feature request: VTGate support for retrieving GTIDs from shards

[gordonyoon]

Feature Description

Expose a VTGate command that retrieves the GTID of a given shard.

Our current solution is to run the following commands:

SHOW vitess_shards;
for each:
  USE keyspace/shard;
  SELECT @@global.gtid_executed;

Use Case(s)

We have a query-cache system that relies on the VGTID passed through VStream. From the query-cache, we get the current GTID for a shard and apply messages from VStream based on the relative position of the VGTID to the one retrieved directly from VTGate.

[gordonyoon]

Queries to SHOW vitess_shards frequently returns the following error:

Code: UNKNOWN
vtgate: http://0765d3618921:15001/: keyspace names fetch error: interrupted: keyspaces

From Jacques:

The timeout for that query (when it hits the topology) is rather short; but the underlying problem is that those entries are cached for a very short amount of time by vtgate. We can boost the cache TTL; which will make it perform better in the general case. But the occasional timeout is still possible (but less likely/frequent as you up the TTL).

[harshit-gangal]

This can be supported with a new query syntax
SHOW GLOBAL VGTID_EXECUTED - This will give VGTID from selected keyspace
SHOW GLOBAL VGTID_EXECUTED FROM <KEYSPACE> - This will give VGTID from provided keyspace

[gordonyoon]

It would be great to also understand the availability of the command.

[harshit-gangal]

@gordonyoon The command is as available as the cluster.

[harshit-gangal]

We have added 2 more commands

SHOW GLOBAL GTID_EXECUTED - This will give GTID from all shards in the selected keyspace
SHOW GLOBAL GTID_EXECUTED FROM <KEYSPACE>- This will give GTID from all shards in provided keyspace