Web API Pipeline Revealed: A True Practical Approach

Let’s verify that how does WebApi respond from ChallengeAsync with Unauthorized status. Press F5 and see WebApi is running after getting below screen shot:



Send the Get request in order to verify the execution cycle as given below. Copy and paste the following Url (http://localhost:57888/api/employees/GetEmp/5) in PostMan tool as depicted below in screen shot.



As soon as you click on the send button it reaches to Custom value provider’s method to execute set of statement and gives you following output in an image shown below:



Model Binders, Value Providers



Model binding uses the request to create values for the parameters of the action .These values are passed to the action when the action is invoked or binding is a mechanism used by the WebApi to mapping request to an object defined in the controller. Model binding is the process of creating .NET objects using the data sent by the browser in an HTTP request. We have been relying on the model binding process each time defines an action method that takes a parameter. the parameter objects are created by model binding. Model binding is ASP.NET mechanism for mapping HTTP request data directly into action method parameters and custom .Net objects.

• For example, when we receive a request for a URL such as "/Home/ Employees/23", the framework must map the details of the request in such a way that it can pass appropriate values or objects as parameters to our action method. A model binder gets raw input values from a value provider and later value provider takes the HTTP request and populates a dictionary of key-value pairs. Further the model binder uses this dictionary to populate the model.

The action invoker, the component that invokes action methods, is responsible for obtaining values for parameters before it can invoke the action method.

Value Providers

Value Providers are the components that feed data to model binders. Feeding the data means installing the data to the Model binder for further use at the action level. Value provider takes the HTTP request and populates a dictionary of key-value pairs which is later used by model binder to populate the model.

The framework contains a few built-in value providers named FormValueProvider, RouteDataValueProvider, QueryStringValueProvider and HttpFileCollectionValueProvider that fetch data from Request.Form, Request.QueryString, Request.Files and RouteData.Values.

The code shown below for CustomValueProvider used at this application level as well as way to use it at action level in an image shown below:

At the time of the model binding the DefaultModelBinder checks with the value providers to determine if they can return a value for the parameter (example empId) by calling the ContainsPrefix method. If none of the value providers registered can return then it checks through CustomtvalueProvider whether such a parameter is stored and if yes it returns the value.

This is one of the best pictorial representations taken from http://asp.net site is shown below which is again self-explanatory.

Image taken from http://asp.net



Kindly refer this link for more understanding on Model Binder Invoke Action with Model Binders and Fetch Header Information Using CustomValueProvider in ASP.NET Web API

IHttpActionInvoker & Action Filters

Once ApiControllerActionInvoker selects an action to handle an HTTP request, and is responsible for producing HttpResponse of your action. The action invoker, the component that invokes action methods, is responsible for obtaining values for parameters before it can invoke the action method. In other words, the invoker gets an instance of HttpActionContext and is expected to produce an HttpResponseMessage out of it. For this purpose it has to invoke an ExecuteAsync method on the

HttpActionDescriptor present on the HttpActionContext

Action Invoker can be a point to manage your exception at global level and returns a response on basis of the result returned by action method of controller. There is an interface shown given below to implement custom action invoker which has method to implement as shown below:
There can be three possible scenarios which can be maintained at ActionInvoker level,

1. You action returns an object than it has to convert into HttpResponseMessgae by IActionResultConverter and run content negotiation with the help of request.createResponse().
   
   
2. If action returns IHttpActionResult than it can be convert into HttpResponseMessage at action invoker level. Which gives privilege to mold your response at this level?
   
   
3. It can return HttpResponseMessage directly from action method.

I’ve created Custom action invoker at my application level to achieve Exception Handling after returning result by the action and later apply code at action invoker level to change it into HttpResponseMessege.

Kindly find a screen shot and code segment to register CustomActionInvoker in an application as shown below:


Action Filters

Custom filters and attributes are an excellent way to inject extra processing logic into the MVC request response pipeline. At some level programmer would be happy to inject some pre-processing or post-processing logic for actions and controllers. In that case we use filters.
 
This filter will be called before and after the action starts executing and after the action has executed. It comes under namespace using System.Web.Http.Filters;

OnActionExecuting occurs just before the action method is called.

OnActionExecuted occurs after the action method is called, but before the result is executed (before the view is rendered).
I’ve also added Exception filter to manage exception at code level an the respective code shown below:
Kindly find a way to register an Exception filter in global.asax file as given below as well as uses at controller’s action level.




IHttpActionInvoker can be applying to manage Exception at action level.

IHttpActionInvoker can be used to read runtime context sent by action and made further amendment to send it further as HttpResponseMessage.You may learn more about Action filters in WebApi from this link.

Result Conversation

As name implies Result conversation is about to convert return value from action in form of HttpResponseMessege.The result returned by the an action can be in multiple form and those forms are given below with pictorial representation taken from

Image taken from http://asp.net

1. Bullet Points. HttpRequest first converts into HttpRequestMessege.
   
   
2. HttpHandler are type of delegate handler and inject to verify the authenticity and to add some preprocessing logic before it goes further in pipeline. Delegate Handler can be creating on per-route basis. If come error/issue occurs at this level can skip the rest of pipeline.
   
   
3. Authentication filters have been introduced in WebApi2.If come error/issue occurs at this level can skip the rest of pipeline.
   
   
4. Mostly classes are derived from using System.Web.Http.
   
   
5. Model Binders and Value Provides plays an important role in parameter binding as well as the added benefit of FormatterParameterBinding.
   
   
6. WebApi2 gives privilege to return IHttpActionResult from action method of controller. Also supports HttpResponseMessege.
   
   
7. You can manage exception at IHttpAcitonInvoker and at ExceptionFilters also.
   
   
8. This is the final structure of an application given below in screen shot.
   
   
   

Hope you would read all contexts and liked it.

Disclaimer

This is all my understanding about WebApi; I’d be happy if you run this sample application and get a chance to share your opinion.