> 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/exception-listeners.md).

# 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.

{% hint style="warning" %}
This feature **does not** **work** for an **in-memory** channel
{% endhint %}

***

### 🔍 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)

***

### 🛠️ How to Create One

To define an exception listener:

1. Implement the `ExceptionListener` interface
2. Decorate the class with `@MessagingExceptionListener()`

#### Example:

```ts
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.
  }
}
```

***

### 🧠 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 |

***

### 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              |


---

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