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Comment by JustLurking2022 | Hacker News Reader

JustLurking2022

Holy hell does that sound awful. In practice, it doesn't even solve the core problem as you'd have to deploy new versions of all binaries precisely in sync to avoid data incorrect because some clients know about the v2 table and others don't. It becomes indistinguishable whether a row is missing because it was intentionally deleted or because it was written by a client who didn't know any the new table version. There are ways to account for this but it's layers of crap on top of crap.

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> it doesn't even solve the core problem as you'd have to deploy new versions of all binaries precisely in sync to avoid data incorrect because some clients know about the v2 table and others don't

its painful but this approach does work as long as every client is migrated between each step.

after 1: v1 is valid, reading v1, writing v1

after 2: v1 is valid, reading v1, writing v1 and v2

after 3: v1 and v2 are valid, reading v1, writing v1 and v2

after 4: v1 and v2 are valid, reading v2, writing v1 and v2

after 5: v2 is valid, reading v2, writing v2

after 6: v2 is valid, reading v2, writing v2

at each point, both the current and the previous version are reading a valid table and writing to a table whose values will make it into v2.

Good luck migrating every single client in the real world (unless you are willing to disable old clients & force an upgrade if they get too old)

These are the same problems you have with any API or package you ship.

Is there really any alternative? Data migrations are always going to be terrible. You need to slowly roll out the incremental changes so that everything is backwards compatible until you the threshold point where you can finally delete the historical route.

With an API, you can often create facades and other patterns to avoid having to migrate the underlying data

You can often also accomplish this with a database view.

In a sensible approach schema migrations are totally decoupled from the API.

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