OnlineSchemaChange VS reshape

I’ve been through multiple incidents where everything worked fine in the testing environment but ended up locking the production database for minutes when deployed. A category of open-source tools called OSC (Online Schema Change) exists to mitigate such pain, like gh-ost used by GitHub and OSC used by Meta. They work by creating a set of "ghost tables" to apply the migrations, copy over old data from the original tables, and catch up with new writes simultaneously. When all old data is migrated, you can trigger a cutover to make the "ghost tables" production. Check the post below for a great introduction and comparison:

For larger companies, with more data and high-traffic tables, schema changes may happen less often, but they still need to worry about things like downtime caused by locking. They require long internal guides on performing schema changes correctly (e.g. GitLab, PayPal), custom tools (e.g. Meta, Square), and they often document incidents or near-incidents caused by schema migrations (e.g. GitHub, Doctolib, GoCardless).

Cool stuff! Do you have any thoughts about how this compares to https://github.com/fabianlindfors/reshape?

From what I know, there is only one project that tries something close to this: the relatively recent Reshape. It uses Postgres views to expose the two versions of the schema and triggers to upgrade/downgrade the new data. It doesn’t do the constraints part as described above, but shows that this approach is possible. Combined with the Xata pull request based workflow, I think the ideal system described above is possible!

My idea is not unique. Reshape is similar, but bigger in scope.

This is precisely what I did for my automated zero-downtime migration tool and it works pretty well: https://github.com/fabianlindfors/reshape. At least for Postgres, simple views like these have almost no overhead as queries are simply rewritten for the underlying table.

Anybody interested in this subject might also be interested in a tool for Postgres I’ve been working on, Reshape: https://github.com/fabianlindfors/reshape. It aims to fully automate away all the pain and manual steps zero-downtime migrations normally requires :)

This post is absolutely terrific and has been been my main reference for Reshape, an automated, zero-downtime schema migration tool: https://github.com/fabianlindfors/reshape