. Does anybody like Kafka, Rabbit-MQ provide this use case.
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Are Kafka stream/redis stream good for reactive architecture?
I am asking this mainly because both redis and kafka seem to be blocking threads when consuming the messages.
Are there any reasons behind this? I was hoping that I could read messages with some callback - so execute when the message was delivered like pub/sub in a reactive manner. Not by blocking thread.
Kafka client is relatively low level, what is "good" as in: it provides you with much flexibility when (and in which thread) you'd do the record processing. In the end, to receive a record, someone needs to block (as actual reading is sending fetch requests over and over). Whether the thread being blocked is a "main-business" thread or some side-i/o dedicated one will depend on developer's choice.
You might take a look at higher-level offerings such as Spring-Kafka or Kafka Stream API / Kafka Connect frameworks that provide "fatter" inversion-of-control containers, effectively answering the above concern.
Currently learning about Kafka architecture and I'm confused as to why the consumer polls the broker. Why wouldn't the consumer simply subscribe to the broker and supply some callback information and wait for the broker to get a record? Then when the broker gets a relevant record, look up who needs to know about it and look at the callback information to dispatch the messages? This would reduce the number of network operations hugely.
Kafka can be used as a messaging service, but it is not the only possible usecase. You could also treat it as a remote file that can have bytes (records) read on demand.
Also, if notification mechanism were to implemented in message-broker fashion as you suggest, you'd need to handle slow consumers. Kafka leaves all control to consumers, allowing them to consume at their own speed.
I'm looking for a pattern and existing implementations for my situation:
I have synchronous soa which uses REST APIs, in fact I implement remote procedure call with REST. And I have got some slow workers which process requests for substantial time (around 30 seconds) and due to some license constraints for some requests I can process them only sequentially (see system setup).
What are recommended ways to implement communication for such case?
How can I mix synchronous and asynchronous communication in the situation when the consumer is behind firewall and I cannot send him notification about completed tasks easily and I might not be able to let consumer use my message broker if I have one?
Workers are implemented in Python using Flask and gunicorn. At the moment I'm using synchronous REST interfaces and allow for the delay as I only had fast workers. I looked at Kafka and RabbitMq and they would fit for backend side
communication, however how does producer communicate with the consumer?
If the consumer fires API request my producer can return code 202, then how shall producer notify consumer that result is available? Will consumer have to poll the producer for results?
Also if I use message brokers and my gateway acts on behalf of consumer, it should have a registry of requests (I already have GUID for every request now) and results, which approach would you recommend for implementing it?
Producer- agent which produces the message
Consumer - agent which can handle the message and implement the logic for processing the message
In our scenario we have a set of micro services which interact with other services by sending event messages. We anticipate millions of messages per day at the peak. Every message is targeted to one or more listener types.
Our requirements are as follows:
Zero lost messages.
Ability to dynamically register multiple listeners of a specific
type in order to increase throughput.
Listeners are not guaranteed to be alive when messages are
dispatched.
We consider 2 options:
Send each message to JMS main queue then listeners of that queue will route the messages to specific queues according to message content, and then target services will listen to those specific queues.
Send messages to a Kafka topic by message type then target services will subscribe to the relevant topic and consume the messages.
What are the cons and pros for using either JMS or Kafka for that purpose?
Your first requirement is "zero lost messages". However, if you want publish-subscribe semantics (i.e. topics in JMS), but listeners are not guaranteed to be alive when messages are dispatched then JMS is a non-starter as those messages will simply be discarded (i.e. lost).
I would suggest to go with Kafka as it has fault tolerance mechanism and even if some message lost or not captured by any listener you can easily retrieve it from Kafka cluster.
Along with this you can easily add new listener / listener in group and kafka along with zookeeper will take care of managing it very well.
In summary, Kafka is a distributed publish-subscribe messaging system that is designed to be fast, scalable, and durable. Like many publish-subscribe messaging systems, Kafkamaintains feeds of messages in topics. Producers write data to topics and consumers read from topics.
Very easy for integration.
With RabbitMQ, is there a way to "push" messages from a queue TO a consumer as opposed to having a consumer "poll and pull" messages FROM a queue?
This has been the cause of some debate on a current project i'm on. The argument from one side is that having consumers (i.e. a windows service) "poll" a queue until a new message arrives is somewhat inefficient and less desirable than the idea having the message "pushed" automatically from the queue to the subscriber(s)/consumer(s).
I can only seem to find information supporting the idea of consumers "polling and pulling" messages off of a queue (e.g. using a windows service to poll the queue for new messages). There isn't much information on the idea of "pushing" messages to a consumer/subscriber...
Having the server push messages to the client is one of the two ways to get messages to the client, and the preferred way for most applications. This is known as consuming messages via a subscription.
The client is connected. (The AMQP/RabbitMQ/most messaging systems model is that the client is always connected - except for network interruptions, of course.)
You use the client API to arrange that your channel consume messages by supplying a callback method. Then whenever a message is available the server sends it to the client over the channel and the client application gets it via an asynchronous callback (typically one thread per channel). You can set the "prefetch count" on the channel which controls the amount of pipelining your client can do over that channel. (For further parallelism an application can have multiple channels running over one connection, which is a common design that serves various purposes.)
The alternative is for the client to poll for messages one at a time, over the channel, via a get method.
You "push" messages from Producer to Exchange.
https://www.rabbitmq.com/tutorials/tutorial-three-python.html
BTW this is fitting very well IoT scenarios. Devices produce messages and sends them to an exchange. Queue is handling persistence, FIFO and other features, as well as delivery of messages to subscribers.
And, by the way, you never "Poll" the queue. Instead, you always subscribe to publisher. Similar to observer pattern. Generally, I would say genius principle.
So it is similar to post box or post office, except it sends you a notification when message is available.
Quoting from the docs here:
AMQP brokers either deliver messages to consumers subscribed to
queues, or consumers fetch/pull messages from queues on demand.
And from here:
Storing messages in queues is useless unless applications can consume
them. In the AMQP 0-9-1 Model, there are two ways for applications to
do this:
Have messages delivered to them ("push API")
Fetch messages as needed ("pull API")
With the "push API", applications have to indicate interest in
consuming messages from a particular queue. When they do so, we say
that they register a consumer or, simply put, subscribe to a queue. It
is possible to have more than one consumer per queue or to register an
exclusive consumer (excludes all other consumers from the queue while
it is consuming).
Each consumer (subscription) has an identifier called a consumer tag.
It can be used to unsubscribe from messages. Consumer tags are just
strings.
RabbitMQ broker is like server that wont send data to consumer without consumer client getting registering itself to server. but then question comes like below
Can RabbitMQ keep client consumer details and connect to client when packet comes?
Answer is no. so what is alternative well then write plugin by yourself that maintain client information in some kind of config. Plugin will pull from RabbitMQ Queue and push to client.
Please give look at this plugin might help.
https://www.rabbitmq.com/shovel.html
Frankly speaking Client need to implement AMQP protocol to receive so and should listen connection on some port for that. This sound like another server.
Regards,
Vishal