Error Handling Architecture

Purpose

Uncaught exceptions crash servers. Leaked errors expose vulnerabilities. This skill creates a central, robust pipeline for capturing, categorizing, logging, and responding to failures without leaking sensitive data, ensuring system resilience and debugging capability.

When to use

• Setting up a new backend service
• Refactoring code with excessive uncoordinated try/catch blocks
• Standardizing API error responses across a team
• Implementing a new microservice with shared error handling

When NOT to use

• Client-side error handling (different context)
• Monitoring/alerting setup (separate concern, use Logging & Observability instead)
• Specific framework error documentation

Inputs required

• Existing backend service codebase
• Framework error handling capabilities (Express middleware, Django signals, etc.)
• Logging infrastructure setup

Workflow

1. Define Error Classes: Create base AppError class with statusCode, isOperational flag, and metadata
2. Categorize Errors: Distinguish Operational (bad input, network) from Programmer (null ptr, logic bug)
3. Centralize Middleware: Add framework-level error handler to catch ALL exceptions globally
4. Log Appropriately: Full stack for 5xx errors; metadata only for 4xx errors
5. Sanitize Output: Strip stack traces and internal details before sending to client
6. Crash on Programmer Error: Unhandled programmer errors MUST crash and restart the process
7. Test Error Paths: Verify error handling works for all failure scenarios

Rules

• MUST NEVER silently swallow exceptions (catch(e) {})
• MUST crash and restart process for unhandled Programmer Errors
• MUST standardize all HTTP error responses to RFC 7807 format
• MUST log full stack traces for 5xx errors
• MUST log only message and context for 4xx errors
• MUST sanitize all error responses (no DB details, SQL, stack traces)
• MUST NEVER throw string literals (always throw Error objects)
• MUST distinguish between Operational and Programmer errors

Anti-patterns

• Throwing Strings: throw "User not found" (throw Error objects always)
• Leaking DB Details: Sending raw SQL constraint violation messages to client
• Silent Catching: catch(e) {} with no logging or action
• 200 OK for Errors: HTTP 200 with { error: true } payload
• Generic Messages: "Something went wrong" (unhelpful for debugging)
• Unhandled Promises: Async functions without catch handlers

Failure conditions

• Unhandled exception occurs (process crashes without logging)
• Error response includes stack trace or internal details
• Silent exception swallowing (error never logged)
• HTTP 200 returned for failed requests
• Database constraint violation message sent to client

Validation checklist

• All errors are Error objects (never strings)
• Global error middleware catches all exceptions
• Error responses use RFC 7807 format with type, title, detail, status
• Stack traces never sent to client
• Sensitive data (DB queries, credentials) never in error responses
• Operational errors return appropriate 4xx status codes
• Programmer errors crash and restart
• Full stack traces logged for 5xx errors
• Request context (user ID, correlation ID) included in logs
• No catch(e) {} blocks with no action

Output format

• Error Object: Contains statusCode, message, isOperational flag, and optional context
• Response Format: RFC 7807 Problem Details JSON
• Log Format: Structured JSON with timestamp, level, context, stack trace (for 5xx)
• Process Behavior: Unhandled programmer errors crash; operational errors continue

Security considerations

• Error messages MUST NOT leak internal architecture (DB names, file paths, versions)
• Stack traces MUST NEVER be sent to clients (log internally only)
• Database error messages MUST be wrapped (never expose raw constraints)
• Sensitive user data MUST NOT appear in error logs (sanitize before logging)
• Error pages MUST NOT reveal system information

Agent execution notes

• Agent MAY: Create error classes, add global error middleware, sanitize responses, implement error logging
• Agent MUST NEVER: Throw strings, catch exceptions silently, leak stack traces, return 200 for errors
• Agent MUST ASK: Before changing error categorization, before modifying global error handler
• Agent MUST VALIDATE: All errors are Error objects, no stack traces in responses, proper categorization

Example

❌ Anti-pattern (Silent catching, leaking details, wrong status):

app.get('/users/:id', async (req, res) => {
  try {
    const user = await db.query(`SELECT * FROM users WHERE id = ${req.params.id}`);
    res.json(user);
  } catch(e) {
    // ANTI-PATTERN: silent catch
    res.json({ status: 'error', message: e.message }); // ANTI-PATTERN: 200 OK
    // ANTI-PATTERN: leaking SQL details
  }
});

✅ Correct pattern (Centralized, sanitized, proper codes):

// 1. Define error class
class AppError extends Error {
  constructor(statusCode, message, isOperational = true) {
    super(message);
    this.statusCode = statusCode;
    this.isOperational = isOperational;
  }
}

// 2. Route handler
app.get('/users/:id', async (req, res, next) => {
  try {
    const user = await db.query('SELECT * FROM users WHERE id = ?', [req.params.id]);
    if (!user) {
      return next(new AppError(404, 'User not found'));
    }
    res.json(user);
  } catch (err) {
    next(err); // Pass to global handler
  }
});

// 3. Global error middleware
app.use((err, req, res, next) => {
  // Log with context
  logger[err.isOperational ? 'warn' : 'error']({
    message: err.message,
    statusCode: err.statusCode,
    stack: err.stack,
    requestId: req.id,
    userId: req.user?.id
  });
  
  // Crash on programmer error
  if (!err.isOperational) {
    process.exit(1);
  }
  
  // Sanitized response
  res.status(err.statusCode || 500).json({
    type: 'https://api.example.com/errors/app-error',
    title: err.statusCode === 404 ? 'Not Found' : 'Server Error',
    detail: err.isOperational ? err.message : 'Internal server error',
    status: err.statusCode || 500
  });
});