# Workload HA

## General Principles of Application High Availability

* Applications adhere to [The Twelve-Factor App](https://12factor.net/zh_cn/)
* Employ Services and deploy applications with multiple replica Pods
* Multiple replicas use anti-affinity to avoid application failure due to single node faults
* Utilize PodDisruptionBudget to prevent application downtime resulting from evictions
* Use preStopHook and health check probes to ensure smooth service updates

## Graceful Shutdown

Configure terminationGracePeriodSeconds for Pods and use a preStop hook to delay the shutting down of container applications to avoid application interruptions during events such as `kubectl drain`:

```yaml
restartPolicy: Always
terminationGracePeriodSeconds: 30
containers:
- image: nginx
  lifecycle:
    preStop:
      exec:
        command: [
          "sh", "-c",
          # Introduce a delay to the shutdown sequence to wait for the
          # pod eviction event to propagate. Then, gracefully shutdown
          # nginx.
          "sleep 5 && /usr/sbin/nginx -s quit",
        ]
```

For the detailed principle, you can refer to the following series of articles:

* [1. Zero Downtime Server Updates For Your Kubernetes Cluster](https://blog.gruntwork.io/zero-downtime-server-updates-for-your-kubernetes-cluster-902009df5b33)
* [2. Gracefully Shutting Down Pods in a Kubernetes Cluster](https://blog.gruntwork.io/gracefully-shutting-down-pods-in-a-kubernetes-cluster-328aecec90d)
* [3. Delaying Shutdown to Wait for Pod Deletion Propagation](https://blog.gruntwork.io/delaying-shutdown-to-wait-for-pod-deletion-propagation-445f779a8304)
* [4. Avoiding Outages in Your Kubernetes Cluster Using PodDisruptionBudgets](https://blog.gruntwork.io/avoiding-outages-in-your-kubernetes-cluster-using-poddisruptionbudgets-ef6a4baa5085)

## Reference Documents

* [Kubernetes Pod Lifecycle](https://kubernetes.io/docs/concepts/workloads/pods/pod-lifecycle/)
* [Kubernetes PodDisruptionBudget](https://kubernetes.io/docs/concepts/workloads/pods/disruptions/)

***

Ensuring that your apps never miss a beat, even while you update your digital orchestra, is what high availability is all about. Let's consider the finely-tuned best practices:

* Treat your applications to the modern methodology of [The Twelve-Factor App](https://12factor.net/zh_cn/), ensuring they're as resilient as they are refined.
* Set up your software symphony using Services and a chorus of multiple replica Pods, each singing the same tune.
* Should one Pod face the music alone, anti-affinity keeps the performance going by avoiding single node failures that hit a sour note.
* Employ PodDisruptionBudget like a metronome, keeping rhythm and preventing evictions from throwing you off-beat.
* PreStopHook and health check probes act like the meticulous conductor, guaranteeing each transition is as smooth as the last.

When it's time for a swan song, here's how you can bring the curtain down on your Pods with elegance and poise:

Set the stage with `terminationGracePeriodSeconds` and cue the preStop hook to dim the lights gently. This way, when the crowd gets rowdy with `kubectl drain` commands, your application can exit stage left without dropping a line.

```yaml
restartPolicy: Always
terminationGracePeriodSeconds: 30
containers:
- image: nginx
  lifecycle:
    preStop:
      exec:
        command: [
          "sh", "-c",
          # This little intermission gives your Pods time to pass the news
          # of their departure. Then, like the grand finale of a symphony,
          # Nginx takes a graceful bow.
          "sleep 5 && /usr/sbin/nginx -s quit",
        ]
```

Intrigued by the mechanics behind the show? Curtain up! Peek behind the scenes with this enlightening series of articles:

* Enlightening [Series 1](https://blog.gruntwork.io/zero-downtime-server-updates-for-your-kubernetes-cluster-902009df5b33): No intermissions in your server performance.
* Graceful [Series 2](https://blog.gruntwork.io/gracefully-shutting-down-pods-in-a-kubernetes-cluster-328aecec90d): Pods that know how to exit the stage properly.
* Timely [Series 3](https://blog.gruntwork.io/delaying-shutdown-to-wait-for-pod-deletion-propagation-445f779a8304): Ensuring every Pod gets the memo before the lights go out.
* Safeguarding [Series 4](https://blog.gruntwork.io/avoiding-outages-in-your-kubernetes-cluster-using-poddisruptionbudgets-ef6a4baa5085): Keep the show going, no matter what happens offstage.

For the theory and technical know-how that underpins your application's resilience, sift through these reference documents:

* [Understanding Pod Lifecycle](https://kubernetes.io/docs/concepts/workloads/pods/pod-lifecycle/): The biology of your digital creatures.
* [Navigating Pod Disruptions](https://kubernetes.io/docs/concepts/workloads/pods/disruptions/): When the digital seas get rough, keep your Pods afloat.


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