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SWC · Interviews · Building an internal platform with swc-interview from scratch: architecture and decision log

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SWr/swc-interview·posted by bob_chen·just nowPostmortem

Building an internal platform with swc-interview from scratch: architecture and decision log

Short version: swc-interview needs almost no tuning at small and medium scale — the point where it starts to hurt is much further out than most people assume. Full measurements below.

We also fixed monitoring along the way: replaced average-based alerts with percentiles and split them per endpoint. False alerts dropped by about seventy percent and the on-call rotation visibly cheered up.

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17 comments

17 comments

M
Wwinter·28 minutes ago

I see point 3 differently. The trade-off depends on your read/write ratio: read-heavy with little writing means caching actually widens the inconsistency window.

411
Nnikic·2 days ago

I just read the swc-interview source — the author actually explains the reasoning in a comment, roughly "so that it degrades into predictable behaviour in extreme cases".

289
Kkite·3 minutes ago

Agreeing with the above. One addition: with this option enabled the GC count in your metrics doubles, so adjust the alert threshold at the same time or it will keep firing.

201
Zzhu_zong·just now

There is actually a simpler fix that needs no architecture change: move this check up to the gateway and the problem disappears. The cost is one extra lookup at the gateway.

376
Sswoole_leeOP·5 hours ago

Has anyone run a controlled experiment? I did, reducing it to a single variable, and the difference was 4% — within noise. So I suspect the main cause is something else.

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Kkite·2 days ago

Sharing our numbers, 8 cores 16GB, same scenario:

| Concurrency | P50 | P99 |
|---|---|---|
| 200 | 12ms | 88ms |
| 500 | 31ms | 340ms |

P99 clearly collapses at 500 concurrency, which lines up with your knee point.

17
Nnikic·yesterday

This matches what we see in production. We only hit it past 3k QPS; the earlier load tests showed nothing — the test traffic was too clean, with no long-tail requests.

295
Rran_bo·28 minutes ago

Sharing our numbers, 8 cores 16GB, same scenario:

| Concurrency | P50 | P99 |
|---|---|---|
| 200 | 12ms | 88ms |
| 500 | 31ms | 340ms |

P99 clearly collapses at 500 concurrency, which lines up with your knee point.

272
Rrase·just now

A question: what changes in a container with a 512Mi memory limit? That is how we run it in production.

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Cchen_dev·28 minutes ago

Has anyone run a controlled experiment? I did, reducing it to a single variable, and the difference was 4% — within noise. So I suspect the main cause is something else.

302
Rran_bo·just now

I just read the swc-interview source — the author actually explains the reasoning in a comment, roughly "so that it degrades into predictable behaviour in extreme cases".

183
Zzhu_zong·28 minutes agoedited

Worth learning from this debugging approach. We went straight at the logs and took a much longer route.

266
Lli_ming·yesterday

Thanks for sharing real numbers — far more useful than the articles that only cover concepts.

435
Rran_boOP·3 minutes ago

We have run this in production for two years without hitting it. That said, we never reached this scale, so our experience is not really evidence here.

1
Zzhu_zong·28 minutes ago

This is not a swc-interview problem, it is a usage problem. The docs say this API is not thread-safe and you must lock around it yourself.

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Cchen_dev·2 days ago

One counter-example: below swc-interview 7.4 the semantics of that code are different, so do not copy it verbatim. We got burned in staging and rolled back once.

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Kkite·28 minutes ago

Saved. I am reworking this area this week — this saves a lot of wrong turns.

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