VReplication: Estimate lag when workflow fully throttled (#16577)

[mhamza15]

Backports vitessio#16577

/cc https://github.com/github/vitess/issues/1431

[Copilot]

Pull Request Overview

This PR backports changes from an upstream PR to improve how replication lag is estimated when a workflow is fully throttled. Key changes include:

• Updating the throttler app name used in tests.
• Adding logic to confirm and validate throttling metrics in tests.
• Refining lag estimation in the vplayer and cleaning up heartbeat injection in the vstreamer.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
go/test/endtoend/vreplication/vreplication_test.go	Updated app name in throttler usage and added validation via confirmVReplicationThrottling.
go/vt/vttablet/tabletmanager/vreplication/vplayer.go	Improved lag estimation logic and updated throttling calculations in applyEvents.
go/vt/vttablet/tabletserver/vstreamer/vstreamer.go	Removed an unused rate limiter and slightly restructured heartbeat injection logic.

Comments suppressed due to low confidence (3)

go/test/endtoend/vreplication/vreplication_test.go:61

• Ensure that changing targetThrottlerAppName from VReplicationName to VPlayerName is intentional and consistent with how throttling is expected to operate in these tests.

targetThrottlerAppName = throttlerapp.VPlayerName

go/vt/vttablet/tabletmanager/vreplication/vplayer.go:510

• Review the logic that excludes throttled heartbeat events from lag computation to ensure that it correctly handles lag estimation when batches include mixed event types.

if !(event.Type == binlogdatapb.VEventType_HEARTBEAT && event.Throttled) {

go/vt/vttablet/tabletserver/vstreamer/vstreamer.go:309

• Confirm that removing the rate limiter for throttled heartbeats does not lead to excessive heartbeat injections during throttling periods.

throttledHeartbeatsRateLimiter.Do(func() error { return injectHeartbeat(true) })

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[mhamza15]

Going to test on vitesst. Control case:

1. Running a MoveTables from vitesst-unsharded to vitesst-sharded
2. Generate writes using sysbench
3. Note throttler metrics healthy and 0 replication lag
4. Stop/disable vttablet on a replica
5. Note throttler metrics unhealthy but still 0 replication lag

Test case:

1. Running a MoveTables from vitesst-unsharded to vitesst-sharded
2. Generate writes using sysbench
3. Note throttler metrics healthy and 0 replication lag
4. Stop/disable vttablet on a replica
5. Note throttler metrics unhealthy and non-zero estimated replication lag

[mhamza15]

Closed in favor of #148