Handling Errors

When to use this skill

• Implementing error handling in new features.
• Designing error-resilient APIs and distributed systems.
• Improving application reliability through Circuit Breakers or Graceful Degradation.
• Standardizing error messages and custom exception hierarchies.

Workflow

1. Categorization

- [ ] Determine if the error is Recoverable (e.g., Network Timeout) or Unrecoverable (e.g., OOM).

1. Strategy Selection

- [ ] Choose a pattern: Fail Fast, Retry, Circuit Breaker, or Graceful Degradation.

1. Implementation

- [ ] Use language-specific best practices (Custom Exceptions, Result Types, or Explicit Returns).

1. Verification

- [ ] Validate that errors are logged with context and resources are cleaned up.

Universal Patterns

Circuit Breaker

Prevent cascading failures by rejecting requests when a service is failing.

# Logic: CLOSED -> failure threshold reached -> OPEN (wait timeout) -> HALF_OPEN (test)

Error Aggregation

Collect multiple errors (e.g., during validation) instead of failing on the first one.

class ErrorCollector {
  private errors: Error[] = [];
  add(error: Error) { this.errors.push(error); }
  throw() { if (this.errors.length) throw new AggregateError(this.errors); }
}

Graceful Degradation

Provide fallback functionality (e.g., fetch from cache if DB is down).

def with_fallback(primary, fallback):
    try: return primary()
    except Exception: return fallback()

Language-Specific Patterns

Python

• Hierarchy: Use ApplicationError(Exception) as a base.
• Cleanup: Use @contextmanager or try/finally.
• Retry: Implement decorators with exponential backoff.

TypeScript/JS

• Results: Use type Result<T, E> = { ok: true; value: T } | { ok: false; error: E }.
• Async: Handle Promise rejections explicitly; avoid swallowing errors in catch.

Rust/Go

• Rust: Prefer Result<T, E> and the ? operator.
• Go: Check if err != nil explicitly; use fmt.Errorf("...: %w", err) for wrapping.

Best Practices

• Fail Fast: Validate early.
• Preserve Context: Log metadata, stack traces, and timestamps.
• Don't Swallow Errors: Log or re-throw; empty catch blocks are forbidden.
• Cleanup: Always close files and connections.

Resources

• [retry-patterns.md](resources/retry-patterns.md)
• [custom-errors.ts](examples/custom-errors.ts)