asp.net core - MediatR (CQRS) Tutorial & Tips 

and if you don't like being constrained by types and have easier access to meta programming try Clojure

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glad you did!

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Yep it’s similar to middleware, though it’s at application level. When the time comes to move your app from server framework to other (Which I think will never come) you just move the mediatr calls. Just a matter of what framework you choose to lean on, but mediatr gives you portability

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Thank you:)

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Thank you, glad to hear )

it's not hard once you've used it for a while

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Cheers, yeah this one is quite fun to use :)

We don’t you create this response

@@RawCoding Sorry, i did not understand. I tried your tutorial with Response . I am getting issue with PipelineBehaviour in DI. This is my DI services.AddTransient(typeof(IPipelineWrapper), typeof(ValidationBehaviour)); /////////////////////////////////////////////// public class ValidationBehaviour :IPipelineWrapper { private readonly IValidationHandler validationhandler; private readonly ILogger logger; public ValidationBehaviour(ILogger logger) { this.logger = logger; } public ValidationBehaviour(ILogger logger, IValidationHandler validationhandler) { this.validationhandler = validationhandler; } public async Task Handle(TIn request, CancellationToken cancellationToken, RequestHandlerDelegate next) { } } /////////////////////////////////////////////////// public interface IPipelineWrapper : IPipelineBehavior { } ////////////////////////////// public class Response { public static Response Failed(IEnumerable message, T data = default) => new Response(data, message, HttpStatusCode.InternalServerError); public static Response OK(T data, IEnumerable message = default, HttpStatusCode code = HttpStatusCode.OK) => new Response(data, message, code); } ///////////////////////// public sealed class Response { public Response(T data, IEnumerable message, HttpStatusCode code) { Data = data; Message = message; Code = code; } public T Data { get; set; } public HttpStatusCode Code { get; set; } public IEnumerable Message { get; set; } } /////////////////

Please share that code on my discord server in the need-help channel

@Raw Coding The issue is some what similar to github.com/jbogard/MediatR/issues/466 , But here it is asking to change DI. Is this wont work in MS DI

@@RawCoding Sure

Ty and I use the internet

@@RawCoding we all use the internet but you are on a different level. :)

Not really :D

ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Q1Tv7vj3Txo.html

I was about to implement sagas to cover a use case, but I thought if I can run away with multiple DbContexts on single physical database.

Do it in the pipe

Thank you!

You can use it like that, but what about retrying if it fails?

For learning MediatR no, your repository should be some kinda data storage abstraction, so it makes sense if you need to change the storage provider.

Make it yourself

@@RawCoding LOL

I can’t recommend it because it makes sense for different projects. Some need it some don’t, abstraction takes time and you have to know if it’s worth investing.

I don’t quite know your use case, but I’d wrap it at service level.

I recommend to watch my video on middleware, link in description

If that’s in visual studio it’s the add new files extension

@@RawCoding thanks

I’ll consider

Cheers

Middleware is tied to asp.net core framework the pipe is tied to Mediatr so it’s a bit closer to business logic. Let’s say you want to transfer functionality from backend to Xamarin app, you can’t shift the middleware but you can shift the pipe.

@@RawCoding aha, I'm getting it a bit..

JetBrains Mono

Пасиб

Never used it so I’ll probably not cover it

Well, after thinking a bit, the solution was declare abstract to BaseRequest ;)

Not sure what you mean

@@RawCoding Hahaha! No worries. I can try to explain it better... In BaseRequest class you have added a new property (UserId). It means that all the clients of this Query (Can't remember if it was on Query or command example) could set UserId during the request. I was wondering how to hide UserId parameter from the clients because it is filled in the Pipeline.

What do you mean hide UserId from the clients?

@@RawCoding Let´s say you want to use GetAllCarsQuery in another Controller. You could do something like this: mediator.Send(GetAllCarsQuery { UserId = 1 }). So, my question was how to make this attribute not accessible for classes that use this Query.

Not only

Sorry I don’t understand your question.

@@RawCoding I practiced the Mediatr pattern in a project that has the typical clear architecture structure of project where there is a dotnet core API project with controllers and an Application project for the business logic where MediatR is used. On the client side I'm using React and Mobx. For the usual CRUD part of the system MediatR is very fine and in case of Queries I send one object that is usually a parameter(s) list and get back - using AutoMapper - a DTO or whatever I need back from the DB. My question is, while i'm on the client side I need a small piece of info from the API that will direct the flow of the program - e.g. display React component A or B - is it ok to use MediatR also for such query or just do that in the controller for example. I'm not sure if MediatR is the right choice to get that info from the DB as we're not returning business objects here it maybe just a yes / no flag! What do you think. I hope I'm clear this time. Or maybe in other words, shall you use CQRS for all communications with the DB or maybe there are other patterns to consider in my case? Thanks for your time.

Mediatr can be used for this yes. Although I still don’t understand why you specifically need Mediatr

I need it to standardise on cqrs for the whole app. Maybe an example will make it clearer if you allow me. I have a grid that shows a list of parties and there is a parties table. The party can either be a person or a company and there are tables for each connected to the main parties table that just contain the common properties of each party. When the user click on a partyid in the grid I want to navigate to a full record screen showing either a person or a company and I use mediatr for that. The point here is that I can pass the partyid to mediatr and determine the party type inside the handler but mediatr need to know what response object type it needs to return and at that moment I still don't know yet. I'm not sure what is the best solution here. Shall I make two calls to the api. One to determine the party type and decide based on that. And if the first check deserve to have a mediatr call by itself to know the part type or what!! Appreciate your help.

So you can put partyid and partytype in the url query, what does this have to do with mediatr?

Hey I am pretty sure I explain that in the video. Pipe will run for all commands and queries, what I do in the video is just specify what to do if it is a command or query

@@RawCoding i think you did not get my point let me clear with an example i want to authenticate the user before processing command or query and also want to log after processing command or query how will i configure this in the DI container. i have been working on MediatR i added validation behaviour like below and it is working perfectly fine but how mediatR know this validation should be run before processing command or query e.g... services.AddMediatR(Assembly.GetExecutingAssembly()); services.AddTransient(typeof(IPipelineBehavior), typeof(ValidationBehavior));

Register the pipe before you register the command/query processing pipe

@@RawCoding yes even in above case i have registered after mediatR service but it is executing before command query anyways thanks :)

> Is there any way to register behaviorPipe on sending the command? Just inlcude that functionality in the command it self > so with bigger projects i am going to end up with 20-30 pipes that get executed with every mediator.send(). dubt you will be able to reach 20-30 I will be suprised if you can give me 10 use cases for using the pipeline. > But what about custom validatiors that need to call db. How will dependency injection work with all the authorization classes and dbcontext and pipes. It should just work > Am i going to end up with a memory leak or something? As long as you lean on the DI, no

Mhmm, I’d say I’m just talking about how to use it rather than when and for what problem

business logic -> other things -> framework -> http -> client the reality is your business logic sits quite far away from http, this wrapper is just a way to bring "structured response" closer to business logic so we can capture it and then the client can have common logic for processing the responses.

@@RawCoding but rather than modifying how an intermediate result is presented to the business logic, in this particular case we are modifying how it is sent to the external client living outside our application. we are sending back a 200 response indicating success in the header, but a boolean in the body suggesting failure. are there cases common use cases where the client would benefit from this kind of response? thanks for your response though

Mhm not sure maybe your missing a using statement

Create custom templates, or swagger

@@RawCoding Custom templates. It's been a few months but I think I'd seen a presentation by someone outlining an end-to-end code gen strategy where even the unit tests were part of the code generation stream.

Umm yea not aware of that

What do you mean?

@@RawCoding What I read about mediator pattern is that in a complex application, the dependency on the mediator object increases, and it becomes God object (when object knows too much or does too much). How to avoid that scenario?

Dependency inject business logic and use handlers as proxies.

@@RawCoding Can you explain a bit further? by giving any example?

The same way you don’t want your logic in the controller and put it in the mediatr, now you don’t want it in mediatr just put it somewhere and inject the logic in to handlers

Mediatr takes care of it

@@RawCoding thanks again

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why not use CreateCommand instead of BookViewModel ? it's just a c# object that you can validate as well. > Does CreateCommand replaces DTO like when I would simply use services for instance _bookService.CreateBook(bookDTO book)? I don't understad DTO's. if you need an class to represent some data make it. > If I'm right I don't need DTOs when using Commands and Queries anymore? Again I don't know or use DTO's, if you need a class to represent some data just make it. > Following along, inside CreateHandler I need to add the book, so I have to map between CreateCommand and Book domain model this time. So the second question is: Am I doing it right? right in what sense? if it works it works. You will know you have a problem when your solution will become hard to maintain untill then it's hard to say if you are doing something right without seeing the rest of the system. I generally think this is enough (try to have as little intermidiate objects as possible) public void Do(CreateCommand command){ ... _ctx.Books.Add(new Book { Title = command.Title ... }) ... }

Thank you for the response. From what you said I understand that I should just keep it simple. >right in what sense? if it works it works. You will know you have a problem when your solution will become hard to maintain untill then it's hard to say if you are doing something right without seeing the rest of the system. In architectural sense. I just wanted to know whether it's a good design flow assuming that we need a separation of concerns (viewmodel strictly for ui purpose, dto to pass only required data, and model for a database). I know it may seem too sophisticated and I could simply use one class for data representation but don't you think that implementing CQRS is also just making our app more complicated? I am sorry for stupid questions, I think's that's because I am beginner yet and just want to make a good software and get simple answers how to do that. Greetings from Poland:)

Not stupid question at all, I’d recommend to 1st keep it simple and only after try to predict realistically where do you need the separation to add the extra classes

Thank you for watching:D

Sorry I didn’t understand the question

@@RawCoding how to insert multiple list by using Mediator pattern & CQRS.

thank you for watching

No

MediatR can’t do that

@@RawCoding Can I write anywhere?

@@RawCoding But MediatR can ouput List of Cars, So I just wanna Output count of Cars

If you want to get a list of cars from the database you’ll need to use some kind a provider native or dapper or ef core. In ef core for example you can .Count() on a table

@@RawCoding OK. ThanX for Answers!!!)))

Mediatr is not meant for that. It’s just to call a handler - your handler then needs to save to database

Cheers

Yup thanks

You haven’t specified where to load handlers from

Inject it in to your handler

@@RawCoding there's a any demo there?

It’s standard dependency injection in dotnet core, if you don’t know anything about dotnet core watch my blog series

@@RawCoding sure thank you I will follow your blogs

I'm not sure what you mean. Just have your controller send a command to the API which MediatR can process

The scope of CQRS and Mediatr(library) is limited to server-side code. In Angular, half of the job(event-based communication) is already done when you use Observables, CQRS is completely dependent on how you shape your models.

if no can you please make other one for the publish event to the event bus and subscribers for that event :)

Don’t think you can, I would make a single handler that calls the other 2

JetBrains mono

@@RawCoding Thanks

Thanks, and it’s Anton

Really liked the last generic part.

@@RawCoding can you make video in docker with .net core api or with anything? also if you can have a video design patterns that you use often would be really helpful sir 🙂

Mmmm maybe some time in the future

What?

@@RawCoding like mediatr design pattern architect like that

Singleton

Some classes are to describe dumb data (entities). Some classes exist purely to hold functions (behaviour). It's generally bad idea to have a hybrid of the 2 - smart data, you don't want this because it harms your reasoning around the code base. Look up Data Context Interaction