Resilience Engineer

resilience-engineer — when something fails, fail well

When to use this skill

Trigger when error handling is the issue. Strong signals:

• "improve the error handling here"
• "add logging / observability"
• "make this more resilient"
• "this should retry on failure"
• "we're swallowing errors somewhere"
• "Sentry isn't getting anything from this service"

Do not trigger for: writing happy-path code that hasn't been written yet (just write it), or for incident response (you need a responder, not a refactor).

The output contract

Error-handling changes that:

1. Make failures observable — every caught error reaches the logging/monitoring layer with enough context to debug from
2. Distinguish recoverable from terminal — transient failures retry with backoff; programmer errors crash loud
3. Type the errors — throw new Error('foo') becomes typed classes with structured fields
4. Surface useful messages — to the user, specific enough to act on; to the developer, specific enough to fix
5. Don't hide bugs — never catch (e) {} without rethrowing or logging

Workflow

1 — Find the silent failures

# Empty catch blocks
rg 'catch\s*\([^)]*\)\s*\{?\s*\}'

# Catch that just returns
rg -A 2 'catch\s*\([^)]*\)\s*\{' | rg -B 1 'return( null| undefined| \[\])?;?$'

# Catch that just logs (no rethrow, no monitoring)
rg -A 3 'catch\s*\([^)]*\)\s*\{' | rg 'console\.(log|warn|error)'

# Promises with no .catch
rg '\.then\([^)]+\)(?!\s*\.catch)' -P

# async functions called without await (fire-and-forget without handler)
rg '^\s*[a-zA-Z_$][\w$]*\(' --no-heading

For each match, decide: real failure mode being swallowed? Or a false positive (e.g., catch that's part of a deliberate fallback strategy)?

2 — Map the error taxonomy

For the module under review, list every distinct kind of failure:

• Transient external — network blip, 503 from upstream, DB connection drop → retry
• Permanent external — 4xx from upstream, invalid API key → fail fast, surface
• User input — validation failed → 422 with clear message
• Programmer error — assertion failed, missing case in switch, impossible state → crash, alert
• Business logic — insufficient funds, expired token, rate-limited → expected, return error to user without alarming logs

For each kind, decide the strategy before touching code:

3 — Introduce typed errors

Replace anonymous Error with a discriminated hierarchy:

// errors.ts
export abstract class AppError extends Error {
  abstract readonly kind: string
  abstract readonly httpStatus: number
  abstract readonly retryable: boolean
  constructor(message: string, public readonly context?: Record<string, unknown>) {
    super(message)
    this.name = this.constructor.name
  }
}

export class ValidationError extends AppError {
  readonly kind = 'validation'
  readonly httpStatus = 422
  readonly retryable = false
}

export class UpstreamUnavailableError extends AppError {
  readonly kind = 'upstream_unavailable'
  readonly httpStatus = 502
  readonly retryable = true
}

export class InsufficientFundsError extends AppError {
  readonly kind = 'insufficient_funds'
  readonly httpStatus = 402
  readonly retryable = false
}

export class NotFoundError extends AppError {
  readonly kind = 'not_found'
  readonly httpStatus = 404
  readonly retryable = false
}

Throw these instead of plain Error('failed'). The shape lets every layer downstream make the right call without parsing message strings.

4 — Add retry where it belongs

For transient failures only — retrying a 401 just wastes effort.

async function withRetry<T>(
  fn: () => Promise<T>,
  opts: { tries?: number; baseMs?: number; maxMs?: number } = {}
): Promise<T> {
  const { tries = 3, baseMs = 500, maxMs = 5000 } = opts
  let lastErr: unknown
  for (let attempt = 1; attempt <= tries; attempt++) {
    try {
      return await fn()
    } catch (err) {
      lastErr = err
      if (err instanceof AppError && !err.retryable) throw err
      if (attempt === tries) throw err
      const delay = Math.min(maxMs, baseMs * 2 ** (attempt - 1)) + Math.random() * baseMs
      await new Promise(r => setTimeout(r, delay))
    }
  }
  throw lastErr
}

Rules:

• Cap the number of attempts (3 is a good default; 5 is the ceiling for user-blocking work)
• Always add jitter — otherwise N clients retry in lockstep and DDoS the upstream
• Respect Retry-After headers when present
• Don't retry on non-idempotent operations (POSTs that create resources) unless an Idempotency-Key is used

5 — Structured logging

Every caught error logs at the right level with structured fields:

logger.error('payment.charge_failed', {
  err: serializeError(err),       // ← message, stack, kind
  user_id: user.id,
  amount_cents: amount,
  payment_method: method.kind,
  request_id: req.id,
})

Rules:

• Use a structured logger (pino, winston, structlog) — not console.log. The fields need to be queryable.
• Always include request_id / trace_id so logs across services join up.
• Never log secrets, full credit card numbers, raw passwords, full session tokens.
• Use stable event names (payment.charge_failed, not 'failed to charge payment for user') — the words in the message will drift; the event name is the query handle.

6 — User-facing messages

Map each error kind to a sentence the user can act on:

function userMessageFor(err: AppError): string {
  switch (err.kind) {
    case 'validation': return err.message
    case 'insufficient_funds': return "Your card was declined for insufficient funds."
    case 'upstream_unavailable': return "We couldn't reach our payment processor. Please try again in a moment."
    case 'not_found': return "We couldn't find what you're looking for."
    default: return "Something went wrong on our end. We've been notified."
  }
}

Rules:

• Never expose stack traces or raw error messages to the user.
• Be specific where you can be ("Your card was declined" beats "Payment failed").
• Include the next step ("try again", "contact support", "check your details").
• Include a support reference ID for the catchall — so the user can give it to support and you can find the trace.

7 — Crash on impossible state

Programmer errors should fail loud and early. Use assert / invariant checks:

function processPayment(p: Payment) {
  if (p.status !== 'pending') {
    throw new Error(`processPayment called on payment in status ${p.status} (id=${p.id}) — this should be impossible`)
  }
  // ...
}

These should reach the pager. The whole point is that they're impossible, so when they happen, you want to find out immediately.

8 — Verify

• Trigger each error path manually or in tests. Confirm it shows up in logs with the expected event name and fields.
• Confirm the user-facing message renders correctly.
• For retries: confirm the retry happens, the backoff is bounded, and the final failure surfaces.

Patterns and anti-patterns

✅ Do:

• Treat unknown errors as the default in catch (TS 4.4+). Narrow before using.
• Cause-chain errors: throw new UpstreamError('Stripe failed', { cause: err }). Preserves the original stack.
• Have one error → HTTP mapping layer. Don't translate errors in every controller.
• Treat error rate as an SLI. Alert on rate change, not on every individual error.

❌ Don't:

• Don't catch (e) { throw e } — pointless. Remove the try.
• Don't return null from a function that could fail in multiple ways. Throw or return a Result type.
• Don't retry forever. There's no 5xx that becomes a 200 on the 17th attempt.
• Don't log and rethrow the same error. The next layer will log it too — you get double-entry noise.
• Don't catch Error broadly when you only meant to handle a specific subtype. You'll swallow real bugs.

Example invocation

User: "Audit error handling in src/payments/ — Sentry has almost nothing from this module."

1. Find silent failures: 11 empty catches, 6 catches that just return null, 4 .then() without .catch().
2. Map taxonomy: provider 5xx (transient, retryable), provider 4xx (permanent, surface), card declined (business logic, return to user), webhook signature invalid (security, log + 401).
3. Introduce PaymentError hierarchy with subclasses for each.
4. Wrap provider calls with withRetry for transients; surface the rest immediately.
5. Replace console.log + ignore with logger.error('payment.<event>', { ... }) with request_id, user_id, amount_cents.
6. Map errors → user messages in the controller layer. "Card declined" gets the real reason from the provider; everything else gets a generic + reference ID.
7. Add invariant: processCharge throws if called on a charge not in pending state.
8. Verify: simulate each path locally; confirm Sentry receives the right events with the right fields; confirm the retry happens on a forced 502.
9. Report: 21 silent failure sites closed, 5 typed error classes added, retries wired for 3 upstream call sites, Sentry now receives 4 named event types from this module.

See also

• code-auditor — finds silent error sites at scale across the codebase
• perf-hunter — when "errors" are actually timeouts caused by slowness
• ship-it — to wire the alert routing on the new structured events