Logging Observability

Logging & Observability

Structured logging, distributed tracing, and metrics for production systems. Covers the full observability stack from log formatting to alert routing.

Decision Points

1. Log Level Assignment by Event Type

Event occurs → 
  ├── System failure?
      ├── YES → Service cannot continue? 
          ├── YES → FATAL (page immediately)
          └── NO → ERROR (operation failed, will retry)
      └── NO → Unexpected condition?
          ├── YES → WARN (circuit breaker, deprecation)
          └── NO → Business event?
              ├── YES → INFO (user action, payment processed)
              └── NO → Debug helper?
                  ├── YES → DEBUG (DB queries, cache hits)
                  └── NO → TRACE (spans, fine-grained flow)

2. Observability Stack Choice by Scale

Request volume →
  ├── < 1000/min → Structured logs + simple metrics
  ├── < 10k/min → Add distributed tracing (10% sampling)
  ├── < 100k/min → Full OTel + head-based sampling
  └── > 100k/min → Tail-based sampling + cardinality limits

3. Alert Threshold Setting

SLI established →
  ├── User-facing service?
      ├── YES → Start with 99% SLO (44min/month error budget)
      └── NO → Start with 95% SLO (36hr/month error budget)
  └── Historical data available?
      ├── YES → Set threshold at 95th percentile of normal operation
      └── NO → Set conservative threshold, tune weekly for 1 month

4. Trace Sampling Decision

Performance impact →
  ├── Latency sensitive service?
      ├── YES → 1-5% sampling rate
      └── NO → 10-20% sampling rate
  └── Error debugging needed?
      ├── YES → Always sample errors (status=error)
      └── NO → Uniform probability sampling

5. PII Handling Strategy

Field contains sensitive data →
  ├── Required for debugging?
      ├── YES → Hash or tokenize (preserve cardinality)
      └── NO → Complete redaction
  └── Regulatory compliance?
      ├── GDPR/CCPA → Allowlist approach only
      └── PCI → Redact payment fields specifically

Failure Modes

1. Alert Fatigue

• Symptom: Teams ignore pages; alerts stay open for hours
• Diagnosis: Alert-to-incident ratio > 3:1, or SLI threshold too sensitive
• Fix: Raise threshold by 10% increments until alerts correlate with real user impact

2. PII Leakage

• Symptom: Compliance audit flags personal data in logs
• Diagnosis: Search logs for patterns like "password":, "ssn":, credit card regex
• Fix: Implement allowlist-only logging; redact at logger config level, not call sites

3. Trace Orphaning

• Symptom: Spans appear disconnected; can't follow requests end-to-end
• Diagnosis: Missing traceparent header propagation on outbound HTTP calls
• Fix: Auto-instrument HTTP clients or manually inject context headers

4. Log-and-Throw Duplication

• Symptom: Same error appears 2-5 times with identical trace IDs
• Diagnosis: Error logged at every stack frame, not just handling boundary
• Fix: Log only where you decide what to do with the error (usually HTTP boundary)

5. Cardinality Explosion

• Symptom: Metrics storage costs spike; query performance degrades
• Diagnosis: Label values exceed 1000 unique values per metric
• Fix: Replace high-cardinality labels (user_id) with bucketed versions (user_tier)

Worked Examples

End-to-End: Distributed Payment Failure Trace

Scenario: Payment service returning 500s sporadically. Need to trace through API Gateway → Payment Service → Bank API.

Step 1: Trace ID Recovery

# Customer reports failed payment at 14:35 UTC
# Find trace ID from customer-facing logs
grep -A5 -B5 "payment_failed" /var/log/api-gateway.log | grep "14:3[0-9]"
# Extract: trace_id: "abc123def456"

Step 2: Cross-Service Trace Following

# Follow trace through each service
kubectl logs payment-service | grep "abc123def456"
# Shows: bank_api_call_failed, status_code: 502, bank_error: "insufficient_funds"

# Verify bank API logs (if accessible)
curl -H "X-Trace-ID: abc123def456" https://bank-api/logs

Decision Point: Sampling trade-off encountered

• Expert notices: Only 10% of traces sampled, but this error trace was captured
• Novice misses: Would increase sampling to 100%, causing performance impact
• Expert decision: Enable error-based sampling (always trace when status=error)

Step 3: Root Cause Analysis

// Found in payment service code
logger.error({ 
  trace_id,
  bank_response_code: 502, 
  bank_error: "insufficient_funds",
  our_retry_count: 3 
}, "payment_processing_failed");

Resolution: Bank API returns 502 for business logic errors (insufficient funds). Change error handling to return 400 instead of retrying on 502.

Setting Up Structured Logging

Node.js Payment Service Implementation:

import pino from 'pino';

const logger = pino({
  level: process.env.LOG_LEVEL ?? 'info',
  redact: {
    paths: ['req.headers.authorization', 'body.cardNumber', '*.ssn'],
    censor: '[REDACTED]'
  }
});

// Correlation middleware
export function correlationMiddleware(req, res, next) {
  const traceId = req.headers['x-trace-id'] ?? randomUUID();
  res.setHeader('x-trace-id', traceId);
  
  // AsyncLocalStorage context
  requestContext.run({ traceId }, () => {
    logger.info({ 
      traceId, 
      method: req.method, 
      path: req.path,
      userAgent: req.headers['user-agent']
    }, 'request_received');
    next();
  });
}

Failure Modes

Alert Noise Syndrome

• Detection: If alert-to-incident ratio exceeds 3:1, threshold too low
• Cause: SLI thresholds set at 90th percentile instead of 95th percentile
• Fix: Raise threshold by 10% increments until alerts predict real user impact

Schema Drift

• Detection: Dashboard queries break after service deployments
• Cause: Log field names change without coordinated dashboard updates
• Fix: Treat log schema as API contract; version field names explicitly

Sampling Blind Spots

• Detection: Cannot find traces for reported user issues
• Cause: Uniform sampling misses rare but critical error paths
• Fix: Implement intelligent sampling (always sample errors, high-value users)

Quality Gates

• All log outputs are valid JSON (no string interpolation)
• PII redaction configured at logger initialization level
• Correlation ID propagated on every outbound HTTP call
• OpenTelemetry SDK initialized before application imports
• Error rate SLI defined with 99% availability target
• Alert runbook linked from every paging notification
• Trace sampling rate documented and tunable via config
• Log retention policy set based on compliance requirements
• Dashboard covers four golden signals (latency, traffic, errors, saturation)
• Cardinality limits enforced (<1000 unique values per metric label)

NOT-FOR Boundaries

This skill handles: Production observability, structured logging, distributed tracing, alerting strategy

Delegate elsewhere:

• Application performance profiling → Use performance-optimization skill instead
• Load testing and capacity planning → Use infrastructure-scaling skill
• Database query optimization → Use database-architect skill
• Security event monitoring → Use security-architect skill
• Cost optimization for observability tools → Use cost-optimization skill