Polly VS MediatR

Polly

This is probably one of the most useful "cloud" patterns out there and it is fairly easy to implement. There are great articles and implementations, like Polly, already on the internet about this pattern so why another one?

Polly Documentation: Polly Official Site Circuit Breaker Design Pattern: Microsoft Learn Microservices Best Practices: Microservices on .NET

Custom HTTP handlers are well known as a mechanism to manage cross-cutting concerns around HTTP requests. The calling application has control over the HTTP handler pipeline, so it can be reconfigured, reordered, or even rebuilt from scratch. Decorating a client with a Token Management Handler or a custom Polly policy is easy... assuming the client accepts an HttpClient parameter in its constructor, and you haven't messed with the natural order of things by obstructing the client customization in some way (I really don't want to show how).

In our applications, we should wrap all requests to remote services in code that implements a retry policy that follows one of the strategies I listed earlier. If you are a .NET developer like myself, you may be familiar with the Polly library. Golang has a library called Retry, and there are numerous third-party libraries for Python and Java.

Another question that rises is, would it be better to use some HttpClient package to handle the requests, like Refit in combination with Polly. But then again, it seems Refit also uses the HttpClient factory, which was a bad thing according to the previous?

D'Oh! Sorry, not PolySharp. I meant Polly. Too many similarly-named libraries!

Everywhere I've worked with AGs, we've worked with the application team to add retry logic to help make things a bit more seamless to end users. There are libraries out there that can make this pretty easy - Polly is one that I've used a few times, but there are others.

Fallacy 1: The network is reliable. If system 2 works perfectly well, but is not accessible for service 1 due to network issues, service 2 is still unavailable. This is why timeouts, service breakers and retry policies exist. A great tool for .NET to handle common network issues is Polly, but even when using a tool like this, the network is still not completely reliable.

I see. Then I don't have any better idea right now, but I do want to suggest that if your script is mostly API calls and you want to be able to deal with failures then take a look at the polly library: https://github.com/App-vNext/Polly

MediatR – GitHub

MediatR GitHub Repository

MediatR: https://github.com/jbogard/MediatR

MediatR simplifies in-process messaging, CQRS, and domain events.

We need something to do the work of calling the tag repository and mapping to our dto. This will be a class known as a handler. The Web UI will pass a request - encapsulated as an object - to an instance of this handler. The handler will talk to the repository, map the result to a dto, and then return a response. We could (quite easily) write the code for all this ourselves. However there is an elegant library called MediatR that does this job already. Lets use it.

You probably noticed that .SetName() returns a Either. You may have come across Unit in libraries like MediatR or Language-Ext. It's a simple construct representing a type with only one possible value. We use it as a placeholder for operations that do not return a value but may return another state. In our example, .SetName() is a Command that does not return a value but may fail. Therefore, the monad Either carries two possible states: Right (without value) or Left (with an Error).

The widely used MediatR library[0] could be used to do that as well, just FYI.

[0]: https://github.com/jbogard/MediatR