Negentropy https://github.com/hoytech/strfry?tab=readme-ov-file#syncing

I'm not sure if it works with other relay implementations (I believe it doesn't) but certainly it's the most efficient way. 

 so relays become part of a federation and then sync between themselves while robosats clients fetch these events normally? 

 yep, every coordinator runs their own relay where they publish their own order.

 Every minute this relay takes cares of synching with the other relays of the federation pushing and pulling the missing orders and eventually consolidating a global order book where all of them contains all existing orders 

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