logging

name	logging
description	Consistent, safe log statements with correct levels and no PII. Use when adding or reviewing log statements.

Skill: Logging

When to load it: When adding any new log statement, or when reviewing log coverage in an existing feature.

---

What This Skill Does

Produces log statements at the correct level, with safe content (no PII, no full request/response bodies), following a consistent pattern across the codebase.

---

Rules

• Use SLF4J (LoggerFactory.getLogger()). Never System.out.println.
• DEBUG — service method entry and exit. Verbose, for local diagnosis.
• INFO — business events. Something meaningful happened (resource created, query returned results).
• WARN — recoverable conditions. Something unexpected but handled (empty result, fallback used).
• ERROR — exception boundaries only. Once per failure, with full stack trace.
• Never log personally identifiable information — names, emails, phone numbers, addresses.
• Never log full request or response bodies in production — log identifiers only.

---

Pattern

private static final Logger log = LoggerFactory.getLogger([ClassName].class);

// Service entry (DEBUG)
log.debug("Finding [resource] id={}", id);

// Business event (INFO)
log.info("Created [resource] id={}", resource.getId());

// Recoverable condition (WARN)
log.warn("No [resource] found for id={}, returning empty result", id);

// Exception boundary (ERROR — once, with stack trace)
log.error("Failed to retrieve [resource] id={}", id, ex);

---

Design Constraints

• Do not use System.out.println or System.err.println
• Do not log at ERROR inside service methods — only at exception boundaries (global handler)
• Do not use string concatenation in log statements — use parameterised placeholders {}
• Do not log PII under any log level
• Do not log full request or response bodies — log the request ID or entity ID only