[Backend] SSE broadcast/heartbeat ignore res.write() backpressure - unbounded buffering on slow clients

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

sse.service.ts:216 (broadcast) and :92 (sendHeartbeat) call client.res.write(...) and discard the boolean return value. When a consumer cannot drain (slow/stalled socket), Node buffers every event in the socket's writable buffer with no high-water-mark check, no drain wait, and no max-buffer disconnect. Under the documented 10k-connection / 1k-events-sec target a few stalled clients cause unbounded heap growth and OOM. broadcast also has an empty catch that does not prune the failed client (unlike sendHeartbeat).

Acceptance criteria

• Check the res.write() return value; when false, stop sending to that client until drain or disconnect it after a bounded writableLength threshold
• On write failure in broadcast(), remove the client (mirror sendHeartbeat)
• Expose a slow-client-dropped counter for observability
• Add a test where a client's write() returns false / throws and the client is removed without blocking others

Files to touch

• backend/src/services/sse.service.ts

Out of scope

• Switching transport from SSE to WebSocket

[Oluwasuyi-Oluwatimilehin-Daniel]

@Oluwasuyi-Timilehin has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello maintainer, please assign me to this task. i would get started immediately
ETA 1hour

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[bakarezainab]

@bakarezainab has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi Maintainer, I am a web3 fullstack developer. I’ve reviewed this issue and I understand the requirements. I’d like to implement this and submit a clean PR. Kindly assign this issue to me

ℹ️ Repo Maintainers: To accept this application, review their application or assign @bakarezainab to this issue.

[grantfox-oss]

🦊 GrantFox — @bakarezainab has been assigned to this issue!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #842)
2. Your PR will be reviewed by the LabsCrypt maintainers

Good luck! Track your progress on GrantFox.

[drips-wave]

Congratulations, @bakarezainab! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @bakarezainab: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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[drips-wave]

This issue has been marked as completed by a Drips Wave moderator for @bakarezainab as part of the Stellar Wave Program's 6th Wave 🥳

Moderator's reason: [Auto-assessed] The contributor successfully addressed the issue by implementing SSE backpressure logic, a slow-client counter, and relevant unit tests in PR #960, which was merged on 2026-07-07. Although a maintainer previously requested changes regarding dependency versions and a duplicate Prisma constraint, the subsequent merge of the PR serves as the definitive signal of acceptance.

😎 @bakarezainab: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here.

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ℹ️ Note: This issue was marked complete via moderation. Points were issued even though the GitHub issue remains open.