Document unconditional update

Signed-off-by: Xudong Sun <[email protected]>

discussion/kubernetes-model/unconditional_update.md

@@ -0,0 +1,18 @@
+## Unconditional update
+
+### Description
+
+Many types of Kubernetes objects allow one to update the objects without providing a resource version, which is called [unconditional update](https://github.com/kubernetes/kubernetes/blob/v1.30.0/staging/src/k8s.io/apiserver/pkg/registry/generic/registry/store.go#L623-L631). During conditional update, API server will not perform resource version checking.
+
+Objects that allow unconditional update include:
+- [ConfigMap](https://github.com/kubernetes/kubernetes/blob/v1.30.0/pkg/registry/core/configmap/strategy.go#L96-L98)
+- [Secret](https://github.com/kubernetes/kubernetes/blob/v1.30.0/pkg/registry/core/secret/strategy.go#L100-L102)
+- [Service](https://github.com/kubernetes/kubernetes/blob/v1.30.0/pkg/registry/core/service/strategy.go#L112-L114)
+- [Pod](https://github.com/kubernetes/kubernetes/blob/v1.30.0/pkg/registry/core/pod/strategy.go#L161-L164)
+- [ReplicaSet](https://github.com/kubernetes/kubernetes/blob/v1.30.0/pkg/registry/apps/replicaset/strategy.go#L179-L181)
+- [Deployment](https://github.com/kubernetes/kubernetes/blob/v1.30.0/pkg/registry/apps/deployment/strategy.go#L167-L169)
+- [DaemonSet](https://github.com/kubernetes/kubernetes/blob/v1.30.0/pkg/registry/apps/daemonset/strategy.go#L183-L186)
+- [StatefulSet](https://github.com/kubernetes/kubernetes/blob/v1.30.0/pkg/registry/apps/statefulset/strategy.go#L189-L192)
+
+Objects that do not allow unconditional update include:
+- [CustomResource](https://github.com/kubernetes/kubernetes/blob/v1.30.0/staging/src/k8s.io/apiextensions-apiserver/pkg/registry/customresource/strategy.go#L271-L274)