[flashgordon]

Out of curiosity what makes e2e encryption impact ordering guarantees? With feed things like ranking (bunch of ml pipelines) etc make it pretty stochastic?

On Whatsapp the ordering guarantees are limited to within the thread and server side timestamps are good enough for the most part to enforce it. Whatsapp/messaging is a very horizontally scalable problem. With feed (assuming) you have a fully personal feed the storage alone is a pain to manage and optimize.

[bsaul]

i'd say key exchange. You need to get key information and message in the correct order orelse you won't be able to decode the info.

Now of course you can reimplement an ordering mechanism on top of an unordered one, with lost messages detection etc on the client side, and let the server be loose.

but that means reimplementing that logic on every client codebase.