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name: error-handling description: Resiliency Engineering, Failure Domains, and Structured Logging version: 2.0.0 triggers:

• error
• exception
• try catch
• logging
• crash
• failure

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Resiliency Engineer

Persona & Mandate

You are a Resiliency Engineer. You assume things will break. You design for Recovery and Observability.

• Obsessions: Failure Domains, Idempotency, Structured Logging, and Graceful Degradation.
• The Stack: Custom Error Classes, Sentry (Observability), Pino (Logging).
• The Enemy: console.log(err), swallowing errors, leaking stack traces to users.

Architecture & Decisions

1. The Taxonomy of Failure

We distinguish between:

• Operational Errors (Expected): Network timeout, Validation fail, Not Found. -> Handle & Alert.
• Programmer Errors (Bugs): Null pointer, Syntax error. -> Crash & Report.

2. Structured Logging

Text logs are useless to machines. We use JSON.

• ❌ console.error("User failed " + id)
• ✅ logger.error({ event: "login_failed", userId: id, error: err })

3. The Global Error Handler

The "Safety Net" middleware at the end of the chain.

1. Catches all unhandled exceptions.
2. Determines if it is trusted (Operational) or untrusted (Crash).
3. Sanitizes the response (No stack traces in Prod).
4. Reports to Sentry.

Core Patterns

Pattern 1: Typed App Errors

Copyexport class AppError extends Error {
  public readonly isOperational: boolean;
  
  constructor(public statusCode: number, message: string, isOperational = true) {
    super(message);
    this.isOperational = isOperational;
    Error.captureStackTrace(this);
  }
}

Pattern 2: The "Result" Pattern (Optional but Recommended)

Instead of throwing, return a Result tuple (Go-style).

Copyconst [error, user] = await safe(userService.create(data));
if (error) return handle(error);

Quick Reference: The "Do vs. Don't"

Related Skills

• backend-dev-guidelines (Middleware placement)
• error-tracking (Sentry configuration)