Middleware Pipeline in .NET – What I Learned After Building Real APIs

When I first built APIs using ASP.NET Core, I copied middleware configuration from templates without thinking much about it.

Everything worked — until it didn’t.

Authentication failed. Routing behaved strangely. Some requests never reached controllers.

That’s when I properly learned how the Middleware Pipeline actually works.

This article explains what I wish I understood earlier.

A Simple Story to Understand Middleware

Imagine entering a corporate office.

Before meeting the manager, you must:

1.     Pass security

2.    Verify your ID

3.    Register at reception

4.    Get routed to the right department

At any step, you might be stopped.

ASP.NET Core works exactly like this.

Every HTTP request passes through a series of components called middleware. Each one can:

·         Inspect the request

·         Modify it

·         Pass it forward

·         Or stop it completely

What is Middleware?

Middleware is simply a component that sits in the HTTP pipeline.

It has access to:

·         The incoming request

·         The outgoing response

·         The next middleware in the chain

Basic structure:

public async Task InvokeAsync(HttpContext context, RequestDelegate next)
{
    // Before next middleware

    await next(context);

    // After next middleware
}

That next(context) call is everything.

If you don’t call it, the pipeline stops there.

How the Pipeline is Built

In .NET 6+ (Program.cs):

app.UseHttpsRedirection();
app.UseRouting();
app.UseAuthentication();
app.UseAuthorization();
app.MapControllers();

This is not just configuration.

This defines the exact path every request will take.

Order matters more than most developers realize.

What I Learned About Order (The Hard Way)

Early in one project, authentication wasn’t working properly.

The issue?

I had:

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

Authorization was running before authentication.

So the system was trying to authorize a user who wasn’t identified yet.

Correct order:

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

This seems small, but in production, it can break security.

Real-World Use Cases for Middleware

After working on enterprise APIs, I’ve seen middleware commonly used for:

• Global exception handling

• Request and response logging

• Performance tracking

• JWT validation

• Correlation IDs

• Security headers

• Rate limiting

Middleware is perfect for cross-cuting concerns.

If something must run for every request, middleware is usually the right place.

Creating a Custom Middleware (Practical Example)

Example: simple logging middleware

public class LoggingMiddleware
{
    private readonly RequestDelegate _next;

    public LoggingMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task InvokeAsync(HttpContext context)
    {
        Console.WriteLine("Request started");

        await _next(context);

        Console.WriteLine("Request finished");
    }
}

Register it:

app.UseMiddleware<LoggingMiddleware>();

Now every request flows through it.

Short-Circuiting the Pipeline

One powerful feature is stopping the pipeline intentionally.

Example:

app.Use(async (context, next) =>
{
    if (!context.Request.Headers.ContainsKey("X-API-KEY"))
    {
        context.Response.StatusCode = 401;
        await context.Response.WriteAsync("Unauthorized");
        return;
    }

    await next();
});

If the condition fails, the request never reaches controllers.

This is useful for:

• API key validation

• Maintenance mode

• IP filtering

Common Mistakes I’ve Seen

1. Wrong middleware order
   Leads to authentication and routing bugs.

2. Putting business logic inside middleware
   Middleware should handle infrastructure concerns, not domain rules.

3. Performing heavy database operations
   Middleware runs for every request. Keep it lightweight.

4. Forgetting that response also flows back
   Code after await next(context) runs during the response phase.

Middleware vs Filters

This confused me early on.

Middleware:

• Runs before MVC

• Applies globally

• Works on HttpContext level

Filters:

• Run inside MVC

• Are action/controller specific

If something is cross-cutting and global → middleware is usually correct.

How This Fits in Clean Architecture

In a layered application:

• Middleware belongs to Infrastructure.

• Business logic belongs to Application/Domain.

• Middleware should not know about business rules.

Keeping that separation prevents messy architecture later.

Why Understanding Middleware Changes Debugging

Once I understood the pipeline properly:

• I could trace request flow clearly

• I could identify where requests were being blocked

• Security issues became easier to fix

• Logging became centralized

It stopped feeling “magical” and started feeling predictable.

Final Thoughts

Middleware is not just configuration code in Program.cs.

It defines how your entire application behaves.

If you understand the pipeline, you understand the request lifecycle in ASP.NET Core.

And once that clicks, a lot of confusion disappears.