MNW #030: How To Publish MediatR Notifications In Parallel

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MediatR is a popular library with a simple mediator pattern implementation in .NET.

Here's a definiton taken from MediatR's GitHub: "In-process messaging with no dependencies."

With the rise in popularity of the CQRS pattern, MediatR became the go-to library to implement commands and queries.

However, MediatR also has support for the publish-subscribe pattern using notifications. You can publish an INotification instance and have multiple subscribers handle the published message.

Until recently, the handlers subscribing to an INotification message could only execute serially, one by one.

In this week's newsletter, I'll show you how to configure MediatR to execute the handlers in parallel.

How Publish-Subscribe Works With MediatR

Before I talk about notification publishing strategies, let's see how publish-subscribe works with MediatR.

You need a class implementing the INotification interface:

Then you need a respective INotificationHandler implementation:

And then you simply publish a message using either IMediator or IPublisher. I prefer using the IPublisher because it's more expressive:

MediatR will invoke all the respective handlers.

Introducing Notification Publisher Strategies

Before MediatR v12, the publishing strategy would invoke each handler individually.

However, there's a new interface INotificationPublisher controlling how the handlers are called.

The default implementation of this interface is ForeachAwaitPublisher:

But now you can also use the TaskWhenAllPublisher:

Here's a comparison between these two strategies.

ForeachAwaitPublisher:

Invokes each handler one by one
Fails when an exception occurs in one of the handlers

TaskWhenAllPublisher:

Invokes all the handlers at the same time
Executes all the handlers regardless of one of them throwing an exception

If you store the task returned by TaskWhenAllPublisher you can access the Task.Exception property, which will contain an AggregateException instance. You can then implement more robust exception handling.

Configuring MediatR Notification Publishing Strategy

How do we configure which INotificationPublisher strategy MediatR will use?

There's a new way to apply configuration options when calling the AddMediatR method.

You supply an Action<MediatRServiceConfiguration> delegate and configure the MediatRServiceConfiguration instance.

If you want to use the TaskWhenAllPublisher strategy, you can either:

Provide a value for the NotificationPublisher property
Specify the strategy type on the NotificationPublisherType property

You can also implement a custom INotificationPublisher instance and use your own implementation instead.

How Is This Useful?

Being able to run notification handlers in parallel provides a significant performance improvement over the default behavior.

However, note that all handlers will use the same service scope.

If you have service instances that don't support concurrent access you may run into problems.

Unfortunately, one such service instance is the EF Core DbContext.

In any case, I think this is a great addition to the already amazing MediatR library.

That's all for today.

See you next week.

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