Meaningful Workload Distribution

No placement rules exist, so you can use OCM to distribute simple workloads statically to the managed nodes. To do so, grab an existing YAML file that describes an application and can be easily rolled out to a cluster with:

kubectl apply -f

I used a plain vanilla NGINX template in my lab. With the clusteradm tool, you can easily delegate existing definition files to a cluster:

clusteradm create work nginx1 -f nginx.yml --clusters one --context kind-hub

The OCM hub then sends the request to the one cluster, which subsequently generates and starts all the resources listed in the YAML file (Figure 2). You can view the status of your application with the hub:

clusteradm get works --cluster one

Figure 2: On the hub, OCM generates a workspace of the same name for each managed cluster. The management tool stores the manifests in the workspace.

The regular kubectl tool also shows the loads, for example, if you type:

kubectl get manifestwork -A --context kind-hub

As an alternative to clusteradm, kubectl generates and rolls out works. All you need to do is extend your existing YAML file. The declaration typically starts with something like:

apiVersion: apps/v1
kind: Deployment
metadata:
      namespace: default
...

If you use a work agent, the whole thing looks a little different (Listing 1). Note that in this example, the manifest defines a static assignment to the one cluster (Figure 3).

apiVersion: work.open-cluster-management.io/v1
kind: ManifestWork
metadata:
      namespace: one
      name: nginx1
spec:
      workload:
           manifests:
                - apiVersion: v1
                     kind: Deployment
                     metadata:
                          namespace: default
...

Figure 3: An NGINX demo rollout running on the managed one cluster as specified by the manifest on the hub cluster. The OCM agent (klusterlet) is responsible for implementing OCM requirements locally.

If you manage more than one cluster with OCM, first create placement rulesets (kind:Placement), and then distribute the workloads to the managed clusters to comply with the rulesets:

clusteradm create work nginx1 -f nginx1.yml --placement <namespace>/<placement-ruleset name>

How placements currently work is documented in detail on the project's website [5]. Describing the many options in detail would go beyond the scope of this article.

Conclusions

OCM provides the appropriate underpinnings for multicluster management for Kubernetes. In addition to the integrated functions for management and placement, the tool also comes with an add-on manager. Other projects extend the functionality, but only commercial implementations offer a GUI for OCM. Fortunately, as its use in hybrid environments increases, it is probably only a matter of time until free Kubernetes user interfaces offer a graphical view of OCM.

If you are interested in managing edge devices that use a miniature version of Kubernetes or just a simple container runtime like Podman, you might look at Flotta (see the "Flotta Alternative" box).