ASP.NET Core Middleware Pipeline

Mind Map Summary

• ASP.NET Core Middleware
  • Definition: A series of software components assembled into a pipeline to handle HTTP requests and responses.
  • The Pipeline Flow
    • Request: Travels down from Middleware 1 -> Middleware 2 -> Final Endpoint.
    • Response: Once generated, it travels back up in reverse: Final Endpoint -> Middleware 2 -> Middleware 1.
  • Middleware Capabilities
    1. Process Request: Read/Modify HttpContext.
    2. Pass Control: Call the next delegate to continue the chain.
    3. Short-Circuit: Exit early and return a response without calling next.
• Order of Execution
  • Registration order in Program.cs is strictly followed.
  • Authentication must precede Authorization.
  • Exception handling should usually be at the very top to catch downstream errors.

Core Concepts

1. What is Middleware?

Middleware is the heart of an ASP.NET Core application’s request processing. Every bit of functionality, from serving static files to routing and security, is implemented as a middleware component. It’s essentially a series of “decorators” around your business logic.

2. The Chain of Responsibility

The pipeline is built as a chain of RequestDelegate objects. When a request arrives, the first middleware is called. It does some work and then (usually) calls the next link. When the last middleware finishes (or someone short-circuits), the logic resumes after the next() call in each middleware, moving back up to the first one.

3. The Importance of Ordering

Ordering isn’t just a suggestion; it’s a requirement for security and stability.

• app.UseExceptionHandler(): Put this first to catch errors in everyone else.
• app.UseStaticFiles(): Put this early so image requests don’t waste time on authentication logic.
• app.UseAuthentication() before app.UseAuthorization(): You must know who is calling before you decide what they can do.

Practice Exercise

Create a custom middleware that logs the request and a second one that short-circuits. Demonstrate how flipping their order changes the application result.

Answer (Custom Middleware & Pipeline Analysis in C#)

1. The Custom Middleware Components

// 1. A simple logging middleware
public class LoggingMiddleware(RequestDelegate next, ILogger<LoggingMiddleware> logger)
{
    public async Task InvokeAsync(HttpContext context)
    {
        logger.LogInformation($"Processing request: {context.Request.Path}");
        await next(context); // Pass to the next middleware
    }
}

// 2. A middleware that short-circuits
public class EarlyExitMiddleware(RequestDelegate next)
{
    public async Task InvokeAsync(HttpContext context)
    {
        context.Response.StatusCode = 200;
        await context.Response.WriteAsync("Stopped early!");
        // We DO NOT call next(context)
    }
}

2. Analyzing the Order in Program.cs

Scenario A: Log then Exit

app.UseMiddleware<LoggingMiddleware>();
app.UseMiddleware<EarlyExitMiddleware>();

// Result:
// 1. Console logs the path.
// 2. Browser shows "Stopped early!".

Scenario B: Exit then Log

app.UseMiddleware<EarlyExitMiddleware>();
app.UseMiddleware<LoggingMiddleware>();

// Result:
// 1. Browser shows "Stopped early!".
// 2. Console LOGS NOTHING.
// The logging middleware was never even reached because the first one short-circuited.

Key Takeaway

Middleware execution is a linear journey. If any component fails to call next(), the pipeline effectively ends there. This is why tools like CORS or Authentication must be placed precisely; if they are too late, they might be bypassed; if they are too early, they might block valid but unauthenticated public static files.