Scaling Realtime Event Delivery for 10,000 Reconnecting Delivery Tracking Maps

For realtime release compatibility in a delivery tracking map, scale event delivery with a durable, ordered log per delivery and treat every browser connection as a disposable projection of that log. Presence can guide fan-out and capacity planning, but it must never decide whether a location update exists. Short answer: release compatibility comes from versioned envelopes, resume cursors, and an explicit resync path; scaling comes from partitioning by delivery ID and coalescing map updates at the edge, not from trusting a long-lived connection to carry every event exactly once. This decision targets an e-commerce tracking experience in which a shopper may open a map, lose connectivity in a tunnel, return on another network, and also join a delivery-specific support chat room. The deciding constraint is presence accuracy: an online indicator is useful only when its expiry rules are understood, while the delivery state must remain correct even when that indicator is late. A green dot isn't a commit log. How should realtime release compatibility scale event delivery in a delivery tracking map? Separate the system into three contracts: durable delivery state, transient room presence, and the connection used to move updates. The first contract owns truth. The second answers a narrower question: which sessions have renewed a lease recently enough to be considered reachable? The third may disappear at any point and should be replaceable without changing either of the other two. For each delivery, append an event with a monotonically increasing sequence within that delivery's partition. The client persists the last applied sequence and includes it when reconnecting. If retained events cover the gap, the server replays them in order; if they don't, the server returns a fresh snapshot plus its sequence. This is at-least-once delivery with idempotent application, which means duplicates are ordinary and gaps are detectable. It does not promise global order across unrelated del