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Running rqlite on Kubernetes
This document provides an example of how to run rqlite as a Kubernetes StatefulSet. Full source for the configuration described below is available in the rqlite Kubernetes Configuration repo.
Creating a cluster
Create Services
The first thing to do is to create two Kubernetes Services. The first service, rqlite-svc-internal, is Headless and allows the nodes to find each other and cluster automatically. It shouldn't be used by rqlite clients. It is the second service, rqlite-svc, that is for clients to talk to the cluster -- this service will get a Cluster IP address which those clients can connect to.
A key difference between rqlite-svc-internal and rqlite-svc is that the second will only contain Pods that are ready to serve traffic. This makes it most suitable for use by end-users of rqlite. In otherwords you should communicate with your cluster using the hostname rqlite-svc.
Download and apply the service configuration like so:
curl -s https://raw.githubusercontent.com/rqlite/kubernetes-configuration/master/service.yaml -o rqlite-service.yaml
kubectl apply -f rqlite-service.yaml
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.
Retrieve the Stateful set configuration, apply it, and a 3-node rqlite cluster will be created:
curl -s https://raw.githubusercontent.com/rqlite/kubernetes-configuration/master/statefulset-3-node.yaml -o rqlite-3-nodes.yaml
kubectl apply -f rqlite-3-nodes.yaml
Note the args passed to rqlite in the YAML file. The arguments tell rqlite to use dns discovery mode, and to resolve the DNS name rqlite-svc-internal 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 setting the replica count to the desired cluster size. For example, to grow the above cluster to 9 nodes, you would issue the following command:
kubectl scale statefulsets rqlite --replicas=9
You could also increase the replicas count in rqlite-3-nodes.yaml and reapply the file. Note that you do not need to change the value of bootstrap-expect. If you do, you may trigger Kubernetes to restart the previously launched Pods during the scaling process. bootstrap-expect only needs to be set to the desired size of your cluster on the very first time you launch it.
⚠️ It is important that your rqlite cluster is healthy and fully functional before scaling up. It is also critical that DNS is working properly too. If this is not the case, some of the new nodes may become standalone single-node clusters, as they will be unavailable to connect to the Leader.
Shrinking the cluster, however, will require some manual intervention. As well reducing replicas, you will also need to explicitly remove the deprovisioned nodes, or the Leader will continually attempt to contact those nodes.
⚠️ 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.