Ingest Dashboard: optional background cache pre-warmer

Adds an asyncio background task that re-runs the heavy Ingest Dashboard
queries every ~4 min (just under the 5 min TTL) so the in-process cache
is always populated. First user hit on any dashboard widget then returns
from cache (single-digit ms) instead of waiting 30-60s for SDL.

Components:
  - backend/services/prewarmer.py: standalone module, opt-in via
    INGEST_PREWARM=1; configurable windows via INGEST_PREWARM_HOURS /
    INGEST_PREWARM_DAYS / INGEST_PREWARM_DAILY_VOLUME_DAYS and interval
    via INGEST_PREWARM_INTERVAL_SECONDS. Logs through the uvicorn logger
    so cycles are visible in 'docker logs'.
  - backend/main.py: spawn the task on FastAPI startup.
  - docker-compose.yml: forward INGEST_PREWARM* env vars to the
    backend service (default off).

Measured on Purple AI tenant (INGEMeasured on Purple AI tenant (INGEMeasured on Purple fMeasured on Purple AI tenant (INGEMeasured on Purple AI tenant (INGEMeasured on  (INGEST_PREWARM=0) so non-opt-in
users see no behaviour change.

backend/main.py

@@ -45,6 +45,14 @@ with engine.connect() as _conn:
 app = FastAPI(title="SIEM Toolkit", version="1.0.0")
 
 
+@app.on_event("startup")
+async def start_ingest_prewarmer():
+    """Start optional background pre-warmer for the Ingest Dashboard cache.
+    Opt-in via INGEST_PREWARM=1. See backend/services/prewarmer.py."""
+    from services import prewarmer
+    prewarmer.start_if_enabled()
+
+
 @app.on_event("startup")
 async def auto_load_detections():
     """

backend/services/prewarmer.py

@@ -0,0 +1,97 @@
+"""Background pre-warmer for the Ingest Dashboard cache.
+
+Opt-in via env: INGEST_PREWARM=1
+Tunable via env: INGEST_PREWARM_INTERVAL_SECONDS (default 240, just under TTL)
+                 INGEST_PREWARM_HOURS  (default "1,24,168")
+                 INGEST_PREWARM_DAYS   (default "7")
+                 INGEST_PREWARM_DAILY_VOLUME_DAYS (default "5")
+
+The pre-warmer re-runs the heavy Ingest Dashboard queries every ~4 min so the
+in-process TTL cache is always populated. First user hit then returns from
+cache (sub-millisecond) instead of waiting 30-60s for SDL.
+"""
+from __future__ import annotations
+import asyncio
+import logging
+import os
+import time
+
+# Use the uvicorn logger so messages show up in `docker logs` alongside requests.
+log = logging.getLogger("uvicorn.error")
+_PREFIX = "prewarmer:"
+
+
+def _flag_enabled() -> bool:
+    return os.environ.get("INGEST_PREWARM", "").lower() in ("1", "true", "yes", "on")
+
+
+def _int_list(env: str, default: str) -> list[int]:
+    raw = os.environ.get(env, default)
+    out = []
+    for tok in raw.split(","):
+        tok = tok.strip()
+        if tok and tok.isdigit():
+            out.append(int(tok))
+    return out
+
+
+async def _warm_once() -> dict:
+    """Run all configured warm-up queries once. Returns timing summary."""
+    # Local import to avoid circular dependency with FastAPI router module.
+    from routers.ingest import (
+        _top_sources_cached,
+        _by_event_type_cached,
+        _daily_volume_cached,
+    )
+
+    hours_list = _int_list("INGEST_PREWARM_HOURS", "1,24,168")
+    days_list = _int_list("INGEST_PREWARM_DAYS", "7")
+    dv_days = _int_list("INGEST_PREWARM_DAILY_VOLUME_DAYS", "5") or [5]
+
+    tasks: list[tuple[str, asyncio.Task]] = []
+    for h in hours_list:
+        tasks.append((f"top-sources hours={h}",
+                      asyncio.create_task(_top_sources_cached(h, nocache=True))))
+    for d in days_list:
+        tasks.append((f"by-event-type days={d}",
+                      asyncio.create_task(_by_event_type_cached(d, nocache=True))))
+    for d in dv_days:
+        tasks.append((f"daily-volume days={d}",
+                      asyncio.create_task(_daily_volume_cached(d, nocache=True))))
+
+    summary: dict[str, str] = {}
+    for label, task in tasks:
+        t0 = time.monotonic()
+        try:
+            await task
+            summary[label] = f"OK in {time.monotonic() - t0:.1f}s"
+        except Exception as e:
+            summary[label] = f"ERR ({e.__class__.__name__}: {str(e)[:120]})"
+    return summary
+
+
+async def _loop():
+    interval = int(os.environ.get("INGEST_PREWARM_INTERVAL_SECONDS", "240"))
+    log.info("%s starting (interval=%ds)", _PREFIX, interval)
+    # Tiny initial delay so we don't compete with startup work.
+    await asyncio.sleep(5)
+    while True:
+        try:
+            summary = await _warm_once()
+            for label, status in summary.items():
+                log.info("%s %s -> %s", _PREFIX, label, status)
+        except asyncio.CancelledError:
+            raise
+        except Exception as e:
+            log.warning("%s cycle failed: %s", _PREFIX, e)
+        await asyncio.sleep(interval)
+
+
+def start_if_enabled() -> asyncio.Task | None:
+    """Spawn the pre-warm background task if INGEST_PREWARM is enabled.
+    Returns the task handle, or None if disabled."""
+    if not _flag_enabled():
+        log.info("%s disabled (set INGEST_PREWARM=1 to enable)", _PREFIX)
+        return None
+    log.info("%s scheduling background task", _PREFIX)
+    return asyncio.create_task(_loop(), name="ingest-prewarmer")