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Jinwoo Hong
0ae42fe594
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## ELI5
Relay servers ("cells") normally get their seat bookings from the front door (the director). If the director goes quiet for a minute or two, each cell switches itself to an older mode that talks to the shared Postgres database for every connection (the "dead-man" trip). Today every cell switches at once, so the database budget must keep room for every cell's full connection pool. This PR adds a per-cell switch that caps how many cells may trip at the same time. A cell that cannot trip yet stays in its normal mode and keeps serving the desktops it already has.
## What Changed
- New cell flag `deadManTripSlots` (integer 0-64). Absent, which is every cell today, keeps the current behaviour: trip as soon as the dead-man window (60-120 s) passes.
- With the flag set, a cell whose window has passed first claims a slot in Postgres. The claim is one short transaction: it takes an advisory lock without waiting, counts the other reserve cells whose own `relay_cell_admit_effective` row says `db`, and writes this cell's row to `db` only if that count is below the flag. Only then does the cell trip (void bookings, re-register its hosts, run on its full pool).
- A cell that gets no slot is "held". It stays in reserve mode, logs `orca_relay_cell_reserve_dead_man_held` once, keeps writing `reserve` to its row, and asks again every 30 s.
- A slot is held while the holder's own row says `db`, its switch still says `reserve`, and the row was refreshed in the last 10 minutes. A holder that restarts or is re-armed writes `reserve` and frees its slot. One that died tripped frees it after 10 minutes. Heartbeats are not used: they reach Postgres through the director, so in a director outage they all go stale and every slot would read as free.
- A tripped cell skips its 15 s row refresh while its pool has waiters. So that a long brownout cannot age its row out of the count, a tripped cell's row older than 5 minutes is refreshed even when the pool is busy.
- A trip stays latched after the director returns, as today. The tripped cells keep their slots until an operator re-arms them with a new reserve write. Until then, a second outage trips nobody new: the other cells stay held. That is the intended bound, but re-arming after an outage now also frees the slots.
- The claim goes through `CellAdmitEffectiveWriter`, which already serialises every write of that row. A `reserve` write still in flight therefore cannot land after the claim and hide the slot from the next claimant.
- Cells keep their existing per-cell window spread (60-120 s by cell-id hash), so the earliest windows claim first.
What a held cell does during a director outage (unchanged code paths, now for longer than the window):
- Desktops already connected keep their sockets. Nothing is closed or re-registered.
- A desktop that drops and redials at the same epoch is admitted from memory (`seatedHereAt`). Any other hello falls through to the normal database check, as it does in reserve mode today.
- The database placement path places no new desktops on it, because its row says `reserve`. It also never re-places the held cell's desktops, because `liveReserveCell` keys on the switch (`relay_cell_admit_modes`), not on this row.
- New placements during the outage go to cells already in database mode (c32, c33 and c34 today) plus the tripped slot holders.
## Why
The database budget (Cloud SQL `orca-cloud-auth-db`, shared with auth, max_connections 500, 490 usable) currently reserves every relay cell's full pool. The only reason for that is the dead-man trip of the whole fleet at once. Spare is 35 on main and about 3 once C36/C37 land, which caps the number of Asia cells.
Budget arithmetic (`relay-cloud-sql-connection-budget.mjs` on orca-cloud main: directors 15, auth 20, API 50, rollout overlap 65, maintenance 5, explicit reserve 10):
| | Cells line | Operating | Spare |
|---|---:|---:|---:|
| Main today: 6 db/legacy cells at 10, 16 US at 8, 7 Asia at 16 | 300 | 455 | 35 |
| Plus C36/C37 at 16 | 332 | 487 | 3 |
| Same fleet, `deadManTripSlots=2`, Asia reserve cells budgeted at 12 (measured peak) | 76 + 128 + 8x12 + 2x4 = 308 | 463 | 27 |
| Plus two more Asia cells at 12 | 332 | 487 | 3 |
So with two slots, two more Asia cells fit in the room C36/C37 leave today. Two caveats keep this honest:
- The bound holds only once every reserve cell runs this image with the same flag value. An older image ignores the flag and trips freely, though its `db` row still takes a slot from the capped cells. With mixed values the cap is the largest value set.
- Counting reserve cells at their measured peak is a softer guarantee than counting their configured pool: the pool is still 16, and a reserve cell could burst past 12. Making it a hard bound needs a pool ceiling that depends on the mode (reserve-mode ceiling 12, the full 16 only for a slot holder or a deliberate db cell). That is a follow-up; this PR does not change pool sizes.
Alternatives considered:
- Longer or ranked trip delays only spread the trips out. A trip is latched until the switch is rewritten, so after the longest delay every cell is still in database mode. That bounds the burst, not the budget.
- A static per-cell "may trip" list in the switch file needs no database coordination, but a listed cell that is down or draining wastes its slot, and an operator has to keep the list current.
- Removing the fallback (cells never trip) is the target design, but it waits on the director stall fix and two weeks of clean running. `deadManTripSlots=0` gives exactly that per cell, so it is the same switch.
Deliberate db flips (for example, flipping reserve cells to db for an image roll) are outside the cap. Budget each Asia cell flipped that way at its full 16. With a two-cell Asia roll batch running during an outage, the slot row above is 316, which leaves room for one more Asia cell rather than two.
Rollout: the flag is dark. Setting it is an `Operate Relay Production Cell Flags` write, per cell, after every reserve cell runs this image.
## Linked Issue
N/A (internal relay capacity work).
## Visual Proof
N/A: relay server behaviour only, no UI.
## Testing
- `cell-reserve-dead-man.test.ts` (new): trips as before without the flag; held until a permit, then latched; slot 0 never trips; a permit for a silence that contact ended, or for an earlier switch generation, is dropped.
- `cell-admit-effective-writer.test.ts`: the claim waits for an in-flight reserve write; a refused claim keeps the reserve row fresh and retries after 30 s; no claim while the pool has waiters or the switch is off reserve; a tripped row is refreshed past a saturated pool after 5 minutes, a reserve row is not.
- `cell-reserve-dead-man-slots.blackbox.test.ts` (new, real cell on SQLite): a director outage with the only slot taken by another cell that writes nothing but its own row: the cell stays reserve across several claims, re-registers nothing, keeps its desktops connected, and lets a redialing desktop back in from memory; once the peer writes `reserve`, the cell trips and re-registers. Also: slot 0 rides out 30 minutes of outage without tripping; no flag trips as before.
- `cell-dead-man-trip-slot-postgres.test.ts` (new, PostgreSQL): eight cells claiming together get exactly two slots; a live holder keeps its slot; a stale one frees exactly one. Removing the advisory lock fails this test.
- Mutation checks: making the slot count always 0, or removing the lock, each fails a test.
- Full relay suite, including the PostgreSQL suites against a local PostgreSQL 17: 173 files, 1564 passed, 3 skipped. The pr.yml static-analysis steps pass locally.
- Two rounds of adversarial review. Round 1 found that the first version counted holders by director-written heartbeats (fixed). Round 2 found the brownout aging (fixed) and the latched slots after an outage (documented above).
- [x] I manually tested these changes locally
- [x] Automated tests added/updated, or explained why not below
## Review
## Agent skill upstream boundary
- [x] Not applicable, or this change copies or mechanically translates no upstream skill-installer source, tests, fixtures, registry entries, path tables, comments, or documentation.
## Notes
No new table or column. The claim reads `relay_cell_admit_effective` and `relay_cell_admit_modes` (one row per cell). Under a fleet-wide trip, held cells add one short transaction each per 30 s; the advisory lock is held for about one round trip.
## Checklist
- [x] This PR is small and focused
- [x] I explained what changed and why (ELI5, the user-facing before/after, the mechanism, and why over the alternatives)
- [x] Before/after screenshots or videos attached for UI changes, or `N/A` with reason
- [x] Self-reviewed for correctness, security, and performance
- [x] Cross-platform, SSH/remote, and path/shortcut impact considered (or N/A)
- [x] `pnpm lint`, `pnpm typecheck`, `pnpm test`, and `pnpm build` pass (or CI will cover; local preferred)