Error Handling Patterns

Ship resilient software. Handle errors at boundaries, fail fast and loud, never swallow exceptions silently.

Error Handling Philosophy

| Principle | Description |

|-----------|-------------|

| Fail Fast | Detect errors early — validate inputs at the boundary, not deep in business logic |

| Fail Loud | Errors must be visible — log them, surface them, alert on them |

| Handle at Boundaries | Catch and translate errors at layer boundaries (controller, middleware, gateway) |

| Let It Crash | For unrecoverable state, crash and restart (Erlang/OTP philosophy) |

| Be Specific | Catch specific error types, never bare catch or except |

| Provide Context | Every error carries enough context to diagnose without reproducing |

---

Error Types

Operational errors — network timeouts, invalid user input, file not found, DB connection lost. Handle gracefully.

Programmer errors — TypeError, null dereference, assertion failures. Fix the code — don't catch and suppress.

// Operational — handle gracefully
try {
  const data = await fetch('/api/users');
} catch (err) {
  if (err.code === 'ECONNREFUSED') return fallbackData;
  throw err; // re-throw unexpected errors
}

// Programmer — let it crash, fix the bug
const user = null;
user.name; // TypeError — don't try/catch this

---

Language Patterns

| Language | Mechanism | Anti-Pattern |

|----------|-----------|-------------|

| JavaScript | try/catch, Promise.catch, Error subclasses | .catch(() => {}) swallowing errors |

| Python | Exceptions, context managers (with) | Bare except: catching everything |

| Go | error returns, errors.Is/As, fmt.Errorf wrapping | _ = riskyFunction() ignoring error |

| Rust | Result, Option, ? operator | .unwrap() in production code |

JavaScript — Error Subclasses

class AppError extends Error {
  constructor(message, code, statusCode, details = {}) {
    super(message);
    this.name = this.constructor.name;
    this.code = code;
    this.statusCode = statusCode;
    this.details = details;
    this.isOperational = true;
  }
}

class NotFoundError extends AppError {
  constructor(resource, id) {
    super(`${resource} not found`, 'NOT_FOUND', 404, { resource, id });
  }
}

class ValidationError extends AppError {
  constructor(errors) {
    super('Validation failed', 'VALIDATION_ERROR', 422, { errors });
  }
}

Go — Error Wrapping

func GetUser(id string) (*User, error) {
    row := db.QueryRow("SELECT * FROM users WHERE id = $1", id)
    var user User
    if err := row.Scan(&user.ID, &user.Name); err != nil {
        if errors.Is(err, sql.ErrNoRows) {
            return nil, fmt.Errorf("user %s: %w", id, ErrNotFound)
        }
        return nil, fmt.Errorf("querying user %s: %w", id, err)
    }
    return &user, nil
}

---

Error Boundaries

Express Error Middleware

app.use((err, req, res, next) => {
  const statusCode = err.statusCode || 500;
  const response = {
    error: {
      code: err.code || 'INTERNAL_ERROR',
      message: err.isOperational ? err.message : 'Something went wrong',
      ...(process.env.NODE_ENV === 'development' && { stack: err.stack }),
      requestId: req.id,
    },
  };

  logger.error('Request failed', {
    err, requestId: req.id, method: req.method, path: req.path,
  });

  res.status(statusCode).json(response);
});

React Error Boundary

import { ErrorBoundary } from 'react-error-boundary';

function ErrorFallback({ error, resetErrorBoundary }) {
  return (
    <div role="alert">
      <h2>Something went wrong</h2>
      <pre>{error.message}</pre>
      <button onClick={resetErrorBoundary}>Try again</button>
    </div>
  );
}

<ErrorBoundary FallbackComponent={ErrorFallback} onReset={() => queryClient.clear()}>
  <App />
</ErrorBoundary>

---

Retry Patterns

| Pattern | When to Use | Config |

|---------|-------------|--------|

| Exponential Backoff | Transient failures (network, 503) | Base 1s, max 30s, factor 2x |

| Backoff + Jitter | Multiple clients retrying | Random ±30% on each delay |

| Circuit Breaker | Downstream service failing repeatedly | Open after 5 failures, half-open after 30s |

| Bulkhead | Isolate failures to prevent cascade | Limit concurrent calls per service |

| Timeout | Prevent indefinite hangs | Connect 5s, read 30s, total 60s |

Exponential Backoff with Jitter

async function withRetry(fn, { maxRetries = 3, baseDelay = 1000, maxDelay = 30000 } = {}) {
  for (let attempt = 0; attempt <= maxRetries; attempt++) {
    try {
      return await fn();
    } catch (err) {
      if (attempt === maxRetries || !isRetryable(err)) throw err;
      const delay = Math.min(baseDelay * 2 ** attempt, maxDelay);
      const jitter = delay * (0.7 + Math.random() * 0.6);
      await new Promise((r) => setTimeout(r, jitter));
    }
  }
}

function isRetryable(err) {
  return [408, 429, 500, 502, 503, 504].includes(err.statusCode) || err.code === 'ECONNRESET';
}

Circuit Breaker

class CircuitBreaker {
  constructor({ threshold = 5, resetTimeout = 30000 } = {}) {
    this.state = 'CLOSED';       // CLOSED → OPEN → HALF_OPEN → CLOSED
    this.failureCount = 0;
    this.threshold = threshold;
    this.resetTimeout = resetTimeout;
    this.nextAttempt = 0;
  }

  async call(fn) {
    if (this.state === 'OPEN') {
      if (Date.now() < this.nextAttempt) throw new Error('Circuit is OPEN');
      this.state = 'HALF_OPEN';
    }
    try {
      const result = await fn();
      this.onSuccess();
      return result;
    } catch (err) {
      this.onFailure();
      throw err;
    }
  }

  onSuccess() { this.failureCount = 0; this.state = 'CLOSED'; }
  onFailure() {
    this.failureCount++;
    if (this.failureCount >= this.threshold) {
      this.state = 'OPEN';
      this.nextAttempt = Date.now() + this.resetTimeout;
    }
  }
}

---

HTTP Error Responses

| Status | Name | When to Use |

|--------|------|-------------|

| 400 | Bad Request | Malformed syntax, invalid JSON |

| 401 | Unauthorized | Missing or invalid authentication |

| 403 | Forbidden | Authenticated but insufficient permissions |

| 404 | Not Found | Resource does not exist |

| 409 | Conflict | Request conflicts with current state |

| 422 | Unprocessable Entity | Valid syntax but semantic errors |

| 429 | Too Many Requests | Rate limit exceeded (include Retry-After) |

| 500 | Internal Server Error | Unexpected server failure |

| 502 | Bad Gateway | Upstream returned invalid response |

| 503 | Service Unavailable | Temporarily overloaded or maintenance |

Standard Error Envelope

{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "The request body contains invalid fields.",
    "details": [
      { "field": "email", "message": "Must be a valid email address" }
    ],
    "requestId": "req_abc123xyz"
  }
}

---

Graceful Degradation

| Strategy | Example |

|----------|---------|

| Fallback values | Show cached avatar when image service is down |

| Feature flags | Disable unstable recommendation engine |

| Cached responses | Serve stale data with X-Cache: STALE header |

| Partial response | Return available data with warnings array |

async function getProductPage(productId) {
  const product = await productService.get(productId); // critical — propagate errors

  const [reviews, recommendations] = await Promise.allSettled([
    reviewService.getForProduct(productId),
    recommendationService.getForProduct(productId),
  ]);

  return {
    product,
    reviews: reviews.status === 'fulfilled' ? reviews.value : [],
    recommendations: recommendations.status === 'fulfilled' ? recommendations.value : [],
    warnings: [reviews, recommendations]
      .filter((r) => r.status === 'rejected')
      .map((r) => ({ service: 'degraded', reason: r.reason.message })),
  };
}

---

Logging & Monitoring

| Practice | Implementation |

|----------|---------------|

| Structured logging | JSON: level, message, error, requestId, userId, timestamp |

| Error tracking | Sentry, Datadog, Bugsnag — automatic capture with source maps |

| Alert thresholds | Error rate > 1%, P99 latency > 2s, 5xx spike |

| Correlation IDs | Pass requestId through all service calls |

| Log levels | error = needs attention, warn = degraded, info = normal, debug = dev |

---

Anti-Patterns

| Anti-Pattern | Fix |

|-------------|-----|

| Swallowing errors catch (e) {} | Log and re-throw, or handle explicitly |

| Generic catch-all at every level | Catch specific types, let unexpected errors bubble |

| Error as control flow | Use conditionals, return values, or option types |

| Stringly-typed errors throw "wrong" | Throw Error objects with codes and context |

| Logging and throwing | Log at the boundary only, or wrap and re-throw |

| Catch-and-return-null | Return Result type, throw, or return error object |

| Ignoring Promise rejections | Always await or attach .catch() |

| Exposing internals | Sanitize responses; log details server-side only |

---

NEVER Do

1. NEVER swallow errors silently — catch (e) {} hides bugs and causes silent data corruption
2. NEVER expose stack traces, SQL errors, or file paths in API responses — log details server-side only
3. NEVER use string throws — throw 'error' has no stack trace, no type, no context
4. NEVER catch and return null without explanation — callers have no idea why the operation failed
5. NEVER ignore unhandled Promise rejections — always await or attach .catch()
6. NEVER cache error responses — 5xx and transient errors must not be cached and re-served
7. NEVER use exceptions for normal control flow — exceptions are for exceptional conditions
8. NEVER return generic "Something went wrong" without logging the real error — always log the full error server-side with request context