64 lines
1.8 KiB
Markdown
64 lines
1.8 KiB
Markdown
# Stateful Sets
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## Introduction
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Like a `Deployment`, a `StatefulSet` manages Pods that are based on an identical container spec. Unlike a `Deployment`, a `StatefulSet` maintains a sticky identity for each the pods. These are created from the same spec, but are not interchangeable: each has a persistent identifier that it maintains across any rescheduling.
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`StatefulSets` are valuable for applications that require one or more of the following.
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* Stable, unique network identifiers, ex: distributed system, like ElasticSearch
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* Stable, persistent storage, ex: MySQL
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* Ordered, graceful deployment and scaling
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* Ordered, automated rolling updates, ex: MySQL Master+Slave
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```yml
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apiVersion: apps/v1
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kind: StatefulSet
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metadata:
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name: web
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spec:
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selector:
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matchLabels:
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app: nginx # has to match .spec.template.metadata.labels
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serviceName: "nginx"
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replicas: 3 # by default is 1
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template:
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metadata:
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labels:
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app: nginx # has to match .spec.selector.matchLabels
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spec:
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containers:
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- name: nginx
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image: k8s.gcr.io/nginx-slim:0.8
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ports:
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- containerPort: 80
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name: web
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volumeMounts:
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- name: www
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mountPath: /usr/share/nginx/html
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volumeClaimTemplates:
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- metadata:
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name: www
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spec:
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accessModes: [ "ReadWriteOnce" ]
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resources:
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requests:
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storage: 1Gi
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```
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As you can see the manifest is very close to the one of a deployment. Apply the manifest [01-statefulset.yml](01-statefulset.yml).
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Look at the pods generated, see how they are generated. Connect to one of the pods:
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```sh
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kubectl exec -ti web-0 /bin/bash
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```
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Write a file in the volume `www`. Terminate the same pod. See what happens. Reconnect to the pod, look at volume `www`. What can you see?
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## Clean up
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```sh
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kubectl delete statefulset,deployment,service,pod --all
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```
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