Currently vtgate has support for filtering tablets, but only by keyspace/shard or key-range:

--tablet_filters strings Specifies a comma-separated list of 'keyspace|shard_name or keyrange' values to filter the tablets to watch.

This RFC proposes that vtgate supports filtering by KVs in the Tags map[string]string map of Tablet topodata.Tablet structs. This filtering would work in addition to the mutually-exclusive filters --tablet_filters and --keyspaces_to_watch

This filtering support would allow users more flexibility in routing queries from vtgate, specifically for testing upgrades, performance of new hardware, etc

I envision this support being controlled by a new tablet vtgate flag that allows a list of KVs, such as:
--tablet-filter-tags "mysql_version:8.0,instance_type:i3.xlarge". Users can apply these tag KVs using this existing vttablet flag (that adds the KVs to the Tags map[string]string of topodata.Tablet):

--init_tags StringMap (init parameter) comma separated list of key:value pairs used to tag the tablet

The existing filtering code can support a condition to filter on tablets that match these tags only

Use Case(s)

• A user that would like to direct traffic to specific tablets using arbitrary tablet tags, for example:
  • To test upgrading to MySQL 8.x could filter by tag mysql_version:8.0
  • To test upgrading tablets to a newer Vitess release, eg: vitess_major_version:19
  • To test new hardware classes, eg: instance_type:i3.xlarge