503 Service Unavailable Error

Set up health check endpoints for load balancers to detect when an instance is unhealthy.

// Simple health check endpoint
app.get("/health", (req, res) => {
  const dbOk = await db.$queryRaw`SELECT 1`
  const memOk = process.memoryUsage().heapUsed < 500 * 1024 * 1024
  res.status(dbOk && memOk ? 200 : 503).json({
    status: dbOk && memOk ? "healthy" : "unhealthy"
  })
})

When returning 503, include a Retry-After header to tell clients when to retry.

app.get("/maintenance", (req, res) => {
  res.status(503)
  res.set("Retry-After", "3600") // 1 hour
  res.json({ error: "Under maintenance, back in ~1 hour" })
})

Use cloud auto-scaling to automatically add instances during traffic spikes.

# AWS CLI: update auto-scaling group
aws autoscaling update-auto-scaling-group \
  --auto-scaling-group-name myapp-asg \
  --min-size 2 --max-size 10 \
  --desired-capacity 4