ASP.NET Core Anatomy (Part 4) – Invoking the MVC Middleware Dissecting and understanding the internals of ASP.NET Core

In the first three parts of this series I looked at what happens when we call AddMvcCore, AddMvc and UseMvc as part of the application startup. Once the MVC services and middleware have been registered in our ASP.NET Core application, MVC is now available to handle HTTP requests.

In this post I want to cover the initial steps that happen when a request flows into the MVC middleware. This is quite a complex area to split up and write about. I’ve broken it out into what I consider a reasonable flow through the code, ignoring for now certain branches of behaviour and details to constrain this post to something that I hope is digestible in one sitting. Where I gloss over deeper implementation details I will highlight these cases and will write about those elements in future posts.

As with my earlier blog posts, I’ve used the original project.json version (1.1.2) of the MVC source code, since I’ve still not found a reliable way to debug through the source of MVC while including source from the other components such as Routing.

So, let’s begin and look at how MVC matches a request through to an available route and finally to an action which can handle that request. As a quick refresher, the middleware pipeline, configured in Startup.cs file of an ASP.NET Core application defines the flow of the request handling. Each middleware component will be invoked in order, until one of the middleware components determines it can provide a suitable response.

I’m using the MvcSandbox, included with the MVC solution to work with MVC while I write this series. The Configure method looks like this:

Assuming none of the earlier middleware handles the request, we reach the MVC pipeline and middleware, included in our pipeline with the UseMvc call. Once the request reaches the MVC pipeline, the middleware we hit is the RouterMiddleware. Its Invoke method looks like this:

The first thing that Invoke does is construct a new RouteContext passing in the current HttpContext object into the constructor.

The HttpContext is used to set a parameter on the RouteContext class and then a new RouteData object is instantiated and set.

Back inside Invoke; the injected IRouter, in this case the RouteCollection that was created during the UseMvc setup, is added to a List of IRouter objects on the RouteContext.RouteData object. Something worth highlighting is that the RouteData object uses a late initialisation pattern for its collections, only allocating them if they are called. This pattern shows the consideration that has to be made for performance inside a large framework such as ASP.NET Core.

For example, here is how the Routers property is defined:

The first time this property is accessed, a new List will be allocated and stored in the backing field.

Back in Invoke; RouteAsync is called on the RouteCollection.

The first thing the RouteAsync implementation does on the RouteCollection is to create a RouteDataSnapshot. As the comment indicates, allocating a RouteData object is not something that should happen for every route being processed. To avoid that, the snapshot of the RouteData object is created once and this allows it to be reset for each iteration. This is another example of where the ASP.NET Core team have coded with performance in mind.

The snapshot is achieved by calling PushState on the RouteData class.

The first thing it does is create a List<IRouter>. To be as performant as possible it only allocates a list if the private field containing the RouteData routers has at least one IRouter in it. If so, a new list is created, with the correct size to avoid internal Array.CopyTo calls occurring to resize the Lists underlying array. Essentially this method now has a copied instance of the RouteData’s internal IRouter list.

Next a RouteDataSnapshot object is created. RouteDataSnapshot is defined as a struct. Its constructor signature looks like this:

The RouteCollection calls PushState with null values for all of the parameters. In cases where the PushState method be called with a non-null IRoute parameter then it gets added to the Routers list. Values and DataTokens are processed in the same way. If any are included on the PushState parameters, the appropriate items in the Values and DataTokens properties on RouteData are updated.

Finally, the snapshot is returned to RouteAsync inside the RouteCollection.

Next a for loop is started which will loop up to the Count property value. Count simply exposes the Count of the Routes (List<IRouter>) on the RouteCollection.

Inside the loop it first gets a route using the value of the loop. This calls to

So this simply returns an IRouter from the List for the required index. In the MvcSandbox example, the first IRouter in the list at index 0 is the AttributeRoute.

Inside a Try/Finally block, the RouteAsync method is called on the IRouter (the AttributeRoute). Ultimately what’s happening here is we’re hoping to find a suitable Handler (a RequestDelegate) that matches the route data.

We’ll look into more detail about what happens inside the AttributeRoute.RouteAsync method in a future post as there’s a lot happening there and at the moment we’ve not defined any AttributeRoutes within the MvcSandbox. In our case, because there are no AttributeRoutes defined, the Handler value remains null.

Inside the finally block, when the Handler is null, the Restore method is called on the RouteDataSnapshot. This method will restore the RouteData object back to its state when the snapshot was created. Because the RouteAsync method may have modified RouteData during processing, this ensures that we are back to an initial state for the object.

Within the MvcSandbox example, the second IRouter in the list is the named “default” route which is an instance of Route. This class does not override the RouteAsync method on the base, so in this case, the code inside the RouteBase abstract class is called.

First it calls a private method, EnsureMatcher which looks like this:

This method will instantiate a new TemplateMatcher, passing in two parameters. Again, this appears to be an allocation optimisation to ensure that this object is only created at the point it is needed, after the properties which are passed to its constructor are available.

In case you’re wondering where those properties are set; that happens inside the constructor of the RouteBase class. This constructor was called when the default Route was first created as a result of calling MapRoute inside the UseMvc extension, called by the Configure method of the MvcSandbox Startup class.

Inside the RouteBase.RouteAsync method, the next call is to EnsureLoggers which looks like this:

This method gets the ILoggerFactory instance from the RequestServices and uses it to setup two ILoggers. The first is for the RouteBase class itself and the second will be used by the RouteConstraintMatcher.

Next it stores a local variable holding the path of the request being made, retrieved from the HttpContext.

Next, TryMatch on the TemplateMatcher is called, passing in the path of the request, along with any route data values. We’ll dive into the internals of TemplateMatcher in another post. For now, assume that TryMatch returns true, which is the case in our example. In cases where a match is not made a TaskCache.CompletedTask is returned. This is just a Task that is already set as completed.

Next, if there are any DataTokens (RouteValueDictionary objects) set, then the values on the context.RouteData.DataTokens are updated as necessary. As the comment suggests, this is only done if there are actually any values to be updated. The DataTokens property on RouteData is only created when its first called (lazily instantiated). Therefore, calling it when there are no values to update would risk allocating a new RouteValueDictionary when it may not be needed.

In our demo using MvcSandbox, there are no DataTokens so MergeValues is not called. However, for completeness, here is its code:

When called it loops any values in the RouteValueDictionary from the DataTokens parameter on the RouteBase class and updates the destination value for the matching key on the context.RouteData.DataTokens property.

Next, back in RouteAsync, RouteConstraintMatcher.Match is called. This static method loops over any IRouteContaints that are passed in and determines if they have all been met. Route constraints allow routes to be configured with additional matching rules. For example, a route parameter can be constrained to only match for integer values. In our case there are no constraints on our demo route so it returns true. We’ll look at a URL with constraints in another post.

A logger entry is made via an extension method on ILogger called MatchedRoute. This is an interesting pattern that makes it simple to reuse more complex formatting of logging messages for specific needs.

This class looks like this:

An action delegate is defined when the TreeRouterLoggerExtensions class is first constructed that defines how the logged message will be formatted.

When the MatchedRoute extension method is called, the route name and template string are passed as parameters. They are then passed to the _matchedRoute Action. This action creates a debug level log entry using the provided parameters. When debugging inside visual studio you will see this appear in the Output window.

Back inside RouteAsync; the OnRouteMatched method is called. This is defined as an abstract method on the RouteBase, so the implementation comes from the inheriting class. In our case it’s the Route class. Its OnRouteMatched override looks like this:

Its IRouter field called _target is added to the context.RouteData.Routers list. In this case it’s an MvcRouteHandler which is the default handler for MVC.

The RouteAsync method is then called on the MvcRouteHandler. The details of what this does are quite involved so I’ll save those to be the topic of a future post. In summary, MvcRouteHandler.RouteAsync will try to establish a suitable action method that should handle the request. One thing that is important to know for the context of this blog post is that when an action is found, the Handler property on the RouteContext is defined via a lambda expression. We can dive into that code another time but to summarise, a RequestDelegate is a function that accepts a HttpContext and can process a request.

Back inside the Invoke method on the RouterMiddleware, we might have a handler that MVC has determined can handle the requested route. If not then _logger.RequestDidNotMatchRoutes() is called. This is another example of the logger extension style we explored earlier. It will add a debug message, indicating that the route was not matched. In that case, the next RequestDelegate in the ASP.NET middleware pipeline is invoked because MVC has determined it cannot handle the request.

In common configurations for client web/api applications you will not have any middleware defined after UseMvc. In that case as we have reached the end of the pipeline a default 404 not found HTTP status code response is returned by ASP.NET Core.

In cases where we have a handler that can handle the route of the request, we flow through into the Invoke methods else block.

Here a new RoutingFeature is instantiated and added to the Features collection on the HttpContext. Briefly; features are a concept of ASP.NET Core that allows the server to define features that the request supports. This includes flowing of data through the lifetime of a request. Middleware such as the RouterMiddleware can add/modify the features collection and can use this as a mechanism to pass data through the request. In our case, the RouteData from the RouteContext is added as part of the IRoutingFeature definition, so that it can be used by other middleware and request handlers.

The method then invokes the Handler RequestDelegate which will ultimately process the request via the appropriate MVC action. At this point I’ll wrap up this post. What happens next, along with some of the other items I skipped over will form the next parts in this series.

Summary

We’ve seen how MVC is actually invoked as part of the middleware pipeline. Upon invocation, the MVC RouterMiddleware determines if MVC knows how to handle the incoming request path and values. If MVC has an action available that is intended to handle the route and route data on the request, then this handler is used to process the request and provide the response.

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