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You should assume it will not work unless you test it regularly. That's a big part of why having active/active multi-region is attractive, even though it's much more complex.

That wouldn't have even caught that, most likely unless they verified they had no incidental tie ins with us-east-1.

The last place I worked actively switched traffic over to the backup nodes regularly (at least monthly) to ensure we could do it when necessary.

We learned that lesson by having to do emergency failovers and having some problems. :)