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Although the solution presented by Marc is also the closest to a consistent backup that I can think of, there are obviously caveats to it. Updates of values which were written after the time of the Scylla snapshot could be present in ES, corrupting the state of the index. Therefore, checking the pure existence of a vertex in Scylla may not be sophisticated enough to guarantee a consistent state. Verifying the property values explicitly can be helpful here, but that still leaves us with the question how to handle mismatches of this kind.
Just keep that in mind when using such a backup strategy in your environment.