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martinothamar / Mediator

A high performance implementation of Mediator pattern in .NET using source generators.
MIT License
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csharp csharp-sourcegenerator dotnet dotnet-core dotnet-standard dotnetcore mediator mediator-pattern source-gen source-generation source-generators sourcegenerator

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Mediator

Note

Version 3.0 is currently being developed. See status and provide feedback here (#98)

This is a high performance .NET implementation of the Mediator pattern using the source generators feature introduced in .NET 5. The API and usage is mostly based on the great MediatR library, with some deviations to allow for better performance. Packages are .NET Standard 2.1 compatible.

The mediator pattern is great for implementing cross cutting concern (logging, metrics, etc) and avoiding "fat" constructors due to lots of injected services.

Goals for this library

In particular, source generators in this library is used to

See this great video by @Elfocrash / Nick Chapsas, covering both similarities and differences between Mediator and MediatR

Using MediatR in .NET? Maybe replace it with this

Table of Contents

2. Benchmarks

This benchmark exposes the perf overhead of the libraries. Mediator (this library) and MediatR methods show the overhead of the respective mediator implementations. I've also included the MessagePipe library as it also has great performance.

See benchmarks code for more details on the measurement.

Warning

A current limitation of this library is that performance degrades significantly for projects with a large number of messages (>500) There is ongoing work on resolving this for version 3.0 (#48).

Requests benchmark

Stream benchmark

3. Usage and abstractions

There are two NuGet packages needed to use this library

You install the source generator package into your edge/outermost project (i.e. ASP.NET Core application, Background worker project), and then use the Mediator.Abstractions package wherever you define message types and handlers. Standard message handlers are automatically picked up and added to the DI container in the generated AddMediator method. Pipeline behaviors need to be added manually (including pre/post/exception behaviors).

For example implementations, see the /samples folder. See the ASP.NET Core clean architecture sample for a more real world setup.

3.1. Message types

As you can see, you can achieve the exact same thing with requests, commands and queries. But I find the distinction in naming useful if you for example use the CQRS pattern or for some reason have a preference on naming in your application.

3.2. Handler types

These types are used in correlation with the message types above.

3.3. Pipeline types

3.3.1. Message validation example

// As a normal pipeline behavior
public sealed class MessageValidatorBehaviour<TMessage, TResponse> : IPipelineBehavior<TMessage, TResponse>
    where TMessage : IValidate
{
    public ValueTask<TResponse> Handle(
        TMessage message,
        CancellationToken cancellationToken,
        MessageHandlerDelegate<TMessage, TResponse> next
    )
    {
        if (!message.IsValid(out var validationError))
            throw new ValidationException(validationError);

        return next(message, cancellationToken);
    }
}

// Or as a pre-processor
public sealed class MessageValidatorBehaviour<TMessage, TResponse> : MessagePreProcessor<TMessage, TResponse>
    where TMessage : IValidate
{
    protected override ValueTask Handle(TMessage message, CancellationToken cancellationToken)
    {
        if (!message.IsValid(out var validationError))
            throw new ValidationException(validationError);

        return default;
    }
}

// Register as IPipelineBehavior<,> in either case
services.AddSingleton(typeof(IPipelineBehavior<,>), typeof(MessageValidatorBehaviour<,>))

3.3.2. Error logging example

// As a normal pipeline behavior
public sealed class ErrorLoggingBehaviour<TMessage, TResponse> : IPipelineBehavior<TMessage, TResponse>
    where TMessage : IMessage
{
    private readonly ILogger<ErrorLoggingBehaviour<TMessage, TResponse>> _logger;

    public ErrorLoggingBehaviour(ILogger<ErrorLoggingBehaviour<TMessage, TResponse>> logger)
    {
        _logger = logger;
    }

    public async ValueTask<TResponse> Handle(
        TMessage message,
        CancellationToken cancellationToken,
        MessageHandlerDelegate<TMessage, TResponse> next
    )
    {
        try
        {
            return await next(message, cancellationToken);
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Error handling message of type {messageType}", message.GetType().Name);
            throw;
        }
    }
}

// Or as an exception handler
public sealed class ErrorLoggingBehaviour<TMessage, TResponse> : MessageExceptionHandler<TMessage, TResponse>
    where TMessage : notnull, IMessage
{
    private readonly ILogger<ErrorLoggingBehaviour<TMessage, TResponse>> _logger;

    public ErrorLoggingBehaviour(ILogger<ErrorLoggingBehaviour<TMessage, TResponse>> logger)
    {
        _logger = logger;
    }

    protected override ValueTask<ExceptionHandlingResult<TResponse>> Handle(
        TMessage message,
        Exception exception,
        CancellationToken cancellationToken
    )
    {
        _logger.LogError(exception, "Error handling message of type {messageType}", message.GetType().Name);
        // Let the exception bubble up by using the base class helper NotHandled:
        return NotHandled;
        // Or if the exception is properly handled, you can just return your own response,
        // using the base class helper Handle().
        // This requires you to know something about TResponse,
        // so TResponse needs to be constrained to something,
        // typically with a static abstract member acting as a consructor on an interface or abstract class.
        return Handled(null!);
    }
}

// Register as IPipelineBehavior<,> in either case
services.AddSingleton(typeof(IPipelineBehavior<,>), typeof(ErrorLoggingBehaviour<,>))

3.4. Configuration

There are two ways to configure Mediator. Configuration values are needed during compile-time since this is a source generator:

services.AddMediator(options =>
{
    options.Namespace = "SimpleConsole.Mediator";
    options.DefaultServiceLifetime = ServiceLifetime.Transient;
});

// or

[assembly: MediatorOptions(Namespace = "SimpleConsole.Mediator", DefaultServiceLifetime = ServiceLifetime.Transient)]

4. Getting started

In this section we will get started with Mediator and go through a sample illustrating the various ways the Mediator pattern can be used in an application.

See the full runnable sample code in the Showcase sample.

4.1. Add package

dotnet add package Mediator.SourceGenerator --version 2.0.*
dotnet add package Mediator.Abstractions --version 2.0.*

or

<PackageReference Include="Mediator.SourceGenerator" Version="2.0.*">
  <PrivateAssets>all</PrivateAssets>
  <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
</PackageReference>
<PackageReference Include="Mediator.Abstractions" Version="2.0.*" />

4.2. Add Mediator to DI container

In ConfigureServices or equivalent, call AddMediator (unless MediatorOptions is configured, default namespace is Mediator). This registers your handler below.

using Mediator;
using Microsoft.Extensions.DependencyInjection;
using System;

var services = new ServiceCollection(); // Most likely IServiceCollection comes from IHostBuilder/Generic host abstraction in Microsoft.Extensions.Hosting

services.AddMediator();
var serviceProvider = services.BuildServiceProvider();

4.3. Create IRequest<> type

var mediator = serviceProvider.GetRequiredService<IMediator>();
var ping = new Ping(Guid.NewGuid());
var pong = await mediator.Send(ping);
Debug.Assert(ping.Id == pong.Id);

// ...

public sealed record Ping(Guid Id) : IRequest<Pong>;

public sealed record Pong(Guid Id);

public sealed class PingHandler : IRequestHandler<Ping, Pong>
{
    public ValueTask<Pong> Handle(Ping request, CancellationToken cancellationToken)
    {
        return new ValueTask<Pong>(new Pong(request.Id));
    }
}

As soon as you code up message types, the source generator will add DI registrations automatically (inside AddMediator). P.S - You can inspect the code yourself - open Mediator.g.cs in VS from Project -> Dependencies -> Analyzers -> Mediator.SourceGenerator -> Mediator.SourceGenerator.MediatorGenerator, or just F12 through the code.

4.4. Use pipeline behaviors

The pipeline behavior below validates all incoming Ping messages. Pipeline behaviors currently must be added manually.

services.AddMediator();
services.AddSingleton<IPipelineBehavior<Ping, Pong>, PingValidator>();

public sealed class PingValidator : IPipelineBehavior<Ping, Pong>
{
    public ValueTask<Pong> Handle(Ping request, MessageHandlerDelegate<Ping, Pong> next, CancellationToken cancellationToken)
    {
        if (request is null || request.Id == default)
            throw new ArgumentException("Invalid input");

        return next(request, cancellationToken);
    }
}

4.5. Constrain IPipelineBehavior<,> message with open generics

Add open generic handler to process all or a subset of messages passing through Mediator. This handler will log any error that is thrown from message handlers (IRequest, ICommand, IQuery). It also publishes a notification allowing notification handlers to react to errors. Message pre- and post-processors along with the exception handlers can also constrain the generic type parameters in the same way.

services.AddMediator();
services.AddSingleton(typeof(IPipelineBehavior<,>), typeof(ErrorLoggerHandler<,>));

public sealed record ErrorMessage(Exception Exception) : INotification;
public sealed record SuccessfulMessage() : INotification;

public sealed class ErrorLoggerHandler<TMessage, TResponse> : IPipelineBehavior<TMessage, TResponse>
    where TMessage : IMessage // Constrained to IMessage, or constrain to IBaseCommand or any custom interface you've implemented
{
    private readonly ILogger<ErrorLoggerHandler<TMessage, TResponse>> _logger;
    private readonly IMediator _mediator;

    public ErrorLoggerHandler(ILogger<ErrorLoggerHandler<TMessage, TResponse>> logger, IMediator mediator)
    {
        _logger = logger;
        _mediator = mediator;
    }

    public async ValueTask<TResponse> Handle(TMessage message, MessageHandlerDelegate<TMessage, TResponse> next, CancellationToken cancellationToken)
    {
        try
        {
            var response = await next(message, cancellationToken);
            return response;
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Error handling message");
            await _mediator.Publish(new ErrorMessage(ex));
            throw;
        }
    }
}

4.6. Use notifications

We can define a notification handler to catch errors from the above pipeline behavior.

// Notification handlers are automatically added to DI container

public sealed class ErrorNotificationHandler : INotificationHandler<ErrorMessage>
{
    public ValueTask Handle(ErrorMessage error, CancellationToken cancellationToken)
    {
        // Could log to application insights or something...
        return default;
    }
}

4.7. Polymorphic dispatch with notification handlers

We can also define a notification handler that receives all notifications.


public sealed class StatsNotificationHandler : INotificationHandler<INotification> // or any other interface deriving from INotification
{
    private long _messageCount;
    private long _messageErrorCount;

    public (long MessageCount, long MessageErrorCount) Stats => (_messageCount, _messageErrorCount);

    public ValueTask Handle(INotification notification, CancellationToken cancellationToken)
    {
        Interlocked.Increment(ref _messageCount);
        if (notification is ErrorMessage)
            Interlocked.Increment(ref _messageErrorCount);
        return default;
    }
}

4.8. Notification handlers also support open generics

public sealed class GenericNotificationHandler<TNotification> : INotificationHandler<TNotification>
    where TNotification : INotification // Generic notification handlers will be registered as open constrained types automatically
{
    public ValueTask Handle(TNotification notification, CancellationToken cancellationToken)
    {
        return default;
    }
}

4.9. Notification publishers

[!IMPORTANT]
This API is currently in the latest 3.0 previews, not available in stable.

Notification publishers are responsible for dispatching notifications to a collection of handlers. There are two built in implementations:

Both of these try to be efficient by handling a number of special cases (early exit on sync completion, single-handler, array of handlers). Below we implement a custom one by simply using Task.WhenAll.

services.AddMediator(options =>
{
    options.NotificationPublisherType = typeof(FireAndForgetNotificationPublisher);
});

public sealed class FireAndForgetNotificationPublisher : INotificationPublisher
{
    public async ValueTask Publish<TNotification>(
        NotificationHandlers<TNotification> handlers,
        TNotification notification,
        CancellationToken cancellationToken
    )
        where TNotification : INotification
    {
        try
        {
            await Task.WhenAll(handlers.Select(handler => handler.Handle(notification, cancellationToken).AsTask()));
        }
        catch (Exception ex)
        {
            // Notifications should be fire-and-forget, we just need to log it!
            // This way we don't have to worry about exceptions bubbling up when publishing notifications
            Console.Error.WriteLine(ex);

            // NOTE: not necessarily saying this is a good idea!
        }
    }
}

4.10. Use streaming messages

Since version 1.* of this library there is support for streaming using IAsyncEnumerable.

var mediator = serviceProvider.GetRequiredService<IMediator>();

var ping = new StreamPing(Guid.NewGuid());

await foreach (var pong in mediator.CreateStream(ping))
{
    Debug.Assert(ping.Id == pong.Id);
    Console.WriteLine("Received pong!"); // Should log 5 times
}

// ...

public sealed record StreamPing(Guid Id) : IStreamRequest<Pong>;

public sealed record Pong(Guid Id);

public sealed class PingHandler : IStreamRequestHandler<StreamPing, Pong>
{
    public async IAsyncEnumerable<Pong> Handle(StreamPing request, [EnumeratorCancellation] CancellationToken cancellationToken)
    {
        for (int i = 0; i < 5; i++)
        {
            await Task.Delay(1000, cancellationToken);
            yield return new Pong(request.Id);
        }
    }
}

5. Diagnostics

Since this is a source generator, diagnostics are also included. Examples below

Missing request handler

Multiple request handlers found

6. Differences from MediatR

This is a work in progress list on the differences between this library and MediatR.

7. Versioning

For versioning this library I try to follow semver 2.0 as best as I can, meaning