[friseurtermin]

Great post! I'm not familiar with all of fly's inner workings but I'm guessing that each app gets its own Redis instance at each location (rather than all of your customers sharing instances) because "global" item replication is a bit cheaper if "global" == locations where the app actually exists. I wonder if there's a way for an application to write to a subset of locations (i.e. locations all over the US, I just want to update something in California). Obv I could have each of my CA locations do a local update but maybe I don't want to

[tptacek]

The Redis instances we're talking about here are "just" Fly.io apps; they're not like a special platform feature or anything. Which is good news; it means you can do whatever you want with Redis. You could, I guess, do a replica-of-replicas tree configuration, with app nodes holding connections (Redis connections are really cheap) to their local node, a macro-regional node, and the global node and selectively synchronizing. Or you could go the other way and lose some Redis servers, keeping Redis servers only in certain regions. We make it sort of straightforward for apps to suss out the topology with DNS queries.