13-understand-self-healing-application

Application Lifecycle Management: Understand the primitives necessary to create a self-healing application

Back to Certified Kubernetes Administrator (CKA) Tutorial

note: Includes the following additional topics:

• Application Lifecycle Management: Know how to scale applications

Test Preparation: Relevant Kubernetes Documentation

Topics to search for:

• replicaset: ReplicaSet

• horizontal pod autoscaler: Horizontal Pod AutoScaler

Script

ReplicaSet

As we learned earlier, Pods are ephemeral, i.e., they can be terminated unexpectedly, e.g., say if a Node goes down (even for maintainence).

Let intoduce the simplest solution to this problem; a ReplicaSet:

A ReplicaSet’s purpose is to maintain a stable set of replica Pods running at any given time. As such, it is often used to guarantee the availability of a specified number of identical Pods.

-Kubernetes-ReplicaSet

A ReplicaSet is defined with fields, including a selector that specifies how to identify Pods it can acquire, a number of replicas indicating how many Pods it should be maintaining, and a pod template specifying the data of new Pods it should create to meet the number of replicas criteria. A ReplicaSet then fulfills its purpose by creating and deleting Pods as needed to reach the desired number. When a ReplicaSet needs to create new Pods, it uses its Pod template.

-Kubernetes-ReplicaSet

Some key observations:

• ownerReferences metadata field on Pod references ReplicaSet

• Any Pod matching the selector (and not otherwise owned) is aquired by the ReplicaSet, e.g., even existing Pods. Has to be the same namespace as the RS though

• Deleting a ReplicaSet, by default, will delete all aquired Pods; --cascade=false option on delete disables this behavior

note: Replaces ReplicationController resource kind.

helm install dev rs

Inspect RS:

kubectl get rs
kubectl describe rs

Inspect Pods; notice name with suffix:

kubectl get pods
kubectl describe pod XXX
kubectl get pod -o yaml XXX

Show RS recreating a deleted Pod:

kubectl delete pod XXX

Show RS accidentally aquiring a Pod:

helm install dev aquiring

Example of scaling ReplicaSet successfullly.

Example of not upgrading Pods in ReplicaSet successfully; i.e., changing Pod template only effects future Pods. Would have to scale to 0 and scale back up to change out version.

Horizontal Pod Autoscaler

A little bit of a side bar.

The Horizontal Pod Autoscaler automatically scales the number of pods in a replication controller, deployment, replica set or stateful set based on observed CPU utilization (or, with custom metrics support, on some other application-provided metrics).

-Kubernetes-Horizontal Pod Autoscaler

The HorizontalPodAutoscaler normally fetches metrics from a series of aggregated APIs (metrics.k8s.io, custom.metrics.k8s.io, and external.metrics.k8s.io). The metrics.k8s.io API is usually provided by metrics-server, which needs to be launched separately.

-Kubernetes-Horizontal Pod Autoscaler

Some observations:

• Implemented as a control loop; managed by kube-controller-manager

• Loop cycle defaults to 15 seconds

• Requires resource requests set for metric; otherwise does nothing

A light touch on resource requests:

When you specify a Pod, you can optionally specify how much of each resource a Container needs. The most common resources to specify are CPU and memory (RAM); there are others.

-Kubernetes-Managing Resources for Containers

helm install dev rs-hpa

note: Could never get HPA to work using documented configuration file. Ended up using imperative command to create and then examine results.

kubectl get hpa
kubectl describe hpa