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.

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🔍 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

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✨ Use Cases

• Logging message flow

• Validating payloads

• Injecting metadata or tracing headers

• Authenticating messages

• Handling common errors or retries

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🛠️ Creating a Middleware

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

Example: Logging Middleware

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.

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🔌 Attaching Middleware to a Channel

Middlewares are assigned per channel in your MessagingModule config:

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 {}

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⚙️ 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.

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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.