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Recently I am trying to use MassTransit in our microservice ecosystem.

According to MassTransit vocabulary and from documents my understanding is :

  • Publish: Sends a message to 1 or many subscribers (Pub/Sub Pattern) to propagate the message.
  • Send: Used to send messages in fire and forget fashion like publish, but instead It is just used for one receiver. The main difference with Publish is that in Send if your destination didn't receive a message, it would return an exception.
  • Requests: uses request/reply pattern to just send a message and get a response in a different channel to be able to get response value from the receiver.

Now, my question is according to the Microservice concept, to follow the event-driven design, we use Publish to propagate messages(Events) to the entire ecosystem. but what is exactly the usage (use case) of Send here? Just to get an exception if the receiver doesn't exist?

My next question is that is it a good approach to use Publish, Send and Requests in a Microservices ecosystem at the same time? like publish for propagation events, Send for command (fire and forget), and Requests for getting responses from the destination.

----- Update I also found here which Chris Patterson clear lots of things. It also helps me a lot.

Mohammad
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  • Might just [read this answer](https://stackoverflow.com/questions/62713786/masstransit-endpointconvention-azure-service-bus/62714778#62714778) as my response would be about the same. – Chris Patterson Sep 10 '21 at 00:01

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Your question is not related to MassTransit. MassTransit implements well-known messaging patterns thoughtfully described on popular resources such as Enterprise Integration Patterns

As Eben wrote in his answer, the decision of what pattern to use is driven by intent. There are also technical differences in the message delivery mechanics for each pattern.

Send is for commands, you tell some other service to do something. You do not wait for a reply (fire and forget), although you might get a confirmation of the action success or failure by other means (an event, for example).

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It is an implementation of the point-to-point channel, where you also can implement competing consumers to scale the processing, but those will be instances of the same service.

With MassTransit using RabbitMQ it's done by publishing messages to the endpoint exchange rather than to the message type exchange, so no other endpoints will get the message even though they can consume it.

Publish is for events. It's a broadcast type of delivery or fan-out. You might be publishing events to which no one is listening, so you don't really know who will be consuming them. You also don't expect any response.

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It is an implementation of the publish-subscribe channel.

MassTransit with RabbitMQ creates exchanges for each message type published and publishes messages to those exchanges. Consumers create bindings between their endpoint exchanges and message exchanges, so each consumer service (different apps) will get those in their independent queues.

Request-response can be used for both commands that need to be confirmed, or for queries.

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It is an implementation of the request-reply message pattern.

MassTransit has nice diagrams in the docs explaining the mechanics for RabbitMQ.

Those messaging patterns are frequently used in a complex distributed system in different combinations and variations.

Alexey Zimarev
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The difference between Send and Publish has to do with intent.

As you stated, Send is for commands and Publish is for events. I worked on a large enterprise system once running on webMethods as the integration engine/service bus and only events were used. I can tell you that it was less than ideal. If the distinction had been there between commands and events it would've made a lot more sense to more people. Anyway, technically one needs a message enqueued and on that level it doesn't matter, which is why a queueing mechanism typically would not care about such semantics.

To illustrate this with a silly example: Facebook places and Event on my timeline that one of my friends is having a birthday on a particular day. I can respond directly (send a message) or I could publish a message on my timeline and hope my friend sees it. Another silly example: You send an e-mail to PersonA and CC 4 others asking "Please produce report ABC". PersonA would be expected to produce the report or arrange for it to be done. If that same e-mail went to all five people as the recipient (no CC) then who gets to do it? I know, even for Publish one could have a 1-1 recipient/topic but what if another endpoint subscribed? What would that mean?

So the sender is responsible, still configurable as subscriptions are, to determine where to Send the message to. For my own service bus I use an implementation of an IMessageRouteProvider interface. A practical example in a system I once developed was where e-mails received had to have their body converted to an image for a content store (IBM FileNet P8 if memory serves). For reasons I will not go into the systems were stopped each night at 20h00 and restarted at 6h00 in the morning. This led to a backlog of usually around 8000 e-mails that had to be converted. The conversion endpoint would process a conversion in about 2 seconds but that still takes a while to work through. In the meantime the web front-end folks could request PDF files for conversion to paged TIFF files. Now, these ended up at the end of the queue and they would have to wait hours for that to come back. The solution was to implement another conversion endpoint, with its own queue, and have the web front-end configured to send the same message type, e.g. ConvertDocumentCommand to that "priority" queue for processing. Pretty easy to do. Now, if that had been a publish how would I do that split? The same event going to 2 different endpoints under different circumstances? Well, you could have another subscription store for your system but now you'd need to maintain both. There could be another answer such as coding this logic into the send bit but that is a design choice and would require coding changes.

In my own Shuttle.Esb service bus I only have Send and Publish. For request/response both the sender and receiver have an inbox and a request would be sent (Send) to the receiver and it in turn could reply (also a Send but uses the sender's URI).

Eben Roux
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  • Thanks, Eben, for your nice answer, So in your opinion with Send we are able to keep messaging responsibly on the sender side. It also helps us to manage messages from the sender. good Idea. So you do prefer to use both Publish and Send in your system? – Mohammad Sep 10 '21 at 15:33
  • @Mohammad, I do use both `Send` and `Publish`, yes. I send a command and publish an event and, by convention is suffix the class with either `Command` or `Event` to make the distinction clear. There may be zero to N subscribers of an event. An event is something interesting that your system informs the world about and anyone could handle that. Typically an event also only has a single logical publisher; else it can become tricky determining authenticity when receiving conflicting values from multiple logical publishers (there may be exceptions). – Eben Roux Sep 11 '21 at 15:12