Error Handling Patterns

error-handling-patterns

Decision framework for picking an error-handling strategy. Catalogues of language-specific code live upstream (links in § Provenance) — this skill is the predicate, not the pattern library. Sunset-policy compliant: large language-specific catalogues stay in authoritative upstream docs.

When to use

• Designing how a new feature, API, or service reports failure.
• Reviewing a diff that introduces a new exception class, Result<T, E>, or sentinel return.
• Debugging production noise that traces back to inconsistent error semantics.
• Choosing between retry, circuit-breaker, fallback, and fail-fast for an external dependency.

Do NOT use when:

• You only need the syntax for a try/catch in language X — read the upstream language guide directly.
• The failure is a single-call Laravel validation error — route to laravel-validation.
• The fix is a one-line null check in existing code — route to bug-analyzer.

Decision framework

Step 1 — Classify the failure

Failure is:
  caller's fault (bad input, missing auth)         → reject at boundary, structured error
  expected operational (timeout, 404, rate-limit)  → Result-type / typed return; retry-aware
  unexpected operational (DB down, OOM, deadlock)  → exception; observability + alert
  programmer bug (null deref, off-by-one)          → crash early; do not catch

Step 2 — Pick the reporting mechanism

IF failure is an EXPECTED, branchable outcome the caller will route on
  → Result type / tagged union / typed error return.
  Forces the caller to handle it; the type system is the proof.

IF failure is UNEXPECTED and most callers cannot do anything useful
  → exception, propagated to a single boundary handler.
  One layer (HTTP, queue, CLI) translates exceptions to user-facing errors.

IF failure is UNRECOVERABLE (invariant violated, data corruption)
  → fail loud, fail fast. No catch-and-continue.
  Log structured context, exit / panic / 500.

IF the language idiom forces one choice (Go: errors are values; Rust: Result;
   Python/PHP/JS: exceptions)
  → follow the idiom. Inventing a foreign mechanism is more cost than the
    correctness it buys.

Step 3 — Pick the resilience strategy

External call?
  Idempotent + transient failure mode  → retry with exponential backoff + jitter, cap.
  Non-idempotent                       → no blind retry; require an idempotency key.
  Repeated failure across instances    → circuit breaker; open → half-open probe → close.
  Optional functionality               → graceful degradation (cached / default / null result).
  Required functionality               → propagate; surface to user with a recovery hint.

Step 4 — Shape the error payload

Every produced error must carry: code (stable string), message (human-readable), cause (chained), context (sanitized inputs), correlation_id (request / trace).

Forbidden: secrets, raw SQL, full stack traces in user-facing surfaces, internal class names leaked through API boundaries.

Step 5 — Define the boundary

Exactly one layer translates internal errors to the egress format (HTTP status + body, queue requeue policy, CLI exit code). Anywhere else doing this duplication is the bug.

Procedure: Apply the framework to a new feature

1. Inspect the feature surface — identify every failure mode (each external call, each invariant, each user input class) and write it down.
2. Run Step 1 of the decision framework against each entry; write the classification next to it.
3. Pick reporting mechanism per Step 2; reject combinations the language idiom rejects.
4. For each external call, run Step 3 and write down the chosen resilience strategy.
5. Sketch the error payload shape (Step 4) and the single boundary (Step 5).
6. Hand the sketch to a reviewer before coding; cite this skill.

Output format

1. The failure-mode table (mode · classification · mechanism · resilience strategy).
2. The shared error payload definition (code, message, cause, context, correlation_id).
3. The single boundary handler (file:line) where internal → egress translation happens.
4. The retry / circuit-breaker config (attempts, base, jitter, breaker thresholds), if any.

Gotcha

• "Catch everything, log it, return null" silently destroys signal — every catch must either rethrow, translate, or recover with a written reason.
• Retries on non-idempotent calls are the second-most-common production incident; insist on idempotency keys before allowing retry.
• Circuit breakers without a half-open probe never close — they degrade to permanent failure.
• Mixing Result types and exceptions in the same module is worse than picking the wrong one — pick one per module and stay in it.
• Upstream pattern catalogues drift; trust the link, not memory. Refresh per refresh_trigger above.

Do NOT

• Do NOT introduce a custom error mechanism that fights the language idiom.
• Do NOT swallow exceptions — every catch has a written purpose.
• Do NOT leak stack traces, secrets, or internal class names across the boundary.
• Do NOT retry without backoff + jitter + cap.
• Do NOT inline language-specific code catalogues into this skill — externalize per Sunset Policy.

Auto-trigger keywords

• error handling strategy
• exceptions vs result
• retry pattern
• circuit breaker
• graceful degradation
• error payload shape

Provenance

• Adopted from: an external reference (internal provenance, redacted) — Sunset Policy applied: a large source reduced to a ~150-line decision framework; language catalogues externalized to the upstream resources below.
• Externalized catalogues:
  • Python: https://docs.python.org/3/tutorial/errors.html · https://docs.python.org/3/library/exceptions.html
  • PHP / Laravel: https://laravel.com/docs/errors · https://www.php.net/manual/en/language.exceptions.php
  • JS / TS: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Control_flow_and_error_handling · https://www.typescriptlang.org/docs/handbook/2/narrowing.html
  • Go: https://go.dev/blog/error-handling-and-go · https://pkg.go.dev/errors
  • Rust: https://doc.rust-lang.org/book/ch09-00-error-handling.html
  • Resilience patterns: https://martinfowler.com/bliki/CircuitBreaker.html · https://aws.amazon.com/builders-library/timeouts-retries-and-backoff-with-jitter/
• Cross-linked: defense-in-depth, laravel-validation, bug-analyzer, api-design.
• Provenance registry: agents/settings/contexts/skills-provenance.yml (entry: error-handling-patterns).
• Iron-Law floor: verify-before-complete, skill-quality, non-destructive-by-default.