> For the complete documentation index, see [llms.txt](https://docs.nestjstools.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.nestjstools.com/components/middlewares.md).

# Middlewares

## 🧵 Middlewares

In `@nestjstools/messaging` **Middlewares** are functions or classes that act on messages **before** they reach their respective handlers. They are ideal for adding **logging, validation, authentication, transformation, or metrics collection** to your messaging flow.

Each **channel** has its own middleware stack, and the middlewares are executed **in order**, just like an HTTP middleware pipeline.

***

### 🔍 What Does a Middleware Do?

A middleware has access to:

* The **incoming message** (`RoutingMessage`)
* The **execution context**
* The ability to:
  * Modify the message
  * Stop message processing
  * Forward the message to the next middleware or the final handler

***

### ✨ Use Cases

* Logging message flow
* Validating payloads
* Injecting metadata or tracing headers
* Authenticating messages
* Handling common errors or retries

***

### 🛠️ Creating a Middleware

Define a class that implements the `Middleware` interface and use the `@MessagingMiddleware()` decorator.

#### Example: Logging Middleware

```ts
import { Injectable } from '@nestjs/common';
import { Middleware, MessagingMiddleware, RoutingMessage, MiddlewareContext } from '@nestjstools/messaging';

@Injectable()
@MessagingMiddleware()
export class TestMiddleware implements Middleware {
  async process(message: RoutingMessage, context: MiddlewareContext): Promise<MiddlewareContext> {
    console.log('!!!! WORKS');
    
    // Continue processing the pipeline
    return await context.next().process(message, context);
  }
}
```

> 🔁 **Important:** Always call `context.next().process(...)` to continue down the middleware chain.

***

### 🔌 Attaching Middleware to a Channel

Middlewares are assigned per channel in your MessagingModule config:

```ts
import { MessagingModule, InMemoryChannelConfig, AmqpChannelConfig, ExchangeType } from '@nestjstools/messaging';
import { TestMiddleware } from './middlewares/test.middleware';

@Module({
  imports: [
    MessagingModule.forRoot({
      buses: [
        {
          name: 'message.bus',
          channels: ['my-channel'],
        },
      ],
      channels: [
        new InMemoryChannelConfig({
          name: 'my-channel',
          middlewares: [TestMiddleware],
        }),
      ],
      debug: true,
    }),
  ],
})
export class AppModule {}
```

***

### ⚙️ Middleware Pipeline

When a message is dispatched to a channel:

1. It passes through the **middleware stack**, in the order they’re defined.
2. Each middleware can:
   * Modify the message
   * Stop further processing
   * Pass the message on via `context.next().process(...)`
3. The final destination is the **message handler**.

***

### Benefits

| Feature                       | Description                                          |
| ----------------------------- | ---------------------------------------------------- |
| 🔄 **Reusable Logic**         | Write once, apply to multiple channels               |
| 🧼 **Separation of Concerns** | Keep logging/validation out of core handler logic    |
| 🔐 **Security & Validation**  | Centralize authentication and schema checks          |
| 🛠 **Customizable**           | Easily extend for metrics, tracing, throttling, etc. |


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