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Kubelet Configuration Directory Merging
When using the kubelet's --config-dir flag to specify a drop-in directory for configuration, there is some specific behavior on how different types are merged.
Here are some examples of how different data types behave during configuration merging:
Structure Fields
There are two types of structure fields in a YAML structure: singular (or a scalar type) and embedded (structures that contain scalar types). The configuration merging process handles the overriding of singular and embedded struct fields to create a resulting kubelet configuration.
For instance, you may want a baseline kubelet configuration for all nodes, but you may want to customize the address and authorization fields. This can be done as follows:
Main kubelet configuration file contents:
apiVersion: kubelet.config.k8s.io/v1beta1 kind: KubeletConfiguration port: 20250 authorization: mode: Webhook webhook: cacheAuthorizedTTL: "5m" cacheUnauthorizedTTL: "30s" serializeImagePulls: false address: "192.168.0.1" Contents of a file in --config-dir directory:
apiVersion: kubelet.config.k8s.io/v1beta1 kind: KubeletConfiguration authorization: mode: AlwaysAllow webhook: cacheAuthorizedTTL: "8m" cacheUnauthorizedTTL: "45s" address: "192.168.0.8" The resulting configuration will be as follows:
apiVersion: kubelet.config.k8s.io/v1beta1 kind: KubeletConfiguration port: 20250 serializeImagePulls: false authorization: mode: AlwaysAllow webhook: cacheAuthorizedTTL: "8m" cacheUnauthorizedTTL: "45s" address: "192.168.0.8" Lists
You can overide the slices/lists values of the kubelet configuration. However, the entire list gets overridden during the merging process. For example, you can override the clusterDNS list as follows:
Main kubelet configuration file contents:
apiVersion: kubelet.config.k8s.io/v1beta1 kind: KubeletConfiguration port: 20250 serializeImagePulls: false clusterDNS: - "192.168.0.9" - "192.168.0.8" Contents of a file in --config-dir directory:
apiVersion: kubelet.config.k8s.io/v1beta1 kind: KubeletConfiguration clusterDNS: - "192.168.0.2" - "192.168.0.3" - "192.168.0.5" The resulting configuration will be as follows:
apiVersion: kubelet.config.k8s.io/v1beta1 kind: KubeletConfiguration port: 20250 serializeImagePulls: false clusterDNS: - "192.168.0.2" - "192.168.0.3" - "192.168.0.5" Maps, including Nested Structures
Individual fields in maps, regardless of their value types (boolean, string, etc.), can be selectively overridden. However, for map[string][]string, the entire list associated with a specific field gets overridden. Let's understand this better with an example, particularly on fields like featureGates and staticPodURLHeader:
Main kubelet configuration file contents:
apiVersion: kubelet.config.k8s.io/v1beta1 kind: KubeletConfiguration port: 20250 serializeImagePulls: false featureGates: AllAlpha: false MemoryQoS: true staticPodURLHeader: kubelet-api-support: - "Authorization: 234APSDFA" - "X-Custom-Header: 123" custom-static-pod: - "Authorization: 223EWRWER" - "X-Custom-Header: 456" Contents of a file in --config-dir directory:
apiVersion: kubelet.config.k8s.io/v1beta1 kind: KubeletConfiguration featureGates: MemoryQoS: false KubeletTracing: true DynamicResourceAllocation: true staticPodURLHeader: custom-static-pod: - "Authorization: 223EWRWER" - "X-Custom-Header: 345" The resulting configuration will be as follows:
apiVersion: kubelet.config.k8s.io/v1beta1 kind: KubeletConfiguration port: 20250 serializeImagePulls: false featureGates: AllAlpha: false MemoryQoS: false KubeletTracing: true DynamicResourceAllocation: true staticPodURLHeader: kubelet-api-support: - "Authorization: 234APSDFA" - "X-Custom-Header: 123" custom-static-pod: - "Authorization: 223EWRWER" - "X-Custom-Header: 345"