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Exception Watcher

The Exception Watcher captures unhandled exceptions and errors in your NestJS application, providing detailed stack traces and context for debugging.

What Gets Captured​

  • Exception name (class name)
  • Error message
  • Stack trace
  • HTTP status code (for HTTP exceptions)
  • Exception context (HTTP, RPC, WebSocket)
  • Request information (method, URL, body)
  • Request correlation ID

Configuration​

NestLensModule.forRoot({
  watchers: {
    exception: {
      enabled: true,
      ignoreExceptions: [
        'NotFoundException',
        'UnauthorizedException',
      ],
    },
  },
})

Configuration Options​

OptionTypeDefaultDescription
enabledbooleantrueEnable/disable exception tracking
ignoreExceptionsstring[][]Exception class names to ignore

Payload Structure​

interface ExceptionEntry {
  type: 'exception';
  payload: {
    name: string;                 // Exception class name
    message: string;              // Error message
    stack?: string;               // Stack trace
    code?: string | number;       // HTTP status or error code
    context: string;              // 'HTTP', 'RPC', 'WebSocket', or raw type
    request: {                    // Request info (always present in HTTP context)
      method: string;
      url: string;
      body?: unknown;
    };
  };
}

Usage Example​

The Exception Watcher automatically captures all unhandled exceptions:

@Controller('users')
export class UserController {
  @Get(':id')
  async findOne(@Param('id') id: string) {
    const user = await this.userService.findOne(id);

    if (!user) {
      // This exception will be captured automatically
      throw new NotFoundException(`User ${id} not found`);
    }

    return user;
  }

  @Post()
  async create(@Body() data: CreateUserDto) {
    try {
      return await this.userService.create(data);
    } catch (error) {
      // Custom exceptions are also captured
      throw new BadRequestException('Failed to create user');
    }
  }
}

Custom Exceptions​

// Custom exception class
export class BusinessRuleException extends HttpException {
  constructor(message: string) {
    super(message, HttpStatus.UNPROCESSABLE_ENTITY);
  }
}

// Will be captured with name: 'BusinessRuleException'
@Post('validate')
async validate(@Body() data: any) {
  if (!this.isValid(data)) {
    throw new BusinessRuleException('Data validation failed');
  }
}

Ignoring Expected Exceptions​

Some exceptions are expected and don't need tracking:

NestLensModule.forRoot({
  watchers: {
    exception: {
      ignoreExceptions: [
        'NotFoundException',        // 404 errors
        'UnauthorizedException',    // 401 errors
        'ForbiddenException',       // 403 errors
      ],
    },
  },
})

Dashboard View​

Exception Detail View

In the NestLens dashboard, exception entries appear in the Exceptions tab showing:

  • Timeline of all exceptions
  • Exception type badges
  • Stack trace viewer
  • Request context
  • Frequency charts (most common exceptions)
  • Error rate trends
  • Correlation with requests

Filters Available​

  • Filter by exception type/name
  • Filter by HTTP status code
  • Filter by context (HTTP, RPC, WS)
  • Search by error message
  • Filter by time range

Exception Analysis​

1. Find Most Common Errors​

The dashboard shows which exceptions occur most frequently:

// If you see "ValidationException" occurring frequently:
// - Check input validation rules
// - Add better client-side validation
// - Improve error messages

2. Debug with Stack Traces​

Each exception includes a full stack trace:

Error: User not found
    at UserService.findOne (user.service.ts:45:13)
    at UserController.getUser (user.controller.ts:23:32)
    at ...

3. Correlate with Requests​

Click on an exception to see:

  • The HTTP request that caused it
  • Query parameters and body
  • Related database queries
  • User who triggered the error

Exception Handling Best Practices​

1. Use Specific Exception Types​

// GOOD: Specific exception types
if (!user) {
  throw new NotFoundException('User not found');
}

if (!canAccess) {
  throw new ForbiddenException('Insufficient permissions');
}

// BAD: Generic errors
if (!user) {
  throw new Error('Something went wrong');
}

2. Include Context in Messages​

// GOOD: Descriptive message
throw new NotFoundException(
  `User with ID ${id} not found in organization ${orgId}`
);

// BAD: Vague message
throw new NotFoundException('Not found');

3. Handle Expected Errors​

@Post('upload')
async upload(@UploadedFile() file: Express.Multer.File) {
  try {
    return await this.processFile(file);
  } catch (error) {
    // Log but don't throw for expected errors
    if (error instanceof UnsupportedFileTypeError) {
      throw new BadRequestException(
        `File type ${file.mimetype} not supported`
      );
    }
    // Re-throw unexpected errors to be captured
    throw error;
  }
}

Exception Filters Integration​

NestLens works with custom exception filters:

@Catch(HttpException)
export class HttpExceptionFilter implements ExceptionFilter {
  catch(exception: HttpException, host: ArgumentsHost) {
    // Your custom exception handling
    const ctx = host.switchToHttp();
    const response = ctx.getResponse();
    const status = exception.getStatus();

    // Exception will still be captured by NestLens
    response.status(status).json({
      statusCode: status,
      message: exception.message,
      timestamp: new Date().toISOString(),
    });
  }
}

// Register the filter
app.useGlobalFilters(new HttpExceptionFilter());

Error Monitoring Alerts​

Use the Exception Watcher data to set up monitoring:

// Example: Check for error rate spike
const recentExceptions = await getExceptions({
  since: Date.now() - 5 * 60 * 1000, // Last 5 minutes
});

if (recentExceptions.length > 100) {
  // Alert: High error rate!
  await sendAlert('High exception rate detected');
}

Context Types​

Exceptions can occur in different contexts:

  • HTTP - From HTTP request handlers
  • RPC - From microservice RPC calls
  • WebSocket - From WebSocket event handlers

Each context provides different metadata in the captured exception.

Immediate Collection​

Exception entries are collected immediately (not batched) because they're critical for debugging. This ensures exceptions are saved even if the application crashes shortly after.