Llmed

A minimal, dependency-light in-process mediator for .NET — MediatR-style request/response with pipeline behaviors, hand-rolled and MIT-licensed.

Targets net8.0. Single runtime dependency: Microsoft.Extensions.DependencyInjection.Abstractions (+ Microsoft.Extensions.Options for the built-in concurrency-limit behavior).

Install

dotnet add package Llmed

Define a request and a handler

public sealed record Ping(string Message) : IRequest<string>;

public sealed class PingHandler : IRequestHandler<Ping, string>
{
    public Task<string> Handle(Ping request, CancellationToken ct) =>
        Task.FromResult($"pong:{request.Message}");
}

Wire it up

var services = new ServiceCollection();
services.AddMediator(typeof(Program).Assembly);
var sp = services.BuildServiceProvider();

var mediator = sp.GetRequiredService<IMediator>();
var result = await mediator.Send(new Ping("hello")); // "pong:hello"

Add a pipeline behavior

public sealed class LoggingBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
    where TRequest : IRequest<TResponse>
{
    public async Task<TResponse> Handle(
        TRequest request,
        RequestHandlerDelegate<TResponse> next,
        CancellationToken ct)
    {
        Console.WriteLine($"-> {typeof(TRequest).Name}");
        var response = await next();
        Console.WriteLine($"<- {typeof(TRequest).Name}");
        return response;
    }
}

services.AddMediator(typeof(Program).Assembly)
        .AddBehavior(typeof(LoggingBehavior<,>));

Behaviors execute in registration order (first registered = outermost in the chain).

Gate concurrency

Useful for serializing calls to a single-forward-pass model resource.

services.AddConcurrencyLimit(maxConcurrency: 1);
services.AddMediator(typeof(Program).Assembly)
        .AddBehavior(typeof(ConcurrencyLimitBehavior<,>));

Isolate two mediators in one process

For Clean Architecture setups where (for example) AI-tool orchestration and domain logic should not share a behavior stack:

services.AddKeyedMediator("ai", typeof(AiHandlers).Assembly)
        .AddBehavior(typeof(AiTracingBehavior<,>));

services.AddKeyedMediator("domain", typeof(DomainHandlers).Assembly)
        .AddBehavior(typeof(DomainValidationBehavior<,>));

// Resolve with [FromKeyedServices("ai")] IMediator mediator

The two mediators do not see each other's handlers or behaviors.

Observability

Each Send emits an ActivitySource("Llmed.Mediator") activity named Mediator.Send with a request.type tag. Attach an ActivityListener (or use OpenTelemetry's AddSource("Llmed.Mediator")) to capture.

License

MIT.