Feature Request: filter topology by shard in vttablet txthrottler

[timvaillancourt]

Feature Description

The txthrottler (on the PRIMARY) uses healthcheck streams to determine the replication lag of replicas in the shard

Today, the txthrottler uses discovery.NewCellTabletsWatcher to watch the topology with a filter on the keyspace only. This means the topology watcher is "watching" more tablets than is necessary

This feature request is to support filtering by keyspace AND shard

Use Case(s)

Standard deploy w/TxThrottler enabled

[frouioui]

@vitessio/cluster-management

[harshit-gangal]

Before was the health checks from other shards ignored? Were they taking part in determining that throttling to be done?

[timvaillancourt]

Before was the health checks from other shards ignored?

@harshit-gangal in v14 and below only the local shard was watched, which I feel is more correct/efficient

This behaviour was changed to filter by keyspace-only here in this PR from v15

Were they taking part in determining that throttling to be done?

I feel the Transaction Throttler (on PRIMARY) should only be concerned about replicas within it's local shard. This will prevent lag in one shard affecting all

[harshit-gangal]

That looks like the side effect of removing the legacy health check. Thank you for digging into that.

[timvaillancourt]

Indirectly resolved by #14412. Thanks @deepthi!