For partial MoveTables, setup reverse shard routing rules on workflow creation

[mattlord]

Description

For normal MoveTables operations — where you're moving tables from one keyspace to another, i.e. the move is at the keyspace level — global routing rules are put in place when the workflow is created so that any attempted access of those tables on the target keyspace are routed to the source keyspace. (In this scenario shard targeting is how you would explicitly get around this routing.)

For partial MoveTables operations, you're moving the tables from one keyspace to another one shard at a time — so the move is per shard rather than per keyspace. In this scenario you want all traffic to go to one side or the other for a given shard, including shard targeted queries. This then offers the same kind of "global" (scoped to a shard) routing at the shard level that you would expect when doing keyspace level MoveTables. This PR implements that functionality.

Note
We have NO documentation on partial MoveTables today (not even the relevant vttablet and vtgate flags). Should I create a new page in the docs here, or continue to defer that part?
Answer: YES. See: vitessio/website#1160

Related Issue(s)

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were updated
• Documentation: Document shard level migrations feature website#1160

[vitess-bot]

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[deepthi]

We have NO documentation on partial MoveTables today (not even the relevant vttablet and vtgate flags). Should I create a new page in the docs here, or continue to defer that part?

I think we should document it. @rohit-nayak-ps thoughts?

[rohit-nayak-ps]

I think we should document it. @rohit-nayak-ps thoughts?

Yes, that is the plan. We will have a link from MoveTables to a separate page highlighting the use cases, shard routing rules and their effect, limitations (need to disable schema tracking, for example) and the fact that it is experimental. Should be ready this week.

[rohit-nayak-ps]

lgtm

[mattlord]

Please note that this will be merged (and backported to release-15.0) just after the v15 RC1 release so that:

1. We don't interfere with that timely release
2. We have a feature complete shard level migration feature in 15.0 (see docs here: Document shard level migrations feature website#1160)