Error Handling Patterns

flowchart TD
    Q1{Is this a programming error\nor contract violation?} -->|Yes| EX[Throw exception\nlet it crash]
    Q1 -->|No| Q2{Is the error part of\nnormal control flow?}
    Q2 -->|Yes| Q3{What is the call site context?}
    Q2 -->|No| Q4{Do callers need to\ndistinguish error types?}
    Q3 -->|Functional / monad-friendly| RT[Result or Either type]
    Q3 -->|Simple script or CLI| EC[Error code + message]
    Q4 -->|Yes| EH[Typed exception hierarchy]
    Q4 -->|No| GE[Generic exception\nwith structured message]
    EX --> NOTE1[Never catch at boundary —\nlet process restart]
    RT --> NOTE2[Compose with map/flatMap;\ncheck references/error-hierarchy-examples.md]
    EH --> NOTE3[See hierarchy design rules below]

flowchart TD
    E[Error occurs] --> C1{Is error transient?\nTimeout, 429, 503, connection reset}
    C1 -->|No| FAIL[Fail immediately\nReturn error to caller]
    C1 -->|Yes| C2{Have we exceeded\nmax retry attempts?}
    C2 -->|Yes| DLQ[Send to dead letter queue\nor return final failure]
    C2 -->|No| C3{Is circuit breaker OPEN?}
    C3 -->|Yes| CB[Return circuit-open error\nDo not attempt request]
    C3 -->|No| WAIT[Wait: exponential backoff\n+ full jitter]
    WAIT --> RETRY[Retry request]
    RETRY --> C1
    CB --> PROBE{After timeout:\nsend probe request}
    PROBE -->|Success| CLOSE[Close circuit\nResume normal traffic]
    PROBE -->|Fail| CB

// Base application error — all domain errors extend this
class AppError extends Error {
  readonly code: string;
  readonly statusCode: number;
  readonly isOperational: boolean; // false = programmer error, crash process

  constructor(message: string, code: string, statusCode: number, isOperational = true) {
    super(message);
    this.name = this.constructor.name;
    this.code = code;
    this.statusCode = statusCode;
    this.isOperational = isOperational;
    Error.captureStackTrace(this, this.constructor);
  }
}

// Domain-specific errors
class ValidationError extends AppError {
  readonly fields: Record<string, string[]>;
  constructor(fields: Record<string, string[]>) {
    super('Validation failed', 'VALIDATION_ERROR', 422);
    this.fields = fields;
  }
}

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

class RateLimitError extends AppError {
  readonly retryAfterMs: number;
  constructor(retryAfterMs: number) {
    super('Rate limit exceeded', 'RATE_LIMIT', 429);
    this.retryAfterMs = retryAfterMs;
  }
}

type Result<T, E = AppError> =
  | { ok: true; value: T }
  | { ok: false; error: E };

// Helpers
const ok = <T>(value: T): Result<T, never> => ({ ok: true, value });
const err = <E>(error: E): Result<never, E> => ({ ok: false, error });

// Usage — caller is forced to handle both cases
async function fetchUser(id: string): Promise<Result<User, NotFoundError | NetworkError>> {
  try {
    const user = await db.users.findById(id);
    if (!user) return err(new NotFoundError('User', id));
    return ok(user);
  } catch (e) {
    return err(new NetworkError('DB unavailable', { cause: e }));
  }
}

// At call site — no silent failures
const result = await fetchUser(userId);
if (!result.ok) {
  if (result.error instanceof NotFoundError) return res.status(404).json(...);
  return res.status(500).json(...);
}
const user = result.value; // typed, safe

class RouteErrorBoundary extends React.Component<Props, State> {
  static getDerivedStateFromError(error: Error): State {
    return { hasError: true, error };
  }

  componentDidCatch(error: Error, info: React.ErrorInfo) {
    // Log to error tracking, not console.error in production
    logger.error('Render error', { error, componentStack: info.componentStack });
  }

  render() {
    if (this.state.hasError) {
      return <ErrorFallback error={this.state.error} onRetry={this.reset} />;
    }
    return this.props.children;
  }
}

class AppError(Exception):
    """Base error. All domain errors subclass this."""
    def __init__(self, message: str, code: str, status: int = 500):
        super().__init__(message)
        self.code = code
        self.status = status

class DatabaseError(AppError):
    def __init__(self, operation: str, cause: Exception):
        super().__init__(f"DB error during {operation}", "DB_ERROR", 503)
        self.__cause__ = cause  # explicit chain

# In application code
try:
    result = db.execute(query)
except psycopg2.OperationalError as e:
    raise DatabaseError("user_fetch", e) from e  # preserves full traceback