# Running rqlite on Kubernetes
This document provides an example of how to run rqlite as a Kubernetes [StatefulSet](https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/).

## Creating a cluster 
### Create a Headless Service
The first thing to do is to create a [Kubernetes _Headless Service_](https://kubernetes.io/docs/concepts/services-networking/service/#headless-services). The Headless service creates the required DNS entries, which allows the rqlite nodes to find each other, and automatically bootstrap a new cluster. 
```yaml
apiVersion: v1
kind: Service
metadata:
  name: rqlite-svc
spec:
  clusterIP: None 
  selector:
    app: rqlite
  ports:
    - protocol: TCP
      port: 4001
      targetPort: 4001
```
Apply the configuration above to your Kubernetes deployment. It will create a DNS entry `rqlite-svc`, which will resolve to the IP addresses of any Pods with the tag `rqlite`.
```bash
kubectl apply -f headless-service.yaml
```
where the file `headless-service.yaml` contains the configuration shown above.

### Create a StatefulSet
For a rqlite cluster to function properly in a production environment, the rqlite nodes require a persistent network identifier and storage. This is what a StatefulSet can provide. The example belows shows you how to configure a 3-node rqlite cluster.
```yaml
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: rqlite
spec:
  selector:
    matchLabels:
      app: rqlite # has to match .spec.template.metadata.labels
  serviceName: rqlite-svc
  replicas: 3 # by default is 1
  template:
    metadata:
      labels:
        app: rqlite # has to match .spec.selector.matchLabels
    spec:
      terminationGracePeriodSeconds: 10
      containers:
      - name: rqlite
        image: rqlite/rqlite
        args: ["-disco-mode=dns","-disco-config={\"name\":\"rqlite-svc\"}","-bootstrap-expect","3"]
        ports:
        - containerPort: 4001
          name: rqlite
        readinessProbe:
          httpGet:
            scheme: HTTP
            path: /readyz?noleader
            port: 4001
          initialDelaySeconds: 1
          periodSeconds: 5
        volumeMounts:
        - name: rqlite-file
          mountPath: /rqlite/file
  volumeClaimTemplates:
  - metadata:
      name: rqlite-file
    spec:
      accessModes: [ "ReadWriteOnce" ]
      storageClassName: "standard"
      resources:
        requests:
          storage: 1Gi
```
Apply this configuration to your Kubernetes system, and a 3-node rqlite cluster will be created.
```bash
kubectl apply -f stateful-set.yaml
```
where the file `stateful-set.yaml` contains the configuration shown above.

Note the `args` passed to rqlite. The arguments tell rqlite to use `dns` discovery mode, and to resolve the DNS name `rqlite-svc` to find the IP addresses of other nodes in the cluster. Furthermore it tells rqlite to wait until three nodes are available (counting itself as one of those nodes) before attempting to form a cluster.

## Scaling the cluster
You can grow the cluster at anytime, simply by increasing the replica count. Shrinking the cluster, however, will require some manual intervention. As well reducing the `replicas` value, you also need to [explicitly remove](https://github.com/rqlite/rqlite/blob/master/DOC/CLUSTER_MGMT.md#removing-or-replacing-a-node) the deprovisioned nodes, or the Leader will continually attempt to contact those nodes.

> :warning: **Be careful that you don't reduce the replica count such that there is no longer a quorum of nodes available. If you do this you will render your cluster unusable, and need to perform a manual recovery.** The manual recovery process is [fully documented](https://github.com/rqlite/rqlite/blob/master/DOC/CLUSTER_MGMT.md#dealing-with-failure).