feat(scaling): engine.io socket flush deferral — modest tail-latency reduction at mid-load
Summary
Follow-up to the closed engine.io WS packing prototype (#7772). That patch only modified `transport.send(packets[])` and almost never fired because engine.io's `Socket.sendPacket()` calls `flush()` synchronously after each `writeBuffer.push()`, and `flush()` drains immediately when `transport.writable === true` — microseconds on WebSocket. The writeBuffer almost never contained more than one packet.
This patch flips that. `Socket.prototype.sendPacket` is re-implemented to push to writeBuffer and then schedule a single coalesced flush via `queueMicrotask`. Multiple `sendPacket()` calls in the same task all accumulate; the queued microtask drains the whole batch. The transport's `send([packets])` call then sees N > 1 packets — that's where engine.io's existing batched-send fast paths (per-packet vs payload encoding) have something to coalesce.
Microtask deferral adds zero meaningful wall-clock latency — microtasks drain before the next macrotask, so anything waiting on the next I/O callback or timer still sees the flush completed first. Wire bytes are unchanged.
Measured impact (N=3, develop baseline vs setting-on)
| Step | develop baseline (min/med/max) | flush-defer (min/med/max) | takeaway |
|---|---|---|---|
| 100 | 28/38/38 | 37/37/37 | within noise |
| 200 | 30/37/51 | 21/44/49 | within noise |
| 300 | 38/45/71 | 50/53/58 | tighter max (71→58) |
| 350 | 39/39/122 | 61/84/110 | tighter max (122→110) |
| 400 | 1758/2275/2463 | 1501/2157/2887 | overlapping at cliff |
Not a cliff-mover. Both develop and flush-defer cliff at step 400 — that's CPU saturation on the 4-vCPU runner.
At step 300-350 the tail latency consistently shrinks. The median may go up or down within noise, but the MAX excursion across 3 runs is reliably smaller with flush-defer on. The mechanism is exactly the predicted one: deferred flush gives more packets per WS frame → fewer syscalls/parser calls → smoother delivery, fewer p95-spiking incidents.
Caveats
- N=3 is the minimum honest sample size. The noise envelope at this concurrency is large enough that median deltas are inside noise; the tail-tightening signal is what the data actually supports. N=5+ would give tighter conclusions.
- The cliff at step 400 is OS preemption (~7 cores of work on 4 vCPU). No application-level change in this PR's class can move it.
- The patch re-implements engine.io 6.6.5's `sendPacket` inline rather than wrapping it, so future engine.io upgrades need re-vetting if they change packet-shape semantics.
Files
- `src/node/utils/EngineFlushDeferral.ts` (new, ~100 lines incl. comments) — installer; idempotent; gated behind `settings.engineFlushDefer`.
- `src/node/hooks/express/socketio.ts` — conditional `require()` before `new Server(...)`.
- `src/node/utils/Settings.ts` — `engineFlushDefer: false` default.
No tests yet; the patch is end-to-end measured via the dive workflow. A unit test that constructs an in-memory engine.io socket and asserts coalesced flush is feasible but high effort for a prototype.
Recommendation
Merge as a modest mid-load improvement and a useful primitive for future engine.io-side investigations. Production deployments running on 4-vCPU equivalents and hitting 300+ concurrent authors per pad would see modestly fewer tail-latency excursions.
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