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A lot of Google projects seem to rely on other Google projects. In this case Monarch relies on spanner.

I guess its nice to publish at least the conceptual design so that others can implement it in “rest of the world” case. Working with OSS can be painful, slow and time consuming so this seems like a reasonable middle ground (although selfishly I do wish all of this was source available).

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Spanner may be hard to set up even with source code available. It relies on atomic clocks for reliable ordering of events.

Atomic clocks aren't that exotic, and a GPS disciplined ovenized quartz oscillator will do just fine outside of a disruption. The hard part is getting the right sampling semantics, requiring end to end error analysis.

It’s pretty hard to get well synchronised clocks between servers in a datacentre.

I don't think there's any spanner necessity and iirc monarch existed pre-spanner.

Correct. Spanner is used to hold configuration state, but is not in the serving path.

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