Add multiplehubs feature. (#104)

Signed-off-by: xuezhaojun <zxue@redhat.com>

enhancements/sig-architecture/100-multiple-bootstrapkubeconfigs/README.md

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+# Support multiple bootstrap kubeconfigs and switching between hubs - MultipleHubs
+
+## Release Signoff Checklist
+
+- [] Enhancement is `implemented`
+- [] Design details are appropriately documented from clear requirements
+- [] Test plan is defined
+- [] Graduation criteria for dev preview, tech preview, GA
+- [] User-facing documentation is created in [website](https://github.com/open-cluster-management-io/open-cluster-management-io.github.io/)
+
+## Summary
+This proposal aims to provide a mechanism to support configuring multiple bootstrapkubeconfigs and that will enable agents to smoothly switch between hubs at scale with fewer manual operations.
+
+## Motivation
+Currently, an agent can only point to one hub, if we want to switch to another hub, we need replace the secret `bootstrap-hub-kubeconfig` in `open-cluster-management-agent` namespace manually.
+
+To address this, we will support configuring multiple bootstrapkubeconfig secrets and provide a simple mechanism for agents to determine which hub they should connect to. And with multiple bootstrap kubeconfigs configured, agents are possible to complete the switching process without manual operations.
+
+This mechanism will be beneficial for various scenarios, including upgrading, disaster recovery, improving availability, etc.
+
+## Goals
+- Trigger the agent to reselect when the current hub it connected to is not available.
+- Trigger the agent to reselect when user explicitly deny the connection to the current hub.
+
+## Non-Goals
+- To ensure bootstrapkubeconfig candidates are valid **during the candidate selection phase** is out of the scope.
+- To determine which bootstrap kubeconfig is suitable for a sepcific hub at any given time. This depends on the user's specific scenario.
+- To synchronize customer workloads and configurations between hubs, ensure that the resources of the hubs are synchronized.
+    - If the resources are not synchronized, the old customer workload may be wiped out after the switch.
+
+## Use cases
+
+### Story1 - Backup and restore
+As a user, I want to switch agents to the new(restored) hub **without manual operations**.
+
+### Story2 - Rolling upgrade
+As a user, I want to switch half of the agents to the upgraded hub to test the functionality of the upgraded components.
+
+### Story3 - High availability architecture
+As a user, I want to configure 2 hubs in active-active mode to improve management availability. In this setup, if one hub goes down, the other hub will continue to provide management capabilities.
+
+## Risk and Mitigation
+N/A
+
+## Design Details
+
+The `MultipleHubs` feature should be controlled by a feature gate of Klusterlet:
+
+```
+apiVersion: operator.open-cluster-management.io/v1
+kind: Klusterlet
+...
+spec:
+  ...
+  registrationConfiguration:
+    ...
+    featureGates:
+      - feature: MultipleHubs
+        mode: Enable
+```
+
+The multiple bootstrapkubeconfigs can be configured in the Klusterlet by the `bootstrapKubeConfigs` field:
+
+```
+apiVersion: operator.open-cluster-management.io/v1
+kind: Klusterlet
+...
+spec:
+  ...
+  registrationConfiguration:
+    ...
+    featureGates:
+      - feature: MultipleHubs
+        mode: Enable
+    bootstrapKubeConfigs:
+      type: "LocalSecrets"
+      localSecretsConfig:
+        kubeConfigSecrets:
+            - name: "hub1-bootstrap"
+            - name: "hub2-bootstrap"
+        hubConnectionTimeoutSeconds: 600
+```
+
+At the initial stage, we only support the `LocalSecrets` type, which means the bootstrapkubeconfig secrets are stored in the managed cluster.
+
+### Workflow
+
+Let’s seen an example, assume we have 2 hubs: `hub1` and `hub2`, and a managed cluster named `cluster1`. We want `cluster1` register to `hub1` first, and then switch to `hub2`.
+
+First, we create 2 secrets on the agent `open-cluster-management-agent` namespace on the managed cluster `cluster1`:
+
+```yaml
+# The kubeconfig points to hub1
+kind: Secret
+apiVersion: v1
+metadata:
+  name: hub1-bootstrap
+  namespace: open-cluster-management-agent
+data:
+  kubeconfig: YXBpVmVyc2lvbjogdjEK...
+```
+
+```yaml
+# The kubeconfig points to hub2
+kind: Secret
+apiVersion: v1
+metadata:
+  name: hub2-bootstrap
+  namespace: open-cluster-management-agent
+data:
+  kubeconfig: YXBpVmVyc2lvbjogdjEK...
+```
+
+And the `bootstrapKubeConfigs` field in the Klusterlet is configured as follows:
+
+```yaml
+bootstrapKubeConfigs:
+  type: "LocalSecrets"
+  localSecretsConfig:
+    kubeConfigSecrets:
+        - name: "hub1-bootstrap"
+        - name: "hub2-bootstrap"
+    hubConnectionTimeoutSeconds: 600
+```
+
+The agent will choose bootstrap kubeconfigs in order from the `kubeConfigSecrets` list. It will first try the kubeconfig from the first secret in the list ("hub1-bootstrap"), and if that fails it will try the second one ("hub2-bootstrap"), and so on. Once a working kubeconfig is found, the agent will use it to start bootstrapping.
+
+In our case, the agent will first register on hub1 successfully. And to trigger it switches to `hub2`, we simply set the `hubAcceptsClient` to `false` on the managed cluster CR on hub1:
+
+```yaml
+apiVersion: cluster.open-cluster-management.io/v1
+kind: ManagedCluster
+...
+spec:
+  ...
+  hubAcceptsClient: false
+```
+
+If the hubAcceptsClient is set to false, the managed cluster currently connected to the hub will immediately disconnect from the hub and try to connect to another hub cluster.
+
+On the other hand, if the agent lost connection to the hub over the `hubConnectionTimeoutSeconds`, it will try to connect to another hub cluster as well.

enhancements/sig-architecture/100-multiple-bootstrapkubeconfigs/metadata.yaml

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+title: multiplehubs
+authors:
+  - "@xuezhaojun"
+reviewers:
+  - "@qiujian16"
+  - "@deads2k"
+approvers:
+  - "@qiujian16"
+  - "@deads2k"
+creation-date: 2023-10-30
+last-updated: 2023-10-30
+status: provisional