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Implement the mediator pattern using MassTransit

The mediator pattern is a behavioral design pattern used in software engineering. It defines an object that encapsulates how a set of objects interact. 

In object-oriented programming, programs often consist of many classes. As more classes are added to a program, especially during maintenance or refactoring, the problem of communication between these classes becomes complex. Direct communication between objects can lead to tight coupling, making the program harder to read, maintain, and change.

The mediator pattern introduces a mediator object that acts as an intermediary between interacting objects. Instead of direct communication, objects now communicate through the mediator. This reduces dependencies between objects and promotes loose coupling.

A popular way to implement the mediator pattern in .NET is through the popular MediatR library. But if you are already using Masstransit,  there is no need to introduce an extra dependency as Masstransit has built-in support for the Mediator pattern.

Similar to MediatR, the MassTransit mediator runs in-process and in-memory. The nice thing is that it builds further on the concepts you already use in MassTransit, like consumers, handlers, sagas, …

Let me show you a quick example on how to get started.

Configure our Mediator pipeline

First we should create a Consumer, we could implement the default IConsumer interface or the MediatorRequestHandler that further simplifies the implementation when using the MassTransit mediator:

We'll use the MediatorRequestHandler in this example:

Now we need to register this handler:

A typical reason to use the mediator pattern is to isolate specific behavior in separate classes. In MassTransit we can use the middleware feature for this. We first need to create our filter:

And then we can configure to apply this filter in our mediator pipeline:

Send a request through the Mediator

Now the configuration is done, the only thing left to do is to send a request through the pipeline. This can be done by injecting an IMediator instance and calling the SendRequest method:

More information

Mediator · MassTransit

MassTransit/Sample-Mediator: A sample solution with various projects showing how to use MassTransit's mediator (github.com)

jbogard/MediatR: Simple, unambitious mediator implementation in .NET (github.com)

Also have a look at the following video:

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