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The mobile app subscribed to every presence event in the community, though it only shows presence for people on screen. It now subscribes only to the people it tracks. ### Validation **Impact:** on an iPhone 12 mini signed in to Block's Builderlab, 5 minutes idle on Messages: | | Presence downloaded | All WebSocket traffic | | --- | --- | --- | | Before | 2.9 MB (8153 events) | 3.1 MB | | After | 3.6 KB (10 events) | 66 KB | That's about 35 MB per hour saved at Builderlab's presence rate (about 27 events per second). Measured on an instrumented profile build of the first revision, [3f789d1](3f789d16b8). The later swap fix doesn't change what is subscribed. **Device runs (automated, profile builds on the 12 mini):** - First revision: cold start, opening and leaving the home list, a Control Center pull, and a resume after 60 s in the background. Presence dots kept updating for people on screen. - Final head [9844c43](https://github.com/block/buzz/commit/9844c438dd): cold start, opening two channels, a 60 s background resume, and scrolling the home list through the DM section. Every DM row showed its presence dot (online and away), including rows newly scrolled into view. Opening a DM and its details sheet worked as before. **Added tests:** - Swapping subscriptions: a late replacement is promoted without clearing anyone's dot and re-reads the newly covered people; a retryable close or an outright failure keeps the current stream; a second track while one is opening doesn't open a third subscription. Each guard was removed in turn and the test failed each time. - A widening that keeps failing is retried each poll, and expired leases are still corrected. - Replacements the relay never answers are dropped and retried, and after the current stream also closes, the next poll covers and reads everyone. - A first subscription still retrying when it opens waits for its ready, then reads everyone and widens. **Updated test:** the filter test now expects no subscription while nobody is tracked, then `authors: [alice]`, then `[alice, bob]`. The full mobile test suite passes. ### How it works - The live subscription filters by `authors`: everyone shown with presence this session. The cache never untracks; it resets on community or account change. Nothing is subscribed while nobody is tracked. - When someone new is tracked, a replacement subscription opens for the wider set. The current one keeps delivering until the replacement's first EOSE, then it's swapped in and the old one closed. Newly covered people get a fresh snapshot at that point. - If the replacement fails or closes first, it's dropped and nothing on screen changes. The 60-second poll retries the widening and keeps reading the people already covered. - One replacement opens at a time; people tracked meanwhile join the next round. ### Regressions - A person tracked mid-session gets live presence after one extra subscription round trip (up to about a second on a slow link). People already on screen keep live presence throughout the swap. - Each widening resends the full tracked list (about 67 bytes per person), so a few hundred people cost tens of KB per widening, against 35 MB per hour saved. The relay indexes `authors` filters up to 4096 authors, so this is cheap for it. --------- Signed-off-by: Tom Brow <tomb@squareup.com> Co-authored-by: Tom Brow <tomb@squareup.com> Co-authored-by: studio <dc2fc481343672df31a689840c09299972e704dac592e0da6f18486646a7ee3e@buzz.block.builderlab.xyz>