Architecture

Overview of the Kubernetes cluster installed by Replicated

The Replicated Kubernetes installer builds on kubeadm, adding support for Pod networking, relocatable Persistent Volumes, and Ingress.

Core Components

Replicated’s install script will first install Docker and a few binaries on the host: kubelet, kubeadm, and kubectl. The kubelet binary runs as a systemd service, while kubeadm and kubectl commands are run manually. Replicated then delegates to kubeadm init to bootstrap the cluster. All system services besides kubelet run as pods in the kube-system namespace, including the Kubernetes API Server, Kube Proxy, Etcd, Kube Controller Manager, the Kube Scheduler, and CoreDNS.

Networking

Replicated adds a Weave DaemonSet to the kube-system namespace to provide pod networking on every node in the cluster. Weave places a binary in /opt/cni/bin that fulfills the CNI interface, which ensures that every pod is assigned an IP address that is routeable across the cluster. The Weave DaemonSet includes a Network Policy Controller to support NetworkPolicy resources in the cluster. All traffic between hosts is encrypted by default.

Storage

Replicated installs the Rook Operator in the rook-ceph-system namespace, which manages the ceph storage system. Replicated creates a Ceph cluster by creating a Cluster config in the rook-ceph namespace and configures the cluster to use the /opt/replicated/rook directory on all nodes automatically. A Pool is created to provide block storage backed by the cluster for PersistentVolumeClaims. Replicated will automatically increase the replication level of this Pool as nodes are added to the cluster, up to a maximum replication factor of 3.

Previous versions of Replicated were bundled with Rook version 0.8. When upgrading Replicated the Rook Operator will not be upgraded to the current version. Manual steps can be followed using this guide.

Ingress

Replicated adds a Contour ingress controller to the heptio-contour namespace. This controller listens on ports 80 and 443 of every node.

Replicated

After a Kubernetes cluster has been brought up, a Deployment is created in the default namespace for Replicated. When a license is uploaded, Replicated will generate a namespace for the application and apply the release yaml to it. There are three cases in which Replicated will directly modify application config yaml:

  1. If running in airgap mode, all Pod images will be rewritten to pull from the registry running in the Replicated daemon pod.
  2. If a pod mounts a Persistent Volume that is included in snapshots, Replicated will add a sidecar to the pod to backup the contents.
  3. All PersistentVolumeClaims will receive the storageClassName property to ensure they are automatically provisioned by Rook.