[improve][pip] PIP-400: Support namespace-level configuration of migratedClusterUrl in blue-green migration feature

pip/pip-400.md

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+# PIP-400: Support namespace-level configuration of migratedClusterUrl in blue-green migration feature
+
+# Background knowledge
+
+The blue-green cluster migration feature supports migrating topic traffic from one cluster to another. 
+Currently, the migration feature can be enabled at both the cluster and namespace levels, 
+and the target migration cluster information can be configured at the cluster level.
+
+# Motivation
+
+However, there are times when we want to migrate different namespaces to different clusters. 
+For example, a certain namespace belongs to a specific business and is currently in a public cluster. 
+We want to migrate this namespace to a dedicated cluster for that business.
+Therefore, we would like to support configuring the target migration cluster information at the namespace level.
+
+# Goals
+
+## In Scope
+
+Support configuring target migration cluster information at the namespace level, 
+allowing different namespaces to be migrated to different clusters.
+
+## Out of Scope
+
+# High Level Design
+
+Allow configuring target migration cluster information at the namespace level, 
+with this configuration having a higher priority than cluster-level configuration.
+
+# Detailed Design
+
+## Design & Implementation Details
+
+- Add a property of type `ClusterUrl` named `migratedClusterUrl` in the `localPolicies` of namespace. This configuration should have a higher priority than the cluster-level configuration.
+- Add or modify the public API and CLI to support updating the value of `migratedClusterUrl`.
+
+## Public-facing Changes
+
+### Public API
+Add a new API:
+```java
+    /**
+     * Update migration state for a namespace.
+     * @param namespace
+     *            Namespace name
+     * @param migrated
+     *            Flag to determine namespace is migrated or not
+     * @param clusterUrl
+     *            Cluster url data
+     * @throws NotAuthorizedException
+     *             Don't have admin permission
+     * @throws NotFoundException
+     *             Namespace does not exist
+     * @throws PulsarAdminException
+     *             Unexpected error
+     */
+    void updateMigrationState(String namespace, boolean migrated, ClusterUrl clusterUrl) throws PulsarAdminException;
+
+    /**
+     * Update migration state for a namespace asynchronously.
+     */
+    CompletableFuture<Void> updateMigrationStateAsync(String namespace, boolean migrated, ClusterUrl clusterUrl);
+```
+
+### Binary protocol
+
+### Configuration
+
+### CLI
+Modify the `update-migration-state` command to add an option for configuring the `ClusterUrl` information.
+```shell
+update-migration-state --migrated --service-url serviceUrl --service-url-secure serviceUrlTls --broker-url brokerServiceUrl --broker-url-secure brokerServiceUrlTls tenant/namespace
+```
+
+### Metrics
+
+# Monitoring
+
+# Security Considerations
+Only superuser can access the admin API.
+
+# Backward & Forward Compatibility
+
+## Upgrade
+
+## Downgrade / Rollback
+
+## Pulsar Geo-Replication Upgrade & Downgrade/Rollback Considerations
+
+# Alternatives
+
+# General Notes
+
+# Links
+
+* Mailing List discussion thread: https://lists.apache.org/thread/bq7g3916fvh5xskvkn6bnynfy8zmr5mo
+* Mailing List voting thread: