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📦 Container Lifecycle in Kubernetes/Docker

A container (or a pod in Kubernetes) goes through several phases from creation to termination. Understanding these phases helps you integrate Spring Boot lifecycle hooks with Kubernetes hooks like postStart, preStop, and readiness probes.

1️⃣ Kubernetes Pod Lifecycle Phases

A Kubernetes pod (one or more containers) has the following high-level phases:

Phase Description
Pending Pod object accepted by Kubernetes, container images being pulled.
ContainerCreating Container is being created and initialized.
Running Container process has started. Pod may or may not be ready for traffic.
Succeeded All containers finished successfully (for jobs).
Failed Containers terminated with failure.
Unknown Kubernetes cannot determine pod state.

2️⃣ Container Lifecycle Hooks

Kubernetes allows you to hook into container start and stop events.

a. postStart Hook

  • Runs immediately after container creation, concurrently with the main process.
  • Does NOT wait for the app to finish initializing. 
lifecycle:
  postStart:
    exec:
      command: ["/bin/sh", "-c", "echo Container started"]
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Use cases:

  • Create temporary files.
  • Log container startup events.
  • Lightweight initialization independent of the app.

b. preStop Hook

  • Runs before the container is terminated.
  • Kubernetes gives a default termination grace period (usually 30s) to finish tasks. 
lifecycle:
  preStop:
    exec:
      command: ["/bin/sh", "-c", "echo Container stopping"]
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Use cases:

  • Deregister from service discovery.
  • Flush caches to external storage.
  • Notify other services before shutdown.

3️⃣ Readiness & Liveness Probes

These are checks Kubernetes uses to manage pod traffic and health.

Probe Purpose When to use
Liveness Checks if container is alive; restarts if failed Crash-loop prevention
Readiness Checks if container can handle traffic Control traffic until initialization is done

Example readiness probe:

readinessProbe:
  httpGet:
    path: /actuator/health/readiness
    port: 8080
  initialDelaySeconds: 5
  periodSeconds: 10
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Tip: Combine Spring Boot ApplicationReadyEvent with readiness probe so the pod only receives traffic after app initialization.

4️⃣ Container Lifecycle Flow Diagram (Simplified) 

Pod scheduled
      │
      ▼
Container image pulled
      │
      ▼
Container created ──▶ postStart hook runs
      │
      ▼
Application starts (e.g., Spring Boot)
      │
      ▼
readinessProbe passes ──▶ Pod marked Ready, receives traffic
      │
      ▼
Container running (serving requests)
      │
      ▼
Shutdown initiated
      │
      ▼
preStop hook runs
      │
      ▼
Termination grace period
      │
      ▼
Container stopped
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5️⃣ Best Practices

  1. Don’t put long-running tasks in postStart hook
    It can delay container readiness and may be killed if it exceeds hook timeout.

  2. Use preStop to gracefully shutdown services
    Ensure Spring Boot shutdown hooks (like @PreDestroy) are triggered.

  3. Combine with Spring Boot lifecycle hooks

  • ApplicationReadyEvent for app-level initialization.
  • @PreDestroy or ContextClosedEvent for cleanup.
  • Kubernetes hooks for container-level concerns.
  1. Always test hooks under pod restart scenarios Ensure idempotency and avoid blocking pod startup.

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