perf(api): right-size SSE per-subscriber channel buffer (currently hardcoded 64, untested)

[EricAndrechek]

Problem

internal/api/stream_sse.go:51:

ch := make(chan []byte, 64) // TODO: need to test how many are actually needed, as this is ~1.6KB per subscriber channel...

The SSE per-subscriber channel buffer is hardcoded to 64. At ~1.6KB per slot per subscriber that's ~100KB per stream connection — fine for hundreds of subscribers, painful for thousands. More importantly: 64 is a guess, not a measurement, and the comment admits as much.

If the buffer is undersized for a real workload, the Hub drops messages to slow subscribers (existing Hub behavior); if it's oversized, memory grows linearly with concurrent subscribers.

Proposed Solution

Two parts:

1. Make it configurable via WH_SSE_CHANNEL_BUFFER / server.sse_channel_buffer (default current value of 64) so operators can tune without recompiling.
2. Load-test to set a defensible default. Drive make dev-obs with N=10/100/1000 concurrent SSE subscribers and a realistic ingest rate, measure (a) drop rate at the Hub, (b) per-subscriber memory, (c) tail latency from publish → flush. Pick the smallest buffer that keeps drop-rate at 0 under expected alpha workloads. Document the experiment in docs/deployment.md so operators can re-run it.

Acceptance criteria

• Config knob shipped (or a deliberate "not configurable yet" decision documented).
• Default backed by measurement, not the original guess.
• Comment at stream_sse.go:51 updated or removed.

Related

• AGENTS.md design decision deps: bump the go-deps group across 1 directory with 15 updates #10 (Active Sweeper / gap-fill) — same subsystem.
• The WS handler (stream_ws.go) likely has the same parameter; check while you're in there.

[coderabbitai]

🔗 Related PRs

#124 - chore(api)!: drop hub wildcard fan-out; SSE/WS use ?table= (closes #100) [closed]

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[EricAndrechek]

Dogfooding confirm (WaveHouse-Stats, 0f8826c): the hardcoded make(chan []byte, 64) is still at internal/api/stream.go:73 (the // TODO: need to test how many are actually needed is unchanged). Filed a sibling reliability issue in the same handler — #226 (no keepalive heartbeat → idle SSE streams reset behind Cloudflare's edge ~2 min). Both touch the stream select loop, so worth fixing together.