perf: single-flight /api/stats; fast-fail health probe

The on-box rerun after the inline fast path showed the cached endpoints
healthy (latest p50 72-160ms, HII ~100ms) but /api/stats at 81% timeouts
and /health at 33%: stats had a cache but NO single-flight, so every
concurrent miss ran the heavy whole-DB counts (~1.7M rows) in parallel,
re-jamming Postgres and the executor — which also dragged uncached
history windows into 60s timeouts. /api/stats now computes through
_ttl_cached_stale (one computation per 5min TTL, stale served on
failure). The health API probe fails fast (5s instead of 30s) and its
cache TTL rises to 30s, so a slow upstream can no longer pin executor
threads longer than the cache lifetime.

src/web_api.py

@@ -64,6 +64,7 @@ FORECAST_TTL = 900  # 15 minutes
 
 DB_STATS_CACHE: Dict[str, tuple] = {}
 DB_STATS_CACHE_LOCK = Lock()
+_DB_STATS_COMPUTE_LOCK = Lock()
 DB_STATS_TTL = 300  # 5 minutes
 
 # Admin API protection. Read/dashboard endpoints stay public; anything that
@@ -1042,20 +1043,9 @@ async def get_database_stats():
     if not scraper or not scraper.db_adapter:
         raise HTTPException(status_code=503, detail="Database not available")
 
-    now = time.monotonic()
-    with DB_STATS_CACHE_LOCK:
-        cached = DB_STATS_CACHE.get("all")
-        if cached and now - cached[0] < DB_STATS_TTL:
-            return cached[1]
-
-    try:
-        stats = await asyncio.to_thread(scraper.db_adapter.get_database_stats)
-    except Exception as e:
-        logger.error(f"Error fetching database stats: {e}")
-        raise HTTPException(status_code=500, detail=str(e))
-
-    if stats is None:
-        raise HTTPException(status_code=503, detail="Database statistics unavailable")
+    cached = _cache_fresh(DB_STATS_CACHE, DB_STATS_CACHE_LOCK, "all", DB_STATS_TTL)
+    if cached is not None:
+        return cached
 
     def hii_totals():
         engine = _hii_engine()
@@ -1079,47 +1069,69 @@ async def get_database_stats():
                 )
             ).one()
 
-    hii = None
+    def compute():
+        # Heavy: full-table counts and coverage over ~1.7M rows. Runs at most
+        # once per TTL thanks to the single-flight; concurrent misses wait for
+        # this one result instead of piling identical queries onto Postgres.
+        stats = scraper.db_adapter.get_database_stats()
+        if stats is None:
+            raise RuntimeError("Database statistics unavailable")
+
+        hii = None
+        try:
+            hii = hii_totals()
+        except Exception as e:
+            logger.warning(f"HII stats unavailable: {e}")
+
+        def as_dt(value):
+            if isinstance(value, str):
+                return datetime.fromisoformat(value)
+            return value
+
+        first_ts = stats["first_timestamp"]
+        last_ts = stats["last_timestamp"]
+        rain_n = wl_n = hii_stations = 0
+        if hii is not None:
+            rain_n, wl_n = hii.rain_n or 0, hii.wl_n or 0
+            hii_stations = (hii.rain_s or 0) + (hii.wl_s or 0)
+            for lo in (as_dt(hii.rain_lo), as_dt(hii.wl_lo)):
+                if lo is not None and lo < first_ts:
+                    first_ts = lo
+            for hi in (as_dt(hii.rain_hi), as_dt(hii.wl_hi)):
+                if hi is not None and hi > last_ts:
+                    last_ts = hi
+
+        return {
+            # Whole-DB totals (RID + HII feeds); breakdown fields alongside
+            "total_measurements": stats["total_measurements"] + rain_n + wl_n,
+            "rid_measurements": stats["total_measurements"],
+            "hii_rainfall_measurements": rain_n,
+            "hii_waterlevel_measurements": wl_n,
+            "station_count": stats["station_count"] + hii_stations,
+            "rid_station_count": stats["station_count"],
+            "hii_station_count": hii_stations,
+            "first_timestamp": first_ts.isoformat(),
+            "last_timestamp": last_ts.isoformat(),
+            "days_spanned": (last_ts.date() - first_ts.date()).days + 1,
+            "coverage_percent": stats["coverage_percent"],
+        }
+
     try:
-        hii = await asyncio.to_thread(hii_totals)
+        data, _ = await asyncio.to_thread(
+            _ttl_cached_stale,
+            DB_STATS_CACHE,
+            DB_STATS_CACHE_LOCK,
+            _DB_STATS_COMPUTE_LOCK,
+            "all",
+            DB_STATS_TTL,
+            compute,
+        )
+        return data
+    except RuntimeError as e:
+        raise HTTPException(status_code=503, detail=str(e))
     except Exception as e:
-        logger.warning(f"HII stats unavailable: {e}")
-
-    def as_dt(value):
-        if isinstance(value, str):
-            return datetime.fromisoformat(value)
-        return value
-
-    first_ts = stats["first_timestamp"]
-    last_ts = stats["last_timestamp"]
-    rain_n = wl_n = hii_stations = 0
-    if hii is not None:
-        rain_n, wl_n = hii.rain_n or 0, hii.wl_n or 0
-        hii_stations = (hii.rain_s or 0) + (hii.wl_s or 0)
-        for lo in (as_dt(hii.rain_lo), as_dt(hii.wl_lo)):
-            if lo is not None and lo < first_ts:
-                first_ts = lo
-        for hi in (as_dt(hii.rain_hi), as_dt(hii.wl_hi)):
-            if hi is not None and hi > last_ts:
-                last_ts = hi
-
-    data = {
-        # Whole-DB totals (RID + HII feeds); breakdown fields alongside
-        "total_measurements": stats["total_measurements"] + rain_n + wl_n,
-        "rid_measurements": stats["total_measurements"],
-        "hii_rainfall_measurements": rain_n,
-        "hii_waterlevel_measurements": wl_n,
-        "station_count": stats["station_count"] + hii_stations,
-        "rid_station_count": stats["station_count"],
-        "hii_station_count": hii_stations,
-        "first_timestamp": first_ts.isoformat(),
-        "last_timestamp": last_ts.isoformat(),
-        "days_spanned": (last_ts.date() - first_ts.date()).days + 1,
-        "coverage_percent": stats["coverage_percent"],
-    }
-    with DB_STATS_CACHE_LOCK:
-        DB_STATS_CACHE["all"] = (now, data)
-    return data
+        logger.error(f"Error fetching database stats: {e}")
+        raise HTTPException(status_code=500, detail=str(e))
 
 
 @app.post("/scrape/trigger", dependencies=[Depends(require_admin_key)])