Add missing tables to globally routed list in schema tracker only if they are not already present in a VSchema

[rohit-nayak-ps]

Description

Follows up on the idea in #16517 by not considering tables already present in the globally routable list of tables.

See RFC for design and implementation details.

Related Issue(s)

#16516

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.66%. Comparing base (9383943) to head (30363c5).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17371      +/-   ##
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- Coverage   67.67%   67.66%   -0.02%     
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  Files        1583     1584       +1     
  Lines      254363   254386      +23     
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- Hits       172140   172121      -19     
- Misses      82223    82265      +42     

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[GuptaManan100]

Just a nomenclature nit, otherwise looks good to me!

[harshit-gangal]

The issue with this approach arises when there are multiple unsharded keyspaces. This makes adding the table to global routing non-deterministic, failing to fully achieve the intended purpose of this PR.

[rohit-nayak-ps]

The issue with this approach arises when there are multiple unsharded keyspaces. This makes adding the table to global routing non-deterministic, failing to fully achieve the intended purpose of this PR.

You mean if there are tables of the same name in multiple unsharded keyspaces (and which are not in the VSchemas).

We could collect all tables names which will be added like this (not in VSchema, but found by schema tracking) first and validate that there are no duplicates, marking ones with duplicates as ambiguous, while others get globally routed.

[rohit-nayak-ps]

Discussed with @harshit-gangal . His point is that:

1. We start with a sharded keyspace Sharded1 and one unsharded keyspace Unsharded1. Unsharded1 has no vschema defined yet.
2. After this PR, schema tracking finds a table ut1 in Unsharded1 that is not in the current VSchema. It then adds it to the global tables list. User starts depending on select * from ut1.
3. Later user adds another unsharded keyspace Unsharded2 which also has table named ut1.
4. Now we will mark ut1 as ambiguous (this logic is not yet part of this PR, but is the enhancement I referred to in Add missing tables to globally routed list in schema tracker only if they are not already present in a VSchema #17371 (comment))
5. Suddenly the user's select * from ut1 query will start erroring out. User won't have visibility as to why this has happened since they have not done anything obvious in Vitess like modifying/adding a VSchema and will have downtime until they drop the other table, which could be difficult to diagnose/fix.

Options here are:

1. Close this PR and not do anything
2. Merge this PR (with enhanced logic to detect ambiguous tables outside of current VSchemas), which will solve the problem for X% of customers who currently have downtime because we don't do this

I guess the question is what is X. Is it high enough to merge this. Or even if X is high, do we want to do this from a principled viewpoint.

[rohit-nayak-ps]

After discussions we decided to go ahead with the implementation as detailed in the RFC

[rohit-nayak-ps]

I have added a vtgate flag, --mark_unique_unsharded_tables_as_global (default true) to allow us the ability to disable the logic added in the PR in case some breaking changes have been introduced. I can remove it if reviewers feel it is not needed.

[rohit-nayak-ps]

1. Why does BuildGlobalTables need to be exported? I couldn't find any references to it outside the package.
   Unexported. It was used in a previous version of the PR.

2. Do we need an empty vschema json file? Can it be removed?
   Removed

3. I assume you will be deleting the flag
   Deleted

[rohit-nayak-ps]

The next test is moved over from vschema_test.go so that it is colocated with the new global routing tests.

[rohit-nayak-ps]

I have added a vtgate flag, --mark_unique_unsharded_tables_as_global (default true) to allow us the ability to disable the logic added in the PR in case some breaking changes have been introduced. I can remove it if reviewers feel it is not needed.

This has been now removed.