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middleware pipeline how to call next middleware ? where is the next() method defined? RRS feed

  • Question

  • User-1248632799 posted

    i've been following the micorosft documentation of asp.net core and i've found that in order to call the next middleware we use the app.use method () and i noticed that in the parameters it takes the next delegate and i cant find the definition of this delegate anywhere ? and how does it work , why when i call this next delegate it automatically calls the next middleware !!

    but how does the next delegate know how to call the next middleware ? from what i've learned when i have a delegate i should assign function to this delegate but the next delegate does not assign anyfunction ? 

    Monday, June 3, 2019 4:09 PM

All replies

  • User-854763662 posted

    Hi Ezio ,

    From ASP.NET Core Middleware ,  you could know that the middlewares are called in a pipeline. That means, there is one middleware which will start and which will pass on to the next middleware in the pipeline, which will then pass on to the next, and so on. Eventually, each middleware has a way to do something afterwards as the pipeline completes. The order that middleware are added in the  app.Use method defines the order in which the middleware are invoked on requests and the reverse order for the response. The order is critical for security, performance, and functionality.This generally looks like this:

    Request
    |            → Middleware1
    |              | run
    |              | next()     → Middleware2
    |              |              | run
    |              |              | next()     → Middleware3
    |              |              |              | run
    |              |              |              | next()     → {}
    |              |              |              | run after
    |              |              |              | return ↵
    |              |              | ←
    |              |              | run after
    |              |              | return ↵
    |              | ←
    |              | run after
    |              | return ↵
    | ←
    | ⇒ Send response

    Right-click on app.Use, "Go To Definition" -- that will have the signature, which will include the delegate type. Refer to here for the source code .

    public static IApplicationBuilder Use(this IApplicationBuilder app, Func<HttpContext, Func<Task>, Task> middleware);

    Note : That Func<Task> is the next parameter. next is a delegate which takes nothing, and returns a Task When the framework calls you middleware, it passes in next. When you call next, you tell the framework to call the next bit of middleware. When the framework calls the next bit of middleware, it will pass in a next. When the next bif of middleware calls next, it tells the framework to call the bit of middleware after that. 

    Reference : How the Next Delegate Works In ASP.NET Core Middleware

    Best Regards ,

    Sherry

    Tuesday, June 4, 2019 2:58 AM
  • User-2054057000 posted

    To invoke the next middleware we use:

    await nextDelegate.Invoke(httpContext);

    But how does the next delegate know how to call the next middleware ?

     The middleware gets the information of all the HTTP REQUEST inside it's Invoke() method:

    public async Task Invoke(HttpContext httpContext)
    {
    //...
    }

    The HttpContext  parameter is the one to look for.

    I Recommend you to read -  Middleware tutorial in ASP.NET CORE to understand it's working.

    Tuesday, June 4, 2019 3:29 AM
  • User-1038772411 posted

    Hii Ezio,

    ASP.NET Core Middleware

    Middleware is software that's assembled into an app pipeline to handle requests and responses. Each component:

    • Chooses whether to pass the request to the next component in the pipeline.
    • Can perform work before and after the next component in the pipeline.
    • The ASP.NET Core request pipeline consists of a sequence of request delegates, called one after the other. The following diagram demonstrates the concept. The thread of execution follows the black arrows.Request processing pattern showing a request arriving, processing through three middlewares, and the response leaving the app. Each middleware runs its logic and hands off the request to the next middleware at the next() statement. After the third middleware processes the request, the request passes back through the prior two middlewares in reverse order for additional processing after their next() statements before leaving the app as a response to the client.
    • Each delegate can perform operations before and after the next delegate. Exception-handling delegates should be called early in the pipeline, so they can catch exceptions that occur in later stages of the pipeline.
    • The simplest possible ASP.NET Core app sets up a single request delegate that handles all requests. This case doesn't include an actual request pipeline. Instead, a single anonymous function is called in response to every HTTP request.
      public class Startup
      {
          public void Configure(IApplicationBuilder app)
          {
              app.Run(async context =>
              {
                  await context.Response.WriteAsync("Hello, World!");
              });
          }
      }

      The first Run delegate terminates the pipeline.

      Chain multiple request delegates together with Use. The next parameter represents the next delegate in the pipeline. You can short-circuit the pipeline by not calling the next parameter. You can typically perform actions both before and after the next delegate, as the following example demonstrates:

      public class Startup
      {
          public void Configure(IApplicationBuilder app)
          {
              app.Use(async (context, next) =>
              {
                  // Do work that doesn't write to the Response.
                  await next.Invoke();
                  // Do logging or other work that doesn't write to the Response.
              });
      
              app.Run(async context =>
              {
                  await context.Response.WriteAsync("Hello from 2nd delegate.");
              });
          }
      }
      When a delegate doesn't pass a request to the next delegate, it's called short-circuiting the request pipeline. Short-circuiting is often desirable because it avoids unnecessary work.
    • The app.use method () can short-circuit the pipeline  (that is, if it doesn't call a next request delegate)

    For more Updates refer the following link

    https://docs.microsoft.com/en-us/aspnet/core/fundamentals/middleware/?view=aspnetcore-2.2

    Thank You.

    Tuesday, June 4, 2019 10:13 AM
  • User585649674 posted

    It is the order in which you configure. In the below example. First I am adding staticfiles to allow css, js files, second fileserver option to allow download of files from server, then the cors, then authorization which will populate the user object for request, then the MVC

    app.UseStaticFiles();

    app.UseFileServer(options);

    app.UseCors(corsoptions);

    app.UseAuthorization();

    app.UseMVC();

    These were some of the out of box middleware from microsoft. We can create custom middleware and insert at the point where you want to perform. e.g. if you want record the request start time. Then add a middleware which will log the current time and insert it above use authorization. If you want to wrap the exception into error message, you should have a middleware which will catch the exception after invoking next delegate. create a UI friendly json and insert above usemvc, You could refer to diagrams in previous post. In custom middleware you can have logic both before and after calling next item in pipeline.

    Wednesday, June 5, 2019 10:39 AM