Exception Listeners

❗ Exception Listeners

In a distributed or asynchronous messaging system, things can go wrong: a handler might throw an error, a payload might be malformed, or some service dependency might fail. That’s where Exception Listeners come in.

The @nestjstools/messaging package provides a powerful way to centrally manage and respond to unhandled exceptions thrown during message processing.

This feature does not work for an in-memory channel

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🔍 What is an Exception Listener?

An Exception Listener allows you to hook into the lifecycle of message processing and respond when any uncaught exception occurs during handling.

This is useful for:

• Logging errors

• Triggering alerting or monitoring workflows

• Performing recovery logic or fallback strategies

• Retrying logic (manual or delegated)

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🛠️ How to Create One

To define an exception listener:

1. Implement the ExceptionListener interface

2. Decorate the class with @MessagingExceptionListener()

Example:

import { Injectable } from '@nestjs/common';
import {
  ExceptionListener,
  MessagingExceptionListener,
  ExceptionContext,
} from '@nestjstools/messaging';

@Injectable()
@MessagingExceptionListener()
export class CustomExceptionListener implements ExceptionListener {
  async onException(context: ExceptionContext): Promise<void> {
    console.log(`Exception caught during message handling:`, context.error);

    // Optional: Log to external services, trigger notifications, retry logic, etc.
  }
}

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🧠 How It Works

• If an exception is thrown in any message handler or middleware during message execution:

  • It is propagated up to the listener

  • The onException(context) method is invoked with detailed metadata

The ExceptionContext includes:

Field	Description
error	The exception that was thrown
message	The message that caused the failure
channelName	The name of the channel where the error occurred
handler	(Optional) The handler class involved, if available
raw	Raw data passed to the channel before deserialization

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Summary

Feature	Description
🧹 Centralized Handling	Avoid scattering try/catch blocks in every handler
🔁 Recovery Support	Trigger fallback, retries, or compensating transactions
📈 Observability	Report to monitoring tools like Sentry, Datadog, or Logstash
🛠 Cleaner Code	Keep message handlers focused on business logic