Architecture

This page is an overview of open cluster management.

Overview

Open Cluster Management (OCM) is a powerful, modular, extensible platform for Kubernetes multi-cluster orchestration. Learning from the past failing lesson of building Kubernetes federation systems in the Kuberentes community, in OCM we will be jumping out of the legacy centric, imperative architecture of Kubefed v2 and embracing the “hub-agent” architecture which is identical to the original pattern of “hub-kubelet” from Kubernetes. Hence, intuitively in OCM our multi-cluster control plane is modeled as a “Hub” and on the other hand each of the clusters being managed by the “Hub” will be a “Klusterlet” which is obviously inspired from the original name of “kubelet”. Here’s a more detailed clarification of the two models we will be frequently using throughout the world of OCM:

  • Hub Cluster: Indicating the cluster that runs the multi-cluster control plane of OCM. Generally the hub cluster is supposed to be a light-weight Kubernetes cluster hosting merely a few fundamental controllers and services.

  • Klusterlet: Indicating the clusters that being managed by the hub cluster, might also called as “managed cluster” or “spoke cluster”. The klusterlet is supposed to actively pulling the latest prescriptions from the hub cluster and consistently reconciles the physical Kubernetes cluster to the expected state.

“Hub-spoke” architecture

Benefiting from the merit of “hub-spoke” architecture, in abstraction we are de-coupling most of the multi-cluster operations generally into (1) computation/decision and (2) execution, and the actual execution against the target cluster will be completely off-loaded into the managed cluster. The hub cluster won’t directly request against the real clusters, instead it just persists its prescriptions delcaratively for each cluster, and the klusterlet will be actively pulling the prescriptions from the hub and doing the execution. Hence, the burden of the hub cluster will be greatly relieved because the hub cluster doesn’t need to either deal with flooding events from the managed clusters or be buried in sending requests against the clusters. Imagine in a world where there’s no kubelet in Kubernetes and its control plane is directly operating the container daemons, it will be extremely hard for a centric controller to manage a cluster of 5k+ nodes. Likewise, that’s how OCM trying to breach the bottleneck of scalability, by dividing and offloading the execution into separated agents. So it’s always feasible for a hub cluster to accept and manage thousand-ish clusters.

Each klusterlet will be working independently and autonomously, so they have a weak dependency to the availability of the hub cluster. If the hub goes down (e.g. during maintenance or network partition) the klusterlet or other OCM agents working in the managed cluster are supposed to keep actively managing the hosting cluster until it re-connects. Additionally if the hub cluster and the managed clusters are owned by different admins, it will be easier for the admin of the managed cluster to police the prescriptions from the hub control plane because the klusterlet is running as a “white-box” as a pod instance in the managed cluster. Upon any accident, the klusterlet admin can quickly cut off the connection with the hub cluster without shutting the whole multi-cluster control plane down.

Architecture diagram

The “hub-agent” architecture also minimized the requirements in the network for registering a new cluster to the hub. Any cluster that can reach the endpoint of the hub cluster will be able to be managed, even a random KinD sandbox cluster on your laptop. That is because the prescriptions are effectively pulled from the hub instead of pushing. In addition to that, OCM also provides a addon named “cluster-proxy” which automatically manages a reverse proxy tunnel for proactive access to the managed clusters by leveraging on the Kubernetes’ subproject konnectivity.

Modularity and extensibility

Not only OCM will bring you a fluent user-experience of managing a number of clusters on ease, but also it will be equally friendly to further customization or second-time development. Every functionality working in OCM is expected to be freely-pluggable by modularizing the atomic capability into separated building blocks, except for the mandatory core module named registration which is responsible for controlling the lifecycle of a managed controller and exporting the elementary ManagedCluster model.

Another good example surfacing our modularity will be the placement, a standalone module focusing at dynamically selecting the proper list of the managed clusters from user’s prescription. You can build any advanced multi-cluster orchestration on the top of placement, e.g. multi-cluster workload re-balancing, multi-cluster helm charts replication, etc. On the other hand if you’re not satisfied by the current capacities from our placement module, you can quickly opt-out and replace it with your customized ones, and reach out to our community so that we can converge in the future if possible.


Concepts

Cluster registering: “double opt-in handshaking”

Practically the hub cluster and the managed cluster can be owned/maintained by different admins, so in OCM we clearly separated the roles and make the cluster registration require approval from the both sides defencing unwelcome requests. In terms of terminating the registration, the hub admin can kick out a registered cluster by denying the rotation of hub cluster’s certificate, on the other hand from the perspective of a managed cluster’s admin, he can either brutally deleting the agent instances or revoking the granted RBAC permissions for the agents. Note that the hub controller will be automatically preparing environment for the newly registered cluster and cleaning up neatly upon kicking a managed cluster.

Clusterset

Cluster namespace

Kubernetes has a native soft multi-tenancy isolation in the granularity of its namespace resources, so in OCM, for each of the managed cluster we will be provisioning a dedicated namespace for the managed cluster and grants sufficient RBAC permissions so that the klusterlet can persist some data in the hub cluster. This dedicated namespace is the “cluster namespace” which is majorly for saving the prescriptions from the hub. e.g. we can create ManifestWork in a cluster namespace in order to deploy some resources towards the corresponding cluster. Meanwhile, the cluster namespace can also be used to save the uploaded stats from the klusterlet e.g. the healthiness of an addon, etc.

Addons

Addon is a general concept for the optional, pluggable customization built over the extensibility from OCM. It can be a controller in the hub cluster, or just a customized agent in the managed cluster, or even the both collaborating in peers. The addons are expected to implement the ClusterManagementAddon or ManagedClusterAddon API of which a detailed elaboration can be found here.


Building blocks

The following is a list of commonly-used modules/subprojects that you might be interested in the journey of OCM:

Registration

The core module of OCM managing the lifecycle of the managed clusters. The registration controller in the hub cluster can be intuitively compared to a broker that represents and manages the hub cluster in terms of cluster registration, while the registration agent working in the managed cluster is another broker that represents the managed cluster. After a successful registration, the registration controller and agent will also be consistently probing each other’s healthiness. i.e. the cluster heartbeats.

Work

The module for dispatching resources from the hub cluster to the managed clusters, which can be easily done by writing a ManifestWork resource into a cluster namespace. See more details about the API here.

Placement

Building custom advanced topology across the clusters by either grouping clusters via the labels or the cluster-claims. The placement module is completely decoupled from the execution, the output from placement will be merely a list of names of the matched clusters in the PlacementDecision API, so the consumer controller of the decision output can reactively discovery the topology or availability change from the managed clusters by simply list-watching the decision API.

Application lifecycle

The application lifecycle defines the processes that are used to manage application resources on your managed clusters. A multi-cluster application uses a Kubernetes specification, but with additional automation of the deployment and lifecycle management of resources to individual clusters. A multi-cluster application allows you to deploy resources on multiple clusters, while maintaining easy-to-reconcile service routes, as well as full control of Kubernetes resource updates for all aspects of the application.

Governance and risk

Governance and risk is the term used to define the processes that are used to manage security and compliance from the hub cluster. Ensure the security of your cluster with the extensible policy framework. After you configure a hub cluster and a managed cluster, you can create, modify and delete policies on the hub and apply policies to the managed clusters.

Registration operator

Automating the installation and upgrading of a few built-in modules in OCM. You can either deploy the operator standalone or delegate the registration operator to the operator lifecyle framework.