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# ASP.NET Core Middleware Explained: Request Pipeline and Custom Middleware
Middleware components form the request pipeline in ASP.NET Core. Each component can inspect a request, perform work, call the next component, and optionally inspect the response.
## The basic idea
Conceptually, a pipeline looks like:
```text
Request
-> Middleware A
-> Middleware B
-> Endpoint
-> Middleware B
-> Middleware A
-> Response
```
Middleware can therefore perform work both before and after the next component runs.
## A simple custom middleware
```csharp
public class RequestTimingMiddleware
{
private readonly RequestDelegate _next;
public RequestTimingMiddleware(RequestDelegate next)
{
_next = next;
}
public async Task InvokeAsync(HttpContext context)
{
var started = DateTime.UtcNow;
await _next(context);
var elapsed = DateTime.UtcNow - started;
Console.WriteLine($"Request took {elapsed.TotalMilliseconds} ms");
}
}
```
Register the middleware with `app.UseMiddleware()`.
## Middleware order matters
Authentication, authorization, exception handling, static files, routing, and endpoints must be placed in an order appropriate to the behavior you want. Changing the order can change the application's behavior.
## Use middleware for cross-cutting concerns
Middleware is a good fit for concerns such as request logging, correlation IDs, exception handling, security headers, and timing.
Avoid putting endpoint-specific business rules into middleware when ordinary services or controllers are a better fit.
## Summary
Middleware is the mechanism that connects HTTP requests to application behavior through an ordered pipeline. Understanding that pipeline makes debugging and designing ASP.NET Core applications much easier.
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