Tracking: deprecation of gh-ost and pt-osc Online DDL strategies

[shlomi-noach]

Vitess (v19) supports these Online DDL strategies:

• vitess, (aka online), based on vreplication.
• gh-ost, using gh-ost.
• pt-osc, using pt-online-schema-change.

Both gh-ost and pt-online-schema-change are external to Vitess, and the Vitess maintainers do not actively participate in supporting these tools. We therefore wish to remove support for the respective strategies. See below.

pt-osc removal (version TBD)

From day 1, pt-osc strategy was marked as experimental. As such, it was never truly supported. The Vitess authors have no information about how community might be using pt-osc as an Online DDL strategy.

• We wish to remove this strategy altogether, along with the code that implements it.
• Specifying --ddl-strategy="pt-osc" will throw an error.

gh-ost

gh-ost is known to be in use in the community, at least on older versions of Vitess. The Vitess maintainers have themselves used gh-ost in production at PlanetScale for a substantial period of time with good results.

We wish to deprecate gh-ost in multiple stages.

Stage 1, unsupport, slated to v20

• Mark gh-ost strategy unsupported and experimental.
• Today, vttablet binary embeds a gh-ost binary. We will remove it. Users will need to supply their own gh-ost binary.
• Remove gh-ost related endtoend tests from CI.
• --ddl-strategy="gh-ost" will still work, nothing changes.

Stage 2, removal (version TBD)

• Remove gh-ost supporting code.
• --ddl-strategy="gh-ost" will throw an error.