C#•14mo ago

✅ What's the point of MediatR?

I haven't done a lot of web/API dev, so this might just be because I've never been in a situation where I've actually needed it, but what is the point of MediatR? I don't really understand it too well, so if someone could give me a rundown, that would be incredible.

81 Replies

Pobiega•14mo ago

Clarification: Do you truly mean the mediator pattern, or do you mean libraries like MediatR etc?

Thinker•14mo ago

Isn't MediatR an implementation or the mediator pattern?

Pobiega•14mo ago

See, that would make sense! But no, its not 🙂

Despite the name, MediatR does not implement the Mediator Pattern at all: it’s a Command Dispatcher.

Thinker•14mo ago

ah yeah okay I had it backwards then, my actual question would be why do people use MediatR?

Pobiega•14mo ago

Well, there are a few benefits and downsides First, having a single handler for a single command hierarchy makes it more focused than an entire service class, while still letting it be mocked out for tests Easy to find what you are looking for and it's not going to be a thousand line service class, so that's good. Also resolves it's own dependencies, so you don't need to do that in the endpoint or whatever caller you are using Third, not common in asp but sometimes you might have a command type hierarchy and not know the concrete implementation. An example is a paser for a command line app, where you just parse and fire away Downsides? Well it's just a glorified method invocation It does add complexity where it might not be needed Oh and most dispatchers use a pipeline, so you can register middleware This lets you handle transactions

rodrigovaz•14mo ago

Another point, a lot of people use MediatR to implement CQRS and not necessarily the Mediator pattern in itself

Thinker•14mo ago

sorry, what's a "command hierarchy"?

Pobiega•14mo ago

Commands related to eachother, either by nature of what they do (two ways to add something) or by actual inheritance (all commands are ICommands) Why is that a benefit of MediatR?

rodrigovaz•14mo ago

Not reinventing the wheel

Pobiega•14mo ago

... what?

Thinker•14mo ago

And a command is just some kind of "action"? Like... send an email or update a db or whatever

Pobiega•14mo ago

So because it can be used to implement CQRS, that counts as a plus ? Wtf Yep It's a glorified method invocation:)

Thinker•14mo ago

right

Pobiega•14mo ago

It does allow your method calls to be serialized thou. Useful for things like undo

rodrigovaz•14mo ago

I meant that you will see the MediatR library in the wild a lot of times because people wanted to implement CQRS (which benefits from using the mediator pattern) and the internet is filled with "implementing CQRS with MediatR" tutorials

Pobiega•14mo ago

I agree with that But how is that "the point" of MediatR? Also, MediatR is not an implementation of the mediator pattern This is important

Thinker•14mo ago

why is it even named that if it doesn't implement the mediator pattern?

Pobiega•14mo ago

No clue But it doesn't. It was just a command dispatcher and then they added a message Hub to it

rodrigovaz•14mo ago

It isn't, it was more of a heads up that the library does much more than simply helping implementing CQRS. I had the same question once and my doubt arose from looking at open source projects that used it for CQRS I really like that in their GitHub page it says "Simple mediator implementation in .NET"

Pobiega•14mo ago

I'm traveling right now, but I'll try to find an excellent article that shows the difference But yeah, CQRS is a big factor of why it's popular for sure. There is a Greg Young talk about CQRS and simplicity that is very good

Tvde1•14mo ago

What MediatR does, is replace classes that end in ...Service or ...Manager with a class that has only one method. whether it's

interface IUserCreationService
{
    Result<User> CreateUser(CreateUserDto request);
}

interface IUserCreationService
{
    Result<User> CreateUser(CreateUserDto request);
}

or whether it's

record CreateUserCommand(string name, ...) : ICommand<Result<User>>;

class CreateUserHandler : IHandler<CreateUserCommand>
{

record CreateUserCommand(string name, ...) : ICommand<Result<User>>;

class CreateUserHandler : IHandler<CreateUserCommand>
{

The result is that you're thinking in small functionalities that handle a command or request, instead of making large classes that do a bunch of inbetween things I want to put this way of thinking into my codebases, without using mediator. I want to see more very very small interfaces and

// some code
Result<User> userRes = _mediatr.Send<CreateUserCommand>(..);

// some code
Result<User> userRes = _mediatr.Send<CreateUserCommand>(..);

and

// some code
Result<User> userRes = _createUserService.Create(...);

// some code
Result<User> userRes = _createUserService.Create(...);

Are roughly similar

Thinker•14mo ago

But you still need these small Command classes and Handler classes

Tvde1•14mo ago

luckily we don't need to watch how many kilobytes of code we write. You usally have a few GBs free on your disk :) whether it's an interface + implementation, or a command + handler, you'll write nearly just as much

Thinker•14mo ago

Ah that's what this meant, instead of calling a method on a service, you use Send<T>(new T(...));

Tvde1•14mo ago

it feels like the service locator antipattern, which makes spaghetti code, but it isn't really you're not specifying a dependency, you just say "I want this command done. I don't care who does it, but I want this result from this input"

Pobiega•14mo ago

yes!

Pobiega•14mo ago

Also, I found the article: https://arialdomartini.github.io/mediatr

You probably don't need MediatR

Personal resume

Pobiega•14mo ago

"The Mediator Pattern" is the section that breaks down how this isn't a mediator Oh, and the primary drawback (at least if you ask me) is that intellisense can't tell you that you have references to the handler

Tvde1•14mo ago

damn that is a very good article

Pobiega•14mo ago

ie, if you want to jump to the AddCommandHandler.Handle method, you need to know that its called such

Tvde1•14mo ago

it reflects the thoughts that I didn't know how to word

Pobiega•14mo ago

Mhm, I felt the same way.

Tvde1•14mo ago

usually you put the command and the handler in the same file

Pobiega•14mo ago

Yeah I know.. unfortunately at work we have stylecop that enforces no more than one class per file and I hate it 😄

Tvde1•14mo ago

that's... unproductive

Thinker•14mo ago

Also if I wanted to write a MediatR handler for like creating a user or whatever, would I inject the EF db context and other related things directly in the handler?

Pobiega•14mo ago

we use a home-rolled command dispatcher thou, not mediatr. but its similar enough yep the idea is that the command would contain all the data needed, and the handler all the code needed.

Thinker•14mo ago

ah, so it's like minimal APIs vs. controllers, where minimal APIs can request the minimum amount of things they actually need

Tvde1•14mo ago

having tiny handlers, all separated from other handlers would also allow you to make refactoring easier. If you switch to a different DB, you don't have to switch out one big layer to another, but you can approach it per handler you can also make controllers with just one endpoint :)

Pobiega•14mo ago

@rodrigovaz https://discord.com/channels/143867839282020352/1155864560859873411/1155883188032380978 thats the article I mentioned So Thinker, do you feel the question has been answered? 🙂

Thinker•14mo ago

Probably, at least I think I got a better understanding of what the point is, to the point I could see why I would use it myself. Actually nevermind, I have another question, why split a request handler and endpoint in the first place? Like if I have an endpoint which one job is to create something, why should I separate that into a request handler which creates the thing and an endpoint which literally just calls the handler with the request object?

Tvde1•14mo ago

you don't want to be tightly coupled to ASP.net what if you decide to be event bus-based, then you want your business logic to be accessible through a message consumer instead of that consumer needing ASP.net dependencies but, you could always refactor that later imo unless you start returning action results in your code imo a business logic csproj should never know it's being called by ASP.net

Thinker•14mo ago

Well, I made this just to test it, but it feels dumb because like these could just be one method

public sealed record CreateArtworkRequest(
    string? Title,
    string? Description,
    HighlightColor HighlightColor,
    LocalDate UploadDate,
    IReadOnlyCollection<int> Categories)
    : IRequest<Artwork>;

public sealed class CreateArtworkHandler(ArtContext context)
    : IRequestHandler<CreateArtworkRequest, Artwork>
{
    public async Task<Artwork> Handle(CreateArtworkRequest request, CancellationToken cancellationToken)
    {
        var categories = await context.Categories
            .Where(x => request.Categories.Contains(x.Id))
            .ToListAsync(cancellationToken);

        var entity = new ArtworkEntity()
        {
            Title = request.Title,
            Description = request.Description,
            HighlightColor = request.HighlightColor,
            UploadDate = request.UploadDate,
            Categories = categories,
        };

        context.Artworks.Add(entity);

        await context.SaveChangesAsync(cancellationToken);

        return entity.ToArtwork();
    }
}

public sealed class CreateArtworkController
{
    [HttpPost("/art")]
    public async Task<IResult> Create(
        [FromServices] IMediator mediator,
        [FromBody] CreateArtworkRequest request)
    {
        var artwork = await mediator.Send(request);
        return Results.Created("/", artwork);
    }
}

public sealed record CreateArtworkRequest(
    string? Title,
    string? Description,
    HighlightColor HighlightColor,
    LocalDate UploadDate,
    IReadOnlyCollection<int> Categories)
    : IRequest<Artwork>;

public sealed class CreateArtworkHandler(ArtContext context)
    : IRequestHandler<CreateArtworkRequest, Artwork>
{
    public async Task<Artwork> Handle(CreateArtworkRequest request, CancellationToken cancellationToken)
    {
        var categories = await context.Categories
            .Where(x => request.Categories.Contains(x.Id))
            .ToListAsync(cancellationToken);

        var entity = new ArtworkEntity()
        {
            Title = request.Title,
            Description = request.Description,
            HighlightColor = request.HighlightColor,
            UploadDate = request.UploadDate,
            Categories = categories,
        };

        context.Artworks.Add(entity);

        await context.SaveChangesAsync(cancellationToken);

        return entity.ToArtwork();
    }
}

public sealed class CreateArtworkController
{
    [HttpPost("/art")]
    public async Task<IResult> Create(
        [FromServices] IMediator mediator,
        [FromBody] CreateArtworkRequest request)
    {
        var artwork = await mediator.Send(request);
        return Results.Created("/", artwork);
    }
}

Pobiega•14mo ago

Agree with all the above points. What if you later must add items not via an endpoint, but via a message queue? or via a CLI app? or a desktop app? if its all in the endpoint, you can't easily.

Tvde1•14mo ago

I have switched a bunch of endpoints to azure service bus consumers lately and some of them were a pain

Pobiega•14mo ago

let the controller/endpoint handle the HTTP specifics and nothing else (except calling the correct command) Yeah, I've had to implement CLI tools that do a lot of business logic from our big ASP application recently

Thinker•14mo ago

So I should have a separate project which contains all the request handlers?

Pobiega•14mo ago

it was trivial, since we use a dispatcher

Tvde1•14mo ago

we have MyApplication.API, MyApplication.Business, MyApplication.Data

Pobiega•14mo ago

imho, yes. that would be your business logic project

Tvde1•14mo ago

etc etc

Pobiega•14mo ago

same here

Tvde1•14mo ago

Data.Database though, cause external dependencies get an e.g. MyApplication.Data.Discord etc

Thinker•14mo ago

I have an API, EF/data, and "common" project (mainly containing models), it already feels somewhat like I'm splitting these up for no reason other than out of principle

Tvde1•14mo ago

be careful with "common" projects D: my codebase has waaay too many and models/DTOs end up not being common but they are used across projects

Thinker•14mo ago

Yeah I just have no idea where else I would put the models which aren't EF entities

Tvde1•14mo ago

some like MyApplication.Domain for their domain models

Pobiega•14mo ago

Common would be stuff like a Result type, or other similar things

Tvde1•14mo ago

love those

Pobiega•14mo ago

we have Domain for domain models, and .Contracts for our public facing DTOs

Tvde1•14mo ago

I like that

Pobiega•14mo ago

that could easily be inside API as well, if you know you dont need it (for a client for example)

Tvde1•14mo ago

I'm drafting an API layout so that we improve on these things (e.g. internal models are given to the outside, and shared through csprojs with microservices) and I want to cut that all out

Thinker•14mo ago

So like - API - Requests/request handlers - Models/domain/whatever - EF/data

Tvde1•14mo ago

you could put requests in your domain too

Thinker•14mo ago

I guess

Pobiega•14mo ago

yup, seems like a good start to me

Tvde1•14mo ago

Thinker have you seen Accord's code?

Thinker•14mo ago

@Accord?

Tvde1•14mo ago

https://github.com/patrickklaeren/Accord

GitHub

GitHub - patrickklaeren/Accord: A C# Discord bot with moderation, X...

A C# Discord bot with moderation, XP/guild participation and utilities - GitHub - patrickklaeren/Accord: A C# Discord bot with moderation, XP/guild participation and utilities

Tvde1•14mo ago

you'll like this

Pobiega•14mo ago

this is primarily for larger or more serious codebases btw. its not worth for a small app or a POC

Thinker•14mo ago

4 projects??? only 4???

Pobiega•14mo ago

😄

Thinker•14mo ago

man MODiX looks like a beast in comparison

Tvde1•14mo ago

at work I have 113 csprojs lol

Tvde1•14mo ago

worst thing is, this is ~15 ""microservice"" APIs and they share so many csprojs

Thinker•14mo ago

what's a microservice again when

Pobiega•14mo ago

its like a unicorn

Tvde1•14mo ago

rare

Pobiega•14mo ago

a magical thing that doesnt exist 😄

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✅ What's the point of MediatR?