Understanding Middleware

Middleware runs around endpoint execution and lets you apply behavior to HTTP requests as they move through a Mach application.

It is useful for concerns that should apply across multiple endpoints, such as logging, request validation, security checks, or response processing.

The Request Pipeline

When a request reaches a Mach application, it passes through the registered middleware pipeline before reaching the matched endpoint.

Each middleware can:

inspect or modify the current request
inspect or modify the current response
continue to the next step in the pipeline
stop the pipeline and produce a response immediately

Conceptually, the flow looks like this:

Request
  ↓
Middleware A
  ↓
Middleware B
  ↓
Endpoint
  ↓
Middleware B
  ↓
Middleware A
  ↓
Response

Middleware therefore runs both before and after the next step in the pipeline, depending on where its logic is placed.

Before and After the Endpoint

A middleware can perform work before continuing:

// Before the next middleware or endpoint

next();

// After the next middleware or endpoint

This makes middleware useful for behavior that surrounds request handling.

For example, a logging middleware could record when a request starts, continue the pipeline, and then record the final response status.

Continuing the Pipeline

Middleware normally continues processing by invoking the next step in the pipeline.

If it does not continue, later middleware and the matched endpoint are not executed.

This is called short-circuiting the pipeline and is useful when a request should be handled immediately, such as when a required condition is not satisfied.

Short-circuiting is covered in detail later in this section.

Middleware Order

Middleware executes in the order in which it is registered on the way into the pipeline.

After the endpoint runs, control returns through the middleware in reverse order.

For example, with:

Middleware A
Middleware B
Endpoint

the execution order is:

A before
B before
Endpoint
B after
A after

Because of this, registration order can affect application behavior.

When to Use Middleware

Middleware is a good fit for behavior that applies across multiple endpoints or needs to run as part of the request pipeline.

Typical examples include:

request and response logging
authentication or authorization checks
CORS handling
CSRF protection
exception handling
request preprocessing
response headers or other shared response behavior

Endpoint-specific business logic generally belongs in the endpoint itself rather than in middleware.

Middleware and Endpoints

Middleware works with both minimal APIs and controllers.

The middleware pipeline is independent of how the matched endpoint is defined, so the same middleware can apply to endpoints created with app.mapGet(), controllers, and other route mappings.

Next, you’ll learn how to create a custom middleware in Mach.