If you have a stateless app you want to use a deployment. However, for a stateful app you might want to use a StatefulSet. Unlike a deployment, the StatefulSet provides certain guarantees about the identity of the pods it is managing (that is, predictable names) and about the startup order. Two more things that are different compared to a deployment: for network communication you need to create a headless services and for persistency the StatefulSet manages a persistent volume per pod.

In order to see how this all plays together, we will be using an educational Kubernetes-native NoSQL datastore.

Let’s start with creating the stateful app, that is, the StatefulSet along with the persistent volumes and the headless service:

$ kubectl apply -f https://raw.githubusercontent.com/mhausenblas/mehdb/master/app.yaml

After a minute or so, you can have a look at all the resources that have been created:

$ kubectl get sts,po,pvc,svc
NAME                     DESIRED   CURRENT   AGE
statefulset.apps/mehdb   2         2         1m

NAME          READY   STATUS    RESTARTS   AGE
pod/mehdb-0   1/1     Running   0          1m
pod/mehdb-1   1/1     Running   0          56s

NAME                                 STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   AGE
persistentvolumeclaim/data-mehdb-0   Bound    pvc-bc2d9b3b-310d-11e9-aeff-123713f594ec   1Gi        RWO            ebs            1m
persistentvolumeclaim/data-mehdb-1   Bound    pvc-d4b7620f-310d-11e9-aeff-123713f594ec   1Gi        RWO            ebs            56s

NAME            TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)    AGE
service/mehdb   ClusterIP   None         <none>        9876/TCP   1m

Now we can check if the stateful app is working properly. To do this, we use the /status endpoint of the headless service mehdb:9876 and since we haven’t put any data yet into the datastore, we’d expect that 0 keys are reported:

$ kubectl run -it --rm jumpod --restart=Never --image=quay.io/mhausenblas/jump:0.2 -- curl mehdb:9876/status?level=full
If you don't see a command prompt, try pressing enter.
0
pod "jumpod" deleted

And indeed we see 0 keys being available, reported above.

Note that sometimes a StatefulSet is not the best fit for your stateful app. You might be better off defining a custom resource along with writing a custom controller to have finer-grained control over your workload.

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