| name | logging-and-observability-principles |
| description | Structured logging and observability patterns: log levels, structured fields, correlation IDs, language-specific logger setup. |
| user-invocable | false |
Logging and Observability Principles
⚠️ All operations MUST be logged per Logging and Observability Mandate GEMINI.md § Logging and Observability Mandate.
Log Levels
| Level | When | Examples |
|---|
| TRACE | Extremely detailed diagnostics | Fn entry/exit, var states (dev only) |
| DEBUG | Detailed flow for debugging | Query exec, cache hits, state transitions |
| INFO | General informational | Request started, task created, login |
| WARN | Potentially harmful | Deprecated API, fallback, retry |
| ERROR | Error, app continues | Request failed, API timeout, validation |
| FATAL | Severe, causes shutdown | DB unreachable, critical config missing |
Rules
1. Every operation logs start + success/error:
log.Info("creating task",
"correlationId", correlationID,
"userId", userID,
"title", task.Title,
)
log.Info("task created successfully",
"correlationId", correlationID,
"taskId", task.ID,
"duration", time.Since(start),
)
log.Error("failed to create task",
"correlationId", correlationID,
"error", err,
"userId", userID,
)
2. Context fields: correlationId (UUID), userId, duration (ms), error.
3. Structured only:
// ✅ Structured
log.Info("user login", "userId", userID, "ip", clientIP)
// ❌ String formatting
log.Info(fmt.Sprintf("User %s logged in from %s", userID, clientIP))
4. Never log: passwords, API keys, tokens, credit cards, PII (sanitize if needed), full req/res bodies.
5. Never log in hot paths: tight loops, per-item batch processing, latency-critical sync paths. Use logger middleware redaction (pino-redact, zap masking).
Language Implementations
Go (slog):
import "log/slog"
logger := slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
Level: slog.LevelInfo,
}))
logger.Info("operation started",
"correlationId", correlationID,
"userId", userID,
)
TypeScript (pino):
import pino from 'pino';
const logger = pino({
level: process.env.LOG_LEVEL || 'info',
});
logger.info({
correlationId,
userId,
duration: Date.now() - startTime,
}, 'task created successfully');
Python (structlog):
import structlog
logger = structlog.get_logger()
logger.info("task_created",
correlation_id=correlation_id,
user_id=user_id,
task_id=task.id,
)
Log Patterns
API: log request received (method, path, correlationId) + request completed (status, duration).
DB: DEBUG level — query start (query text) + complete (rows, duration) + error (query, error).
External API: INFO start (service, endpoint). WARN retry (attempt, error). WARN circuit breaker (failureCount).
Background Jobs: INFO start (jobId, type) + progress (periodic, not per-item) + complete (duration, items).
Errors: ERROR recoverable (sanitized input). FATAL critical dependency (action: shutting down).
Environment Config
| Env | Level | Format | Destination |
|---|
| Development | DEBUG | Pretty (colored) | Console |
| Staging | INFO | JSON | Stdout → cloud logs |
| Production | INFO | JSON | Stdout → cloud logs |
func configureLogger() *slog.Logger {
var handler slog.Handler
level := slog.LevelInfo
if os.Getenv("ENV") == "development" {
level = slog.LevelDebug
handler = slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{
Level: level,
})
} else {
handler = slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
Level: level,
})
}
return slog.New(handler)
}
Testing Logs
Capture and assert on log output:
func TestUserLogin(t *testing.T) {
var buf bytes.Buffer
logger := slog.New(slog.NewJSONHandler(&buf, nil))
service := NewUserService(logger, mockStore)
err := service.Login(ctx, email, password)
require.NoError(t, err)
logs := buf.String()
assert.Contains(t, logs, "user login successful")
assert.Contains(t, logs, email)
}
Monitoring
Correlation IDs: generate at ingress, propagate through all services, include in all logs/errors/traces. UUID v4.
Aggregation: centralized (CloudWatch/GCP/Datadog). Index: correlationId, userId, level, timestamp. Alert on ERROR/FATAL. Dashboard: request rates, error rates, latency.
Checklist
Related
- Logging and Observability Mandate GEMINI.md § Logging and Observability Mandate
- Monitoring and Alerting Principles @.gemini/skills/monitoring-and-alerting-principles/SKILL.md
- Error Handling Principles GEMINI.md § Error Handling Principles
- Security Mandate GEMINI.md § Security Mandate
- Security Principles GEMINI.md § Security Principles
- API Design Principles @.gemini/skills/api-design-principles/SKILL.md
