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The Wolverine community fields a lot of questions from people who are moving to Wolverine from their previous MediatR usage. A quite natural response is to try to use Wolverine as a pure drop in replacement for MediatR and even try to use the existing MediatR idioms they’re already used to. However, Wolverine comes from a different philosophy than MediatR and most of the other “mediator” tools it’s inspired and using Wolverine with its idioms might lead to much simpler code or more efficient execution. Inspired by a conversation I had online today, let’s just into an example that I think shows quite a bit of contrast between the tools.
We’ve tried to lay out some of the differences between the tools in our Wolverine for MediatR Users guide, including the section this post is taken from.
Here’s an example of MediatR usage I borrowed from this blog post that shows the usage of MediatR within a shopping cart subsystem:
public class AddToCartRequest : IRequest<Result>{ public int ProductId { get; set; } public int Quantity { get; set; }}public class AddToCartHandler : IRequestHandler<AddToCartRequest, Result>{ private readonly ICartService _cartService; public AddToCartHandler(ICartService cartService) { _cartService = cartService; } public async Task<Result> Handle(AddToCartRequest request, CancellationToken cancellationToken) { // Logic to add the product to the cart using the cart service bool addToCartResult = await _cartService.AddToCart(request.ProductId, request.Quantity); bool isAddToCartSuccessful = addToCartResult; // Check if adding the product to the cart was successful. return Result.SuccessIf(isAddToCartSuccessful, "Failed to add the product to the cart."); // Return failure if adding to cart fails. }} public class CartController : ControllerBase{ private readonly IMediator _mediator; public CartController(IMediator mediator) { _mediator = mediator; } [HttpPost] public async Task<IActionResult> AddToCart([FromBody] AddToCartRequest request) { var result = await _mediator.Send(request); if (result.IsSuccess) { return Ok("Product added to the cart successfully."); } else { return BadRequest(result.ErrorMessage); } }}Note the usage of the custom Result<T> type from the message handler. Folks using MediatR love using these custom Result types when you’re passing information between logical layers because it avoids the usage of throwing exceptions and communicates failure cases more clearly.
See Andrew Lock on Working with the result pattern for more information about the Result pattern.
Wolverine is all about reducing code ceremony and we always strive to write application code as synchronous pure functions whenever possible, so let’s just write the exact same functionality as above using Wolverine idioms to shrink down the code:
public static class AddToCartRequestEndpoint{ // Remember, we can do validation in middleware, or // even do a custom Validate() : ProblemDetails method // to act as a filter so the main method is the happy path [WolverinePost("/api/cart/add"), EmptyResponse] public static Update<Cart> Post( AddToCartRequest request, // This usage will return a 400 status code if the Cart // cannot be found [Entity(OnMissing = OnMissing.ProblemDetailsWith400)] Cart cart) { return cart.TryAddRequest(request) ? Storage.Update(cart) : Storage.Nothing(cart); }}There’s a lot going on above, so let’s dive into some of the details:
I used Wolverine.HTTP to write the HTTP endpoint so we only have one piece of code for our “vertical slice” instead of having both the Controller method and the matching message handler for the same logical command. Wolverine.HTTP embraces our Railway Programming model and direct support for the ProblemDetails specification as a means of stopping the HTTP request such that validation pre-conditions can be validated by middleware such that the main endpoint method is really the “happy path”.
The code above is using Wolverine’s “declarative data access” helpers you see in the [Entity] usage. We realized early on that a lot of message handlers or HTTP endpoints need to work on a single domain entity or a handful of entities loaded by identity values riding on either command messages, HTTP requests, or HTTP routes. At runtime, if the Cart isn’t found by loading it from your configured application persistence (which could be EF Core, Marten, or RavenDb at this time), the whole HTTP request would stop with status code 400 and a message communicated through ProblemDetails that the requested Cart cannot be found.
The key point I’m trying to prove is that idiomatic Wolverine results in potentially less repetitive code, less code ceremony, and less layering than MediatR idioms. Sure, it’s going to take a bit to get used to Wolverine idioms, but the potential payoff is code that’s easier to reason about and much easier to unit test — especially if you’ll buy into our A-Frame Architecture approach for organizing code within your slices.
Validation Middleware
As another example just to show how Wolverine’s runtime is different than MediatR’s, let’s consider the very common case of using Fluent Validation (or now DataAnnotations too!) middleware in front of message handlers or HTTP requests. With MediatR, you might use an IPipelineBehavior<T> implementation like this that will wrap all requests:
public class ValidationBehaviour<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse> where TRequest : IRequest<TResponse>
{
private readonly IEnumerable<IValidator<TRequest>> _validators;
public ValidationBehaviour(IEnumerable<IValidator<TRequest>> validators)
{
_validators = validators;
}
public async Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate<TResponse> next)
{
if (_validators.Any())
{
var context = new ValidationContext<TRequest>(request);
var validationResults = await Task.WhenAll(_validators.Select(v => v.ValidateAsync(context, cancellationToken)));
var failures = validationResults.SelectMany(r => r.Errors).Where(f => f != null).ToList();
if (failures.Count != 0)
throw new ValidationException(failures);
}
return await next();
}
}I’ve seen plenty of alternatives out there with slightly different implementations. In some cases folks will use service location to probe the application’s IoC container for any possible IValidator<T> implementations for the current request. In all cases though, the implementations are using runtime logic on every possible request to check if there is any validation logic. With the Wolverine version of Fluent Validation middleware, we do things a bit differently with less runtime overhead that will also result in cleaner Exception stack traces when things go wrong — don’t laugh, we really did design Wolverine quite purposely to avoid the really nasty kind of Exception stack traces you get from many other middleware or “behavior” using frameworks like Wolverine’s predecessor tool FubuMVC did 
Let’s say that you have a Wolverine.HTTP endpoint like so:
public record CreateCustomer( string FirstName, string LastName, string PostalCode){ public class CreateCustomerValidator : AbstractValidator<CreateCustomer> { public CreateCustomerValidator() { RuleFor(x => x.FirstName).NotNull(); RuleFor(x => x.LastName).NotNull(); RuleFor(x => x.PostalCode).NotNull(); } }}public static class CreateCustomerEndpoint{ [WolverinePost("/validate/customer")] public static string Post(CreateCustomer customer) { return "Got a new customer"; } [WolverinePost("/validate/customer2")] public static string Post2([FromQuery] CreateCustomer customer) { return "Got a new customer"; }}In the application bootstrapping, I’ve added this option:
app.MapWolverineEndpoints(opts =>{ // more configuration for HTTP... // Opting into the Fluent Validation middleware from // Wolverine.Http.FluentValidation opts.UseFluentValidationProblemDetailMiddleware();}Just like with MediatR, you would need to register the Fluent Validation validator types in your IoC container as part of application bootstrapping. Now, here’s how Wolverine’s model is very different from MediatR’s pipeline behaviors. While MediatR is applying that ValidationBehaviour to each and every message handler in your application whether or not that message type actually has any registered validators, Wolverine is able to peek into the IoC configuration and “know” whether there are registered validators for any given message type. If there are any registered validators, Wolverine will utilize them in the code it generates to execute the HTTP endpoint method shown above for creating a customer. If there is only one validator, and that validator is registered as a Singleton scope in the IoC container, Wolverine generates this code:
public class POST_validate_customer : Wolverine.Http.HttpHandler
{
private readonly Wolverine.Http.WolverineHttpOptions _wolverineHttpOptions;
private readonly Wolverine.Http.FluentValidation.IProblemDetailSource<WolverineWebApi.Validation.CreateCustomer> _problemDetailSource;
private readonly FluentValidation.IValidator<WolverineWebApi.Validation.CreateCustomer> _validator;
public POST_validate_customer(Wolverine.Http.WolverineHttpOptions wolverineHttpOptions, Wolverine.Http.FluentValidation.IProblemDetailSource<WolverineWebApi.Validation.CreateCustomer> problemDetailSource, FluentValidation.IValidator<WolverineWebApi.Validation.CreateCustomer> validator) : base(wolverineHttpOptions)
{
_wolverineHttpOptions = wolverineHttpOptions;
_problemDetailSource = problemDetailSource;
_validator = validator;
}
public override async System.Threading.Tasks.Task Handle(Microsoft.AspNetCore.Http.HttpContext httpContext)
{
// Reading the request body via JSON deserialization
var (customer, jsonContinue) = await ReadJsonAsync<WolverineWebApi.Validation.CreateCustomer>(httpContext);
if (jsonContinue == Wolverine.HandlerContinuation.Stop) return;
// Execute FluentValidation validators
var result1 = await Wolverine.Http.FluentValidation.Internals.FluentValidationHttpExecutor.ExecuteOne<WolverineWebApi.Validation.CreateCustomer>(_validator, _problemDetailSource, customer).ConfigureAwait(false);
// Evaluate whether or not the execution should be stopped based on the IResult value
if (result1 != null && !(result1 is Wolverine.Http.WolverineContinue))
{
await result1.ExecuteAsync(httpContext).ConfigureAwait(false);
return;
}
// The actual HTTP request handler execution
var result_of_Post = WolverineWebApi.Validation.ValidatedEndpoint.Post(customer);
await WriteString(httpContext, result_of_Post);
}
}I should note that Wolverine’s Fluent Validation middleware will not generate any code for any HTTP endpoint where there are no known Fluent Validation validators for the endpoint’s request model. Moreover, Wolverine can even generate slightly different code for having multiple validators versus a singular validator as a way of wringing out a little more efficiency in the common case of having only a single validator registered for the request type.
The point here is that Wolverine is trying to generate the most efficient code possible based on what it can glean from the IoC container registrations and the signature of the HTTP endpoint or message handler methods while the MediatR model has to effectively use runtime wrappers and conditional logic at runtime.
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