You might find yourself in situations where you believe that a logic implementation should occur only if and when your Operator is running on a specific Kubernetes platform. So, you probably want to know how to get the cluster vendor from the operator. In this article, we will discuss why relying on the vendor is not a good idea. Also, we will show how to solve this kind of scenario.
Why not develop solutions based on the vendor?
Let's think about the scenario where we have an Operator that will take further actions only if the Operator Lifecycle Manager (OLM) is installed in the cluster. Then, could we not check whether the cluster is an OpenShift platform because the OLM is provided by default?
No, we cannot. Note that besides the OLM being installed and available in OpenShift 4.X via the default installation, it also might not always be truthful. And then, it is essential to highlight that the OLM can be installed on other vendors as well.
Thus, this example provides a pretty good idea of the problems that can be caused by relying on such assumptions.
What's the best approach, then?
The best approach to handle this scenario is to look for the exact API resource that is required for the solution, instead of using a resource that assumes the Operator is running or not on a specific Kubernetes platform.
In the above case, we were looking to see if the OLM's Custom Resources Definitions (API Resource), which are essential to the solution, are present or not.
If you want to create a Route resource when the project is running in OpenShift and an Ingress (instead of, for example, only if it is running in the Minikube), then we should be checking whether the specific v1.Route is installed or not.
How to implement this approach
Let's start by creating the discovery client:
// Get a config to talk to the apiserver
cfg, err := config.GetConfig()
if err != nil {
log.Error(err, "")
os.Exit(1)
}
// Create the discoveryClient
discoveryClient, err := discovery.NewDiscoveryClientForConfig(cfg)
if err != nil {
log.Error(err, "Unable to create discovery client")
os.Exit(1)
}Note: The cfg *rest.Config is created in the main.go, by default in all projects built with Operator-SDK.
Now, we can search for all API resources as follows:
// Get a list of all API's on the cluster
apiGroup, apiResourceList , err := discoveryClient.ServerGroupsAndResources()
if err != nil {
log.Error(err, "Unable to get Group and Resources")
os.Exit(1)
}
Note that by using the kubectl api-resources command, it is possible to check the resources available in the cluster as follows:
$ kubectl api-resources NAME SHORTNAMES APIGROUP NAMESPACED KIND bindings true Binding componentstatuses cs false ComponentStatus configmaps cm true ConfigMap endpoints ep true Endpoints events ev true Event limitranges limits true LimitRange namespaces ns false Namespace nodes no false Node persistentvolumeclaims pvc true PersistentVolumeClaim persistentvolumes pv false PersistentVolume pods po true Pod podtemplates true PodTemplate replicationcontrollers rc true ReplicationController resourcequotas quota true ResourceQuota secrets true Secret serviceaccounts sa true ServiceAccount services svc true Service mutatingwebhookconfigurations admissionregistration.k8s.io false MutatingWebhookConfiguration validatingwebhookconfigurations admissionregistration.k8s.io false ValidatingWebhookConfiguration customresourcedefinitions crd,crds apiextensions.k8s.io false CustomResourceDefinition apiservices apiregistration.k8s.io false APIService ...
Now, we can check and verify if the resource is or is not on the cluster:
// Looking for group.Name = "apiextensions.k8s.io"
for i := 0; i < len(apiGroup); i++ {
if apiGroup[i].Name == name {
// found the api
}
}Note: It is possible to use other attributes, such as the Group, Version, and Kind. For further information, check the APIGroup GoDoc.
It is possible to use the result of apiResourceList:
// Looking for Kind = "PersistentVolume"
for i := 0; i < len(apiResourceList); i++ {
if apiResourceList[i].Kind == kind {
// found the Kind
}
}Note: Check the APIResourceList GoDoc to see other options.
How to get the cluster version information
Let's follow a code implementation as an example that can be used to get the cluster version by using the DiscoveryClient as well:
// getClusterVersion will create and use an DiscoveryClient
// to return the cluster version.
// More info: https://godoc.org/k8s.io/client-go/discovery#DiscoveryClient
func getClusterVersion(cfg *rest.Config) string {
discoveryClient, err := discovery.NewDiscoveryClientForConfig(cfg)
if err != nil {
log.Error(err, "Unable to create discovery client")
os.Exit(1)
}
sv, err := discoveryClient.ServerVersion()
if err != nil {
log.Error(err, "Unable to get server version")
os.Exit(1)
}
return sv.String()
}Keep in mind that these ideas can be beneficial and allow you to manage your solutions dynamically and programmatically. See that they can be also useful to handle issues on specific scenarios as well. An then, used beyond the installation and configuration process.
Also, before finishing, I'd like to thank @Joe Lanford and @Jeff McCormick, who collaborated and provided feedback and input for this article.
Last updated: June 29, 2020