Observability

---
name: observability
description: "Use when Production observability mastery. Structured logging (Pino/Winston), OpenTelemetry tracing, metrics (Prometheus/Grafana), SLIs/SLOs/error budgets, distributed tracing, alerting design, health checks, and AI observability. Use when setting up monitoring, debugging production issues, or designing observable distributed systems."
version: 5.0.0
last-updated: 2026-09-13
skills:
  - devops-incident-responder
  - nodejs-best-practices
  - backend-security-expert
tools: Read, Grep, Glob, Bash, Edit, Write
scripts-binding:
  - .agent/scripts/lint_runner.js
  - .agent/scripts/verify_all.js
---

# Observability — Production Monitoring Mastery

---

## 🛠️ Technical Architecture & Reference Recipes

---

---

## The Three Pillars

```
Logs    → WHAT happened (structured events)
Traces  → WHERE it happened (request flow across services)
Metrics → HOW MUCH is happening (counters, histograms, gauges)

All three are needed. Logs alone are not observability.
```

---

## Structured Logging

```typescript
import pino from 'pino';

// ✅ Structured JSON logging
const logger = pino({
  level: process.env.LOG_LEVEL ?? 'info',
  timestamp: pino.stdTimeFunctions.isoTime,
  ...(process.env.NODE_ENV === 'development' && {
    transport: { target: 'pino-pretty' },
  }),
});

// ✅ GOOD: Structured with context
logger.info({ userId: user.id, action: 'login', ip: req.ip }, 'User logged in');
logger.error({ err, orderId: order.id, paymentGateway: 'stripe' }, 'Payment failed');
logger.warn({ queueDepth: 1500, threshold: 1000 }, 'Queue depth exceeding threshold');

// ❌ BAD: Unstructured string logging
console.log('User ' + user.id + ' logged in from ' + req.ip);
console.log('Error: ' + error.message);

// ❌ HALLUCINATION TRAP: console.log is NOT production logging
// - No severity levels (info/warn/error)
// - No structured fields (can't search/filter)
// - No timestamps in ISO format
// - Can't be collected by log aggregators
// ✅ Use Pino (Node.js) or structlog (Python) for production
```

### Log Levels

```
fatal → App is crashing, immediate attention required
error → Operation failed, needs investigation
warn  → Something unexpected, but app continues
info  → Business events (user login, order placed, deploy)
debug → Technical details (query timing, cache hit/miss)
trace → Verbose debugging (only in development)

Rules:
- Production default: info
- Never log PII (names, emails, SSNs) at any level
- Never log secrets (tokens, passwords, API keys)
- Log request IDs for correlation
- Log durations for performance tracking
```

### Request Context / Correlation

```typescript
import { AsyncLocalStorage } from 'node:async_hooks';

const requestContext = new AsyncLocalStorage<{ requestId: string; userId?: string }>();

// Middleware: set context per request
app.use((req, res, next) => {
  const requestId = req.headers['x-request-id']?.toString() ?? crypto.randomUUID();
  res.setHeader('x-request-id', requestId);
  requestContext.run({ requestId, userId: req.user?.id }, next);
});

// Child logger with context
function getLogger() {
  const ctx = requestContext.getStore();
  return logger.child({
    requestId: ctx?.requestId,
    userId: ctx?.userId,
  });
}

// Every log from this request includes requestId and userId
const log = getLogger();
log.info('Processing order'); // { requestId: "abc-123", userId: "42", msg: "Processing order" }
```

---

## Distributed Tracing (OpenTelemetry)

```typescript
import { NodeSDK } from '@opentelemetry/sdk-node';
import { getNodeAutoInstrumentations } from '@opentelemetry/auto-instrumentations-node';
import { OTLPTraceExporter } from '@opentelemetry/exporter-trace-otlp-http';

// Initialize OpenTelemetry
const sdk = new NodeSDK({
  traceExporter: new OTLPTraceExporter({
    url: process.env.OTEL_EXPORTER_OTLP_ENDPOINT ?? 'http://localhost:4318/v1/traces',
  }),
  instrumentations: [
    getNodeAutoInstrumentations({
      '@opentelemetry/instrumentation-http': { enabled: true },
      '@opentelemetry/instrumentation-express': { enabled: true },
      '@opentelemetry/instrumentation-pg': { enabled: true },
      '@opentelemetry/instrumentation-redis': { enabled: true },
    }),
  ],
});

sdk.start();

// Manual span for custom business logic
import { trace } from '@opentelemetry/api';

const tracer = trace.getTracer('order-service');

async function processOrder(order: Order) {
  return tracer.startActiveSpan('processOrder', async span => {
    try {
      span.setAttribute('order.id', order.id);
      span.setAttribute('order.total', order.total);
      span.setAttribute('order.items.count', order.items.length);

      const result = await executeOrder(order);
      span.setStatus({ code: SpanStatusCode.OK });
      return result;
    } catch (error) {
      span.setStatus({ code: SpanStatusCode.ERROR, message: error.message });
      span.recordException(error);
      throw error;
    } finally {
      span.end();
    }
  });
}
```

---

## Metrics

```typescript
import { metrics } from '@opentelemetry/api';

const meter = metrics.getMeter('api-server');

// Counter — things that only go up
const requestCounter = meter.createCounter('http.requests.total', {
  description: 'Total HTTP requests',
});

// Histogram — request durations
const requestDuration = meter.createHistogram('http.request.duration_ms', {
  description: 'HTTP request duration in milliseconds',
  unit: 'ms',
});

// Gauge — current values
const activeConnections = meter.createUpDownCounter('db.connections.active', {
  description: 'Active database connections',
});

// Middleware to record metrics
app.use((req, res, next) => {
  const start = performance.now();
  res.on('finish', () => {
    const duration = performance.now() - start;
    requestCounter.add(1, {
      method: req.method,
      path: req.route?.path ?? req.path,
      status: res.statusCode.toString(),
    });
    requestDuration.record(duration, {
      method: req.method,
      status: res.statusCode.toString(),
    });
  });
  next();
});
```

### Key Metrics to Track

```
RED method (for services):
  Rate     → requests per second
  Errors   → error rate (4xx, 5xx)
  Duration → latency percentiles (P50, P95, P99)

USE method (for resources):
  Utilization → CPU %, memory %, disk %
  Saturation  → queue depth, thread pool saturation
  Errors      → disk failures, OOM kills

Business metrics:
  - Sign-ups per hour
  - Orders processed per minute
  - Revenue per day
  - API calls per customer
```

---

## SLIs, SLOs & Error Budgets

```
SLI (Service Level Indicator) → What you measure
  "99.2% of requests complete in <500ms"

SLO (Service Level Objective) → Your target
  "99.9% of requests should complete in <500ms"

SLA (Service Level Agreement) → Your contract (with penalties)
  "99.95% uptime or we refund 10%"

Error Budget = 100% - SLO
  SLO: 99.9% → Error budget: 0.1% → 43 min downtime/month
  SLO: 99.5% → Error budget: 0.5% → 3.6 hours downtime/month

Rules:
- Burn error budget too fast → freeze deployments
- Error budget remaining → ship features faster
- Don't set SLOs you can't measure
- SLOs should be slightly below actual performance
```

---

## Health Checks

```typescript
// Liveness: Is the process running?
app.get('/health/live', (req, res) => {
  res.status(200).json({ status: 'ok' });
});

// Readiness: Can it accept traffic?
app.get('/health/ready', async (req, res) => {
  try {
    await db.raw('SELECT 1'); // database check
    await redis.ping(); // cache check
    res.status(200).json({
      status: 'ready',
      checks: { database: 'ok', cache: 'ok' },
    });
  } catch (error) {
    res.status(503).json({
      status: 'not ready',
      checks: { database: error.message },
    });
  }
});

// ❌ HALLUCINATION TRAP: Liveness ≠ Readiness
// Liveness fails → container restarts (only for unrecoverable states)
// Readiness fails → stop sending traffic (temporary — DB down, etc.)
// Making liveness check the DB → DB outage restarts all containers → cascade failure
```

---

## Alerting

```
Alert design rules:
1. Alert on SYMPTOMS, not causes (high latency, not "CPU is 80%")
2. Every alert must have a runbook link
3. Every alert must be ACTIONABLE — if you can't do anything, it's a notification
4. Use severity levels:
   - Critical → page on-call (customer-facing outage)
   - Warning  → Slack notification (degraded, not broken)
   - Info     → dashboard only (awareness)
5. Avoid alert fatigue — fewer, meaningful alerts beat many noisy ones
```