A high-concurrency MMORPG event system built with Flutter + Firebase.
With 10,000 concurrent chatters, each client's Firestore listener is
limited to the most recent 50 messages using .limit(50), so each
connection only reads 50 documents on initial load rather than the
full history. New messages trigger incremental snapshot updates rather
than full re-reads, keeping ongoing costs proportional to message
volume rather than user count. For extreme scale, chat would be
sharded into regional sub-collections (e.g. chats/{regionId}/messages)
so listener load is distributed and no single collection becomes a hotspot.