k8s lifecycle doc

Containerization-Orchestration/Kubernetes/workloads/15-lifecycle.md

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+# Kubernetes Lifecycle Hooks: `postStart` and `preStop`
+
+## Overview
+
+In Kubernetes, **lifecycle hooks** allow you to run specific code when a container starts or stops. These hooks are part of a container’s lifecycle and are useful for performing setup and cleanup tasks.
+
+The two main lifecycle hooks are:
+
+* **`postStart`** – Executed immediately after the container starts.
+* **`preStop`** – Executed just before the container stops.
+
+These hooks can be defined inside the container specification in a Pod manifest.
+
+---
+
+## 1. `postStart` Hook
+
+### Description
+
+The `postStart` hook runs **immediately after a container is created**, but before it is marked as *Running*. It can be used to perform initialization tasks that need to occur right after startup.
+
+The `postStart` hook supports two types of actions:
+
+* `exec` – Run a command inside the container.
+* `httpGet` – Send an HTTP request to a specified endpoint inside the container.
+
+### Notes
+
+* The `postStart` hook and the container’s main process run **concurrently**.
+* If the hook fails, the container is considered to have failed and will be restarted according to its restart policy.
+
+### Example
+
+```yaml
+lifecycle:
+  postStart:
+    exec:
+      command: ["/bin/sh", "-c", "echo Container started at $(date) >> /var/log/startup.log"]
+```
+
+Using an HTTP GET action:
+
+```yaml
+lifecycle:
+  postStart:
+    httpGet:
+      path: /startup
+      port: 8080
+```
+
+### Common Use Cases
+
+* Initializing configuration files or environment data.
+* Sending startup notifications to monitoring systems.
+* Preparing application dependencies.
+
+---
+
+## 2. `preStop` Hook
+
+### Description
+
+The `preStop` hook runs **just before a container is terminated**. Kubernetes will execute the hook and wait for it to complete before sending the `SIGTERM` signal to the container’s main process.
+
+This hook is typically used to ensure a **graceful shutdown** of the application.
+
+Like `postStart`, it supports:
+
+* `exec`
+* `httpGet`
+
+### Notes
+
+* The hook must complete within the time defined by `terminationGracePeriodSeconds`.
+* If the hook does not finish within that time, the container will be forcibly terminated.
+
+### Example
+
+```yaml
+lifecycle:
+  preStop:
+    exec:
+      command: ["/bin/sh", "-c", "echo Shutting down... >> /var/log/shutdown.log && sleep 5"]
+```
+
+### Common Use Cases
+
+* Closing active connections.
+* Flushing logs or metrics.
+* Deregistering the service from a load balancer or discovery system.
+* Notifying external systems about the shutdown event.
+
+---
+
+## 3. Full Example
+
+```yaml
+apiVersion: v1
+kind: Pod
+metadata:
+  name: lifecycle-demo
+spec:
+  containers:
+  - name: demo-container
+    image: nginx
+    lifecycle:
+      postStart:
+        exec:
+          command: ["/bin/sh", "-c", "echo 'Container started!' > /usr/share/nginx/html/index.html"]
+      preStop:
+        exec:
+          command: ["/bin/sh", "-c", "echo 'Container stopping...' >> /usr/share/nginx/html/index.html && sleep 10"]
+    terminationGracePeriodSeconds: 15
+```
+
+---
+
+## 4. Best Practices for DevOps Engineers
+
+1. **Handle Signals in Your Application**
+   Ensure your application properly handles `SIGTERM` and `SIGKILL` for clean shutdowns.
+
+2. **Combine Hooks with Grace Periods**
+   Use `preStop` together with `terminationGracePeriodSeconds` to allow sufficient cleanup time.
+
+3. **Monitor Hook Execution**
+   Log hook outputs for debugging and verification.
+
+4. **Avoid Long-Running or Blocking Hooks**
+   Hooks should be lightweight to avoid delaying container lifecycle events.
+
+---
+
+## 5. Summary
+
+| Hook Type   | Trigger Time                       | Purpose           | Common Actions                                    |
+| ----------- | ---------------------------------- | ----------------- | ------------------------------------------------- |
+| `postStart` | Immediately after container starts | Initialization    | Load configs, warm cache, notify service          |
+| `preStop`   | Before container termination       | Graceful shutdown | Flush data, close connections, deregister service |
+