I don't think you understand what HA means.
The app would look up in both databases. If it exists in any, there would be a session.
Thisnis strictly different from partitioning which I think you are mixing it up with.
Paritioning is for performance not HA
> The app would look up in both databases. If it exists in any, there would be a session.
And if you find the session with differing values in both databases, how do you know which one is up-to-date?
You need an algorithm to pick which data is right, such as electing a master instance.
And that brings us back to the original discussion: to manage sessions (unlike caches) in a highly available way, you need to setup HA (or reimplement it, which obviously is a bad idea). You can't read round robin from multiple non-HA instances.
Yes, you are pointing out exactly how HA is difficult.
There is a whole slew of downstream things you need to take into consideration.
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That’s the precise point I’m making