I am new to RabbitMQ and trying to figure out the difference between a broker and an exchange.
From what I've read, the terms seem to almost be used interchangeably and in the diagrams, a broker seems to encompass both the exchange and the queues.
From "RabbitMQ Succinctly" book:
Exchanges are AMQP entities where messages are sent to the message
broker. Exchanges take a message and then route it to one or more
queues
So what is a broker? In the RabbitMQ management there is a tab for "Exchanges", but not for brokers. Can I interact with a broker directly or is this only done by the exchange?
"Broker" is a generic term for the type of messaging system that RabbitMQ is. It's a centralized messaging system, with a server that handles the routing and delivery of messages, etc.
This paper from ZeroMQ is good to understand the differences between brokered and brokerless: http://zeromq.org/whitepapers:brokerless (although this paper is fairly biased toward brokerless model, both are good and both have uses. i tend to prefer rabbitmq / brokered system, but not always)
From the other perspective, here is RabbitMQ's broker vs brokerless post: https://www.rabbitmq.com/blog/2010/09/22/broker-vs-brokerless/
For the most part, just substitute "rabbitmq server" in your mind, when you see the work "broker" and you'll be good to go.
The exchange, as you've noted, the thing through which you publish a message, in RabbitMQ. It handles the bindings and routing of the messages, depending on the exchange type.
A broker stands between producer(s) and consumer(s).
Here is the post-office analogy to understand the components in the Rabbitmq-based messaging system without going into the details.
An exchange is like a parcel-delivery man. A queue is the recipient of the parcel. A producer is the sender of the parcel. A set of rules that an exchange follows to deliver a parcel (that is, the message) to a queue are called "bindings". The routing key and/or the header are like the address on the parcel. The exchange determines which queue a message goes to based on the routing key/header. The producer sends messages to the exchange, not to the queue.
This entire business of accepting messages from the producers and delivering them to the consumers is what the "broker" does.
Rabbitmq is the implementation of AMQP protocol (an application-layer protocol) which is an asynchronous message-brokerage middleware.
Related
I want to create a UI to see all the messages that are flowing through all exchanges in RabbitMQ server (of course other than the management console).
I am also using Mass Transit over rabbit but i am not sure if this matters.
Is this at all possible without having to code a consumer for each one of them one by one? If yes, any starting points?
The message exchanges used for publishing, as well as sending, are all bound to an exchange that has the same name as the queue for message delivery. So you could bind your own wire tap exchange on the broker to any queue exchange, and wiretap the messages to another queue of your choosing.
You can view the RabbitMQ topology layout in the documentation.
It was specifically done this way to make it easy to wiretap any endpoint, since all messages flow through a single fanout exchange.
This is a pretty broad question because it's not entirely obvious what you mean by "see", but regardless, you could create an observer on your bus. It's documented here and I think it's fairly straightforward: https://masstransit-project.com/MassTransit/usage/observers.html
In the observer you can handle various events when any message hits the MT message bus, and perform some kind of operation (like print the message, add logging, metrics, etc). If you have a microservice scenario it might be a good idea to add an observer to your shared library and add it to the bus in your individual applications.
I am integrating several .Net modules using pub/sub messaging using RabbitMQ and MassTransit. Most of the message subscription shall be durable. But some shall be transient. When a consumer dies the messages shall not be stored and already queued messages shall be discarded.
In each module I create 1 bus with 2 receive endpoints. One is configured as durable and non-auto-delete. The other one is configured as non-durable and auto-delete. Each gets its own set of consumers. This works as expected.
Now I am trying to implement request/response messages. Here comes the problem because now the sender has to decide to which exchange to route to. And that is wrong as I want receiver to decide whether to use durable or transient queue.
My questions:
Is there a better way how to support durable and transient subscription at the same time?
Why is MassTransit binding message exchange to an endpoint exchange that is bound to an endpoint queue? Why cannot the message exchange be directly bound to the endpoint queue?
Lets assume that all request consumers in one module are either durable or transient. Is it possible to declare one "module"-exchange which is then bound to either durable or transient queue? So the sender addresses the module exchange and module decides to which queue to bind. How to convince MassTransit to do so?
A module is using durable subscriptions that survive through restarts of module and also broker. After some time, admin (so in run-time of the system) decides to disconnect this module from the system. Can the module somehow unsubscribe everything and let MassTransit to remove the durable exchanges and queues?
Your question starts with request/response sent to an unknown endpoint, and ends with removing exchanges. These are different things, I suppose.
I cannot answer point-by-point, just will try to clear up things.
Request/response by definition requires you to know where you send stuff. As per MassTransit convention, the endpoint address is always an exchange/queue pair address. Therefore, you cannot let receiver decide who will handle this message, it will be delivered to the exchange/queue of the endpoint where you send it to.
About the "unsubscribe" - MassTransit deletes nothing. You have to clean up the binding that is not being used manually or by using the management API.
In discussing the differences between Kafka and RabbitMQ, "dumb broker" and "smart broker" keeps popping up in their interactions with consumers. Kafka is described as having a dumb broker while RabbitMQ is said to have a smart broker/dumb consumer model.
What exactly does this mean? I'm familiar with the basics of Kafka and a little bit more about RabbitMQ. However, what features of RabbitMQ makes the broker smarter than Kafka's?
This is a question that bothered me for sometime too :) Here's what I have understood so far...
In the case of RabbitMQ the broker makes sure the messages are delivered to the consumers and dequeue them only when it gets acknowledgement from all the consumers that need that message. It also keeps track of consumer state.
Kafka does not keep track of "which messages were read by consumers". The Kafka broker keeps all messages in queues for a fixed duration of time and it's the responsibility of the consumer to read it from the queue. It also does not have this overhead operation of keeping track of consumer state.
You can read more about it in this awesome Pivotal blog post comparing RabbitMQ and Kafka.
The point about Kafka using a dumb broker while Rabbit MQ using a smart broker is one of the points used while deciding which Messaging System to use. Since RabbitMQ is a smart broker implementing global startegies for retry is far easier and listener agnostic than in Kafka.
Given a set of microservices accessed through an API gateway I believe that the above point, combined with the advantages of Rabbit MQ being much more maintainable and the knowledge that the data passed across microservices will never amount to the same load as that of streaming data, makes Rabbit MQ a far better choice than Kafka for Inter Service Communication
Dumb vs Smart broker means that the Broker can be smart to route messages based on certain conditions.
In the case of RabbitMQ, producer sends message to Exchange and Exchange routes the message to Queue. Here "Exchange" does the routing and thats what they call as Smart broker. Again people have made Brokers really smart and ended up with ESB which we all know what happened and Industry is moving away from Bloated ESB's.
In the case of Kafka, broker doesn't route messages. It is up to the user to create topics, producers partition the events into topic-partitions, and consumer groups and decide which consumer groups listens to which topic.
Smart vs Dumb broker has nothing to do with Message acknowledgment. In case of RabbitMQ, it tracks the status of each message to see whether it is consumer or not. In the case of Kafka, it happens but differently by using offsets on partitions and offset is stored in Kafka itself ( consumer can also store). But both provide the functionality.
I have been learning AMQP using rabbitMQ and I came across this concept called fanout exchanges. From the illustration diagram, all I could see is that it's some kind of load balancer. Could anyone please explain what is it's actual purpose?
I assume that you mean that only one queue will get a message once it arrives to fanout exchange. So from that point of view:
No, I don't think its a load-balancer (I admit that terminology can be confusing).
In Rabbit MQ there are different types of exchanges, its true and fanout exchange is only one type of them. The basic model of Rabbit MQ assumes that you can connect as many queues as you want to the same exchange. Now, all the queues that are connected to the exchange will get the message (Rabbit MQ just replicates the message) - so exchange can't act as a load balancer.
The only difference between the exchange types is an algorithm of matching routing key. It's like a "to" field in a regular envelope. When a message arrives to exchange, it checks the routing key (a.k.a. binding) and depending on type of exchange "finds" to which queue the message should be routed.
When queue gets registered to exchange it always uses this binding. It like queue says to the binding "hey, all messages which are supposed to arrive to John Smith (its a routing key), please pass them to me". Then when the message arrives, it always has a "to" field in the envelope - so exchange checks whether the message is intended to be sent to John Smith, and if so - just routes it to the queue.
It's possible that there will be many queues interested to get a message from John Smith, in this case the message will be replicated. As for fanout exchange - it just doesn't pay any attention to the routing key and instead just sends the message to all the connected queues.
Now, there is another abstraction called consumer. Consumers can be connected to the single queue (again, many consumers can be connected to the queue).
The trick is that only one consumer can get the message for processing at a time.
So if you want a load balancer - you can use a single queue, connected to your exchange (it can be fanout of course), but then connect many consumers to that queue, and rabbit will send the message to the first consumer (it uses round robin internally to pick the first consumer) - if the consumer can't handle it, the message will be re-queued and rabbit will attempt to send it to another consumer.
I need to build a system that uses a Publish/Subscribe bus (e.g. Mule, ZeroMQ, RabbitMQ), but the literature all implies that subscriber applications are reliably available to receive messages from topics to which they subscribe as soon as the Pub/Sub bus is able to deliver the message.
I have a system where some of the applications will be reliably connected to the Publish/Subscribe bus, but other applications will not be active or connected to the bus all the time.
The obvious solution is to have some sort of "presence" protocol between the unreliable application and the Publish/Subscribe bus so that "present" applications get their messages delivered immediately, and "not present" applications have their messages queued up in a persistent buffer of some kind, and as soon as they complete the "presence handshake", the queued messages are delivered to the newly present application.
Are there any Publish/Subscribe buses which have this kind of feature built in, or are there any open-source add-ons which do this? Can you point me to any URLs which describe this?
You can achieve this behaviour quite easily with any AMQP-compliant broker (such as RabbitMQ).
Choose the correct exchange type for your usage model. You'll want to use a direct exchange if you're always sending to absolutely named destinations, something like chat.messages.
If you want to do pattern-based routing, you'll want to use topic exchange. Then you can route based on patterns such a chat.messages.*.
Routing is described in more detail in the RabbitMQ Tutorials.
To create the kind of persistent subscription that you mention, have each subscriber create a queue that is private to that subscriber. The queue is then bound to the relevant routing keys on your chosen exchange.
Since each subscriber has its own queue, messages will be consumed by the subscriber when active and stored when subscriber is inactive or disconnected.
You haven't mentioned your language of choice, but in Java you can accomplish this with JMS using durable subscribers. Any implementation of JMS (there are many, including the aforementioned RabbitMQ) will support this feature.