I am struggling to understand when to use each of these:

1) Message Bus: Used to send integration events between Microservices. For example, Microservice A could publish an integration event, which is handled by Microservice B and Microservice C. The benefit of a Message Bus e.g. RabbitMQ is that it could be durable meaning if one of the Microservices is down then it can handle the event later. It also guarantees delivery.

2) Mediatr (Mediator pattern): Using CQRS the Mediatr pattern can be used to decouple commands and events making the MVC controller/service thinner.

I see how both of these patterns can be used in the same environment. I then see code like this:

3) In Memory Bus: https://github.com/gregoryyoung/m-r/blob/master/SimpleCQRS/FakeBus.cs. It is an In Memory bus.

What is the difference between using Mediatr and an In Memory Bus? The way I am thinking at the moment is that Mediatr is more appropriate when using the Observer pattern and an In Memory Bus is more suitable when using the publisher/subsciber pattern. Have I understood this correctly?

Is it appropriate to use an InMemory Bus for a single Microservice (for domain events) and a durable Message bus for integration events (between Microsservices)?

  • Presumably one is synchronous and the other is not. – Telastyn Sep 28 '18 at 10:34

A mediator pattern creates a flexible decoupled interface between two microservices. Messages are sent to and from each microservice without one necessarily having to know all the explicit workings of the other.

This pattern is ideal if you think the relationship between the two might change significantly in the future. The mediator could be standalone and it could persist messages in case of failed delivery, so potentially it can be just as stabile as the message bus.

A publish-subscribe pattern which is basically your message bus, isn't just for two interfaces. It is potentially for multiple interfaces, allowing for maximum flexibility. The flexibility also extends to the message itself, meaning the message must be easily sent and received by anyone interested in receiving said message when an event occurs. This also implies you may have problems down the road with versioning if you're not careful, as changes in messages being passed must be consistently interpreted across microservices, unlike the mediator. When the message changes, then only the mediator absolutely must also be updated for everything to work properly.

An in-memory bus isn't really a pattern so much as a message bus with no persistence. It is faster, but also not ideal when there are unexpected failures, as you don't know where the error occurred and would have serious difficulty trying to restore the previous situation. In any production context, I wouldn't recommend this, even if it wrote to a log file.

I hope that answers your question!

  • So a in memory bus is a bit like udp, right? You just send out and don't care if it gets lost. So it could have practical applications just as udp has. – findusl Sep 28 '18 at 12:38
  • @findusl Well you've seen how it is written. If the computer shuts off and the message was received by the memory message bus, message is lost. This may be fine if you don't care too much about guaranteeing the delivery of the message. So yes, I suppose you could look at it that way. It is the UDP of message buses, whereas a persistent message bus would be more like TCP. – Neil Sep 28 '18 at 12:46
  • @Neil, thanks. I understand the use of a message bus for communication between Microservices (integration events). However, say you have Domain Events inside a Micro service, then would you have a second message bus inside the Microservice? + 1 for: "It is potentially for multiple interfaces" – w0051977 Sep 28 '18 at 12:55
  • @w0051977 No, not unless by using the main message bus, you'd have major architectural problems. Generally you'd use just the one (if anything, you can better organize the events into namespaces to modularize, i.e. datalayer.connectionEstablished). – Neil Sep 28 '18 at 12:59
  • Thanks, so why does Greg Young use one here: github.com/gregoryyoung/m-r/blob/master/SimpleCQRS/FakeBus.cs – w0051977 Sep 28 '18 at 13:00

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