It looks like the lack of persistent storage for the federated activity queue is leading to instances running out of memory in a matter of hours. See my comment for more details.
Furthermore, this leads to data loss, since there is no other consistency mechanism. I think it might be a high priority issue, taking into account the current momentum behind growth of Lemmy…
a large part of the queue going unbounded is due to the retry queue and missing checks if the receiving servers are actually available. quick fix is disabling the retry queue which is currently making it not go unbounded on lemmy.world
storing the queue persistently is somewhat of a separate issue since that doesn’t much affect whether or not it can be processed in time.
also a ton of the memory use was (and is) due to inefficient sql queries.
I guess that works as an emergency measure. Persistent storage doesn’t affect whether the updates are processed in time, but it would act as a sort of swap to keep the memory usage manageable.
For scalability, perhaps, you could run dijkstra and route the updates using the shortest path to each federated node, in a multicast sort of way? That would make the updates scale in a
O(log(N))way, provided that activity isn’t too centralised. It would also be great to run periodic “deep scrubs” between instances to sync up each other’s activities and provide actual eventual consistency. I guess that’s kind of a liberal interpretation of ActivityPub, but I think that’s the only way to ensure real scalability.