[MB-64652] FTS Nodes Upgrade Issue from 7.6.0–7.6.3 to 7.6.4

Product: Couchbase Server
Component: fts-service
Issue Link: MB-64652
Affects Version(s): 7.6.4
Fix Version(s): 7.6.5

Summary

• Upgrading FTS nodes from Couchbase Server 7.6.0–7.6.3 to 7.6.4 can cause the cluster to enter a state where rebalances fail due to FTS version mismatches.
• Due to this issue, we recommend skipping the upgrade to version 7.6.4 and proceeding directly to version 7.6.5, where the problem has been resolved.

Symptoms 

Rebalances will consistently fail with the reported error planner: version too low: 5.7.0

Rebalance exited with reason {service_rebalance_failed,fts,
{worker_died,
{'EXIT',<0.11569.2>,
{task_failed,rebalance,
{service_error,
<<"planner: version too low: 5.7.0">>}}}}}.
Rebalance Operation Id = 74e30896c8278e9aea8d58eb6d746388

Triggers

Adding a 7.6.4 node running FTS to a 7.6.x cluster (x<4) with FTS nodes triggers the issue. Once the state is hit, all rebalances fail from that point onwards.

Verification

• The message planner: version too low: 5.7.0 is a distinctive symptom.
• The following is visible in ns_server debug logs:

[ns_server:debug,2024-12-16T11:13:01.354Z,ns_1@node1:service_agent-fts<0.656.0>:service_agent:cleanup_service:641]Cleaning up stale tasks:

• You’ll also see the same error in the FTS logs:

2024-12-16T11:27:39.593+00:00 [WARN] ctl: PlannerSteps, err: planner: version too low: 5.7.0 -- ctl.(*Ctl).startCtlLOCKED.func2() at ctl.go:1256

Workarounds

• There is currently no good workaround available. Please contact the Couchbase Support Team to discuss potential options.