[server] Support DescribeTabletServers API for safe scaling and upgrades#3743
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[server] Support DescribeTabletServers API for safe scaling and upgrades#3743morazow wants to merge 1 commit into
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…upgrades Add a read-only DescribeTabletServers RPC to the Coordinator, following the pattern of GetClusterHealth (apache#3399). It returns, per tablet server, the same four counters getClusterHealth() reports cluster-wide, scoped to that server: numReplicas, inSyncReplicas, numLeaderReplicas and activeLeaderReplicas. This lets operational tooling (e.g. the Fluss Kubernetes Operator): - gate scale-in on a server being empty (numReplicas == 0), - gate rolling upgrades on a per-server green predicate (inSyncReplicas == numReplicas && activeLeaderReplicas == numLeaderReplicas), - report per-server tablet load in cluster status. The counters are computed from in-memory CoordinatorContext state on the coordinator event thread via AccessContextEvent. Tablet servers forward the request to the coordinator. Live and shutting-down servers are always reported, even with zero replicas, and dead servers that still hold assigned replicas remain visible so a scale-in gate cannot treat them as drained. Closes apache#3570
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Summary
DescribeTabletServersRPC to the Coordinator, following the pattern ofGetClusterHealth([server] Support Cluster Health API for safe rolling upgrades #3399 / [server] Add Cluster Health API implementation #3400).Admin.describeTabletServers()(@PublicEvolving) returns, per tablet server, the same four countersgetClusterHealth()reports cluster-wide, scoped to that server:numReplicas,inSyncReplicas,numLeaderReplicas,activeLeaderReplicas.CoordinatorContextstate on the coordinator event thread (viaAccessContextEvent); tablet servers forward the request to the coordinator over the internal listener.serverGreen(serverId)rolling-upgrade gate, the scale-in safety gate (numReplicas == 0), and per-server tablet load reporting.Fixes #3570
Test Plan
DescribeTabletServersTest(fluss-server) covering: empty cluster, full ISR, out-of-ISR replicas, inactive leaders, leaderless buckets, evacuated servers, dead-but-assigned servers, partitioned tables, and reconciliation of per-server sums withcomputeClusterHealth.FlussAdminITCase#testDescribeTabletServersDuringRollingUpgrade: healthy cluster is green and sums match cluster health; a stopped server stays reported, non-empty and non-green; after restart all servers become green again.mvn spotless:checkclean on fluss-rpc, fluss-server, fluss-client, fluss-flink-common.🤖 AI-assisted changes - reviewed by human developer