Health Check Endpoints

Implement health checks for monitoring service availability and readiness.

Probe Types

Probe	Purpose	Failure Action
Liveness	Is process alive?	Restart container
Readiness	Can handle traffic?	Remove from LB
Startup	Has app started?	Delay other probes
Deep	All deps healthy?	Trigger alerts

Implementation (Express)

class HealthChecker {
  async checkDatabase() {
    const start = Date.now();
    try {
      await db.query('SELECT 1');
      return { status: 'healthy', latency: Date.now() - start };
    } catch (err) {
      return { status: 'unhealthy', error: String(err?.message || err) };
    }
  }

  async checkRedis() {
    try {
      await redis.ping();
      return { status: 'healthy' };
    } catch (err) {
      return { status: 'unhealthy', error: err.message };
    }
  }

  async getReadiness() {
    const checks = await Promise.all([
      this.checkDatabase(),
      this.checkRedis()
    ]);
    const healthy = checks.every(c => c.status === 'healthy');
    return { healthy, checks };
  }
}

// Liveness - lightweight
app.get('/health/live', (req, res) => {
  res.json({ status: 'ok', timestamp: new Date().toISOString() });
});

// Readiness - check dependencies
app.get('/health/ready', async (req, res) => {
  const health = await healthChecker.getReadiness();
  res.status(health.healthy ? 200 : 503).json(health);
});

Kubernetes Configuration

livenessProbe:
  httpGet:
    path: /health/live
    port: 3000
  initialDelaySeconds: 15
  periodSeconds: 10
  failureThreshold: 3

readinessProbe:
  httpGet:
    path: /health/ready
    port: 3000
  initialDelaySeconds: 5
  periodSeconds: 10

Best Practices

• Keep liveness checks minimal (no external deps)
• Check only critical systems in readiness
• Return 200 for healthy, 503 for unhealthy
• Set reasonable timeouts to prevent cascading failures
• Include response time metrics

Additional Implementations

See references/implementations.md for:

• Python Flask complete health checker
• Java Spring Boot Actuator
• Full Kubernetes deployment config

Never Do

• Make liveness depend on external services
• Return 200 when dependencies are down
• Skip dependency checks in readiness