vtctl ApplySchema supports '-request_context' flag

[shlomi-noach]

Description

An online DDL migration is associated with both a unique UUID as well as a migration_context, aka the "context".

The context is unique to the scope/origin of the request.

• from VTGate, you can run multiple migrations, each has a unique UUID, but all share the same context which is the @@session_uuid.
• In a single vtctl ApplySchema command you may specify multiple DDLs; each has its own unique UUID but all share the same context.

The context is useful to logically grouping migrations.

Until today, vtctl would auto-generate the migration context. This PR allows the user to optionally override that, and specify their own context. It is the user's responsibility to handle uniqueness of the context.

The recommendation is to use a UUID format, preceded with an identity of the originator, as is the status today. Examples:

• vtgate:a8352418-8d55-11eb-815f-f875a4d24e90
• vtctl:fa4a2e13-8a46-11eb-85f5-f875a4d24e90

At this time the migration context is strictly a logical tag, used to search/filter migrations. Supplying a non-unique context does not break the migrations in any way.

Related Issue(s)

• Tracking: Online DDL: tracking issue #6926

Checklist

• Should this PR be backported?
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving
• VReplication
• Cluster Management
• Build/CI
• VTAdmin

[ajm188]

lg2m!