How do you achieve zero-downtime deployments for a Go microservice in Kubernetes?

Zero-downtime deployment means in-flight requests complete before old pods terminate, and new pods are ready before traffic is routed to them. This requires coordination between the Go service and Kubernetes lifecycle hooks.

// Key components:

// 1. Graceful shutdown in the Go service
func main() {
    srv := &http.Server{Addr: ":8080", Handler: router}

    go func() { srv.ListenAndServe() }()

    quit := make(chan os.Signal, 1)
    signal.Notify(quit, syscall.SIGTERM) // k8s sends SIGTERM
    <-quit

    // Give in-flight requests time to complete
    ctx, cancel := context.WithTimeout(context.Background(), 25*time.Second)
    defer cancel()
    srv.Shutdown(ctx)   // stop accepting; drain active connections
    db.Close()          // close DB connections cleanly
    log.Println("shutdown complete")
}

// 2. Kubernetes deployment configuration
// deployment.yaml (relevant sections):
// spec.template.spec.containers:
//   lifecycle:
//     preStop:
//       exec:
//         command: ["sleep", "5"]
//   # SIGTERM is sent AFTER preStop
//   terminationGracePeriodSeconds: 30  # must be > service shutdown timeout

// 3. Rolling update strategy
// strategy:
//   type: RollingUpdate
//   rollingUpdate:
//     maxSurge: 1        # start 1 new pod before killing old
//     maxUnavailable: 0  # never kill old until new is Ready

// 4. Readiness probe — k8s only sends traffic when this returns 200
// readinessProbe:
//   httpGet:
//     path: /readyz
//     port: 8080
//   initialDelaySeconds: 5   # wait for startup
//   periodSeconds: 5
//   failureThreshold: 3

preStop hook timing: when Kubernetes terminates a pod, it sends SIGTERM and simultaneously removes the pod from the Service endpoints. There is a propagation delay (~5s) before kube-proxy and ingress controllers stop routing. The preStop sleep 5 delays SIGTERM so the graceful shutdown starts after traffic has stopped arriving.

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