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efitenandClaude Opus 5
eb3d71f8f6
Addresses #1910. The Observers page hangs on "Loading..." for 10-20s; the reporter measured `/stats` at 10-17s under the mixed load that page produces, while the same endpoint stays under 70ms at 8x concurrency when it is the only one being hit. ## Cause Two cache layers guard the expensive work and **neither has single-flight**: | | | | |---|---|---| | `handleStats` | 10s cache | releases `statsMu` before rebuilding (`routes.go:774`) | | `GetStoreStats` | 30s cache | releases `statsCacheMu` before scanning (`store.go:2048`) | Both do check, release, then work. The moment either window expires, **every in-flight request does the whole thing itself**. The expensive part is a range scan over 24h of `observations` with two `SUM(CASE...)` over it. The column is indexed (`idx_observations_timestamp`), but the scan still visits every row in the window, and at 18k observers that is millions. The pool is `SetMaxOpenConns(4)` (`db.go:111`), and the page fires stats, observers, nodes, channels and clock-skew at once, so one cache miss turns a single scan into a queue of them. That is exactly the reported profile: fast alone, slow only when mixed. ## Changes 1. **Single-flight both layers.** Concurrent callers that miss the cache wait for the first one's result instead of each running the same queries. 2. **Serve the observation counts stale while refreshing in the background.** An expired cache answers from the previous value and kicks off one refresh, so a miss is never a wait. Single-flight alone would not have fixed the hang: the first caller still waits for the full scan. The second change is what removes it. ## Contract change, stated plainly `TestGetStoreStats_CacheExpiry` asserted that an expired cache returns **fresh DB values on the same call**. It no longer does. For `packetsLastHour` / `packetsLast24h` on a dashboard, answering with a value up to ~30s older instead of blocking for seconds looks like the right trade to me. But that is a judgement, not a bug fix, and **a reviewer should be able to reject it**. I did not quietly delete the test: it now asserts what still has to hold, that the refresh happens, and the new behaviour is pinned separately by `TestGetStoreStats_StaleCacheServedWithoutBlocking`. If you would rather keep the old contract, drop change 2 and keep change 1; the diff separates cleanly. ## Verification The new test **fails without the change**: ``` stale cache not served: got (0, 2), want (424242, 434343). An expired cache must answer from the previous value and refresh in the background, not block the request on the observations scan ``` `gofmt` clean, `go vet` clean, `cmd/server` suite ok in 267s. ## Two things I did not verify **The race detector.** It needs cgo and there is no gcc on this machine, so `go test -race` cannot run here. This change adds a background goroutine writing the cache under `statsCacheMu`, so that check matters. CI runs `go test -timeout 20m -race` for `cmd/server` (`deploy.yml:134`), which covers it before merge. **The 10-17s itself.** I have no database with 18k observers. The mechanism above explains the reported profile, including why the endpoint is fast in isolation, but I did not measure the figure. @dborup, if you can run a build from this branch, the number to watch is `/stats` under the same mixed-load command from your issue. Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>