The ASP.NET Core Web API request lifecycle

 The ASP.NET Core Web API request lifecycle involves several stages that handle the incoming HTTP request, process it, and generate a response. Here's a detailed breakdown of the request lifecycle in ASP.NET Core:

1. HTTP Request Received

The lifecycle starts when an HTTP request is received by the ASP.NET Core web server, which could be Kestrel, IIS, or another hosting model.

2. Hosting Layer

• Web Server (Kestrel/IIS): The web server listens for incoming HTTP requests. If the application is hosted behind IIS, the request first goes through IIS, which then forwards it to the Kestrel server.

3. Middleware Pipeline (Request Processing Pipeline)

ASP.NET Core uses a middleware pipeline to process requests. Middleware components are configured in the Startup class.

• Request Delegate: Each middleware component can either handle the request or pass it to the next middleware in the pipeline.

Here’s a typical middleware pipeline:

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler("/Home/Error");
        app.UseHsts();
    }

    app.UseHttpsRedirection();
    app.UseStaticFiles();
    app.UseRouting();
    app.UseAuthentication();
    app.UseAuthorization();
    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllers();
    });
}

4. Routing

• Endpoint Routing: Middleware uses routing to match the incoming request to a route defined in the application. If a route matches, the request is routed to the appropriate endpoint (e.g., a controller action).

5. MVC Middleware

If the request is routed to an MVC controller:

• Model Binding: The framework binds the incoming request data to the parameters of the action method.
• Action Filters: Any action filters (like Authorize, ValidateAntiForgeryToken, etc.) are executed.
• Action Execution: The action method on the controller is executed.
• Result Filters: Any result filters are executed.
• Result Execution: The result (usually an IActionResult, like Ok, NotFound, etc.) is executed to generate the response.

6. Model Validation

If model binding is successful, model validation is performed. Validation attributes (e.g., [Required], [Range]) are checked, and validation errors are collected if any.

7. Action Execution

The controller action method is executed, performing the necessary operations (e.g., querying the database, business logic) and returning an IActionResult.

8. Action Result Execution

The IActionResult returned from the controller action is executed to create the HTTP response. This could be a JSON result, a view, or any other content type.

9. Middleware Pipeline (Response Processing Pipeline)

The response generated by the action result is passed back through the middleware pipeline. Middleware components can inspect, modify, or short-circuit the response.

10. HTTP Response Sent

The final HTTP response is sent back to the client by the web server.

Summary of Middleware Pipeline

The middleware pipeline is a crucial aspect of the ASP.NET Core request lifecycle. Each middleware component has the opportunity to handle the request and the response. Middleware components are added in the Configure method of the Startup class and are executed in the order they are added.

Example Middleware Pipeline

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    app.Use(async (context, next) =>
    {
        // Do something before the next middleware
        await next.Invoke();
        // Do something after the next middleware
    });

    app.UseRouting();

    app.UseAuthentication();
    app.UseAuthorization();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllers();
    });
}