backend/Dockerfile

@@ -26,8 +26,8 @@ RUN chmod +x /app/deploy/entrypoint.sh
 # Docker seeds them from this image content -- including this ownership --
 # so they stay writable.
 RUN useradd --system --create-home --uid 10001 thermograph \
-    && mkdir -p /app/data /app/logs \
-    && chown -R thermograph:thermograph /app
+    && mkdir -p /app/data /app/logs /state \
+    && chown -R thermograph:thermograph /app /state
 
 USER thermograph
 WORKDIR /app

backend/lake_app.py

@@ -24,6 +24,7 @@ Scale-out safe: replicas share nothing — each keeps its own cache under
 THERMOGRAPH_LAKE_CACHE (a per-task local volume), so 1..2 replicas behind the
 Swarm VIP need no coordination.
 """
+import io
 import os
 import re
 import threading
@@ -31,7 +32,7 @@ import time
 
 import polars as pl
 from fastapi import FastAPI, HTTPException
-from fastapi.responses import FileResponse
+from fastapi.responses import FileResponse, Response
 from pydantic import BaseModel
 
 from data import era5lake
@@ -106,10 +107,20 @@ def history(lat: float, lon: float):
                 df = era5lake._read_bucket(i, j, cfg)
         except era5lake.LakeUnavailable as e:
             raise HTTPException(status_code=404, detail=str(e)) from e
+        try:
             os.makedirs(CACHE_DIR, exist_ok=True)
             tmp = f"{path}.{os.getpid()}.partial"
             df.write_parquet(tmp)
-        os.replace(tmp, path)   # atomic: concurrent requests see whole files only
+            os.replace(tmp, path)   # atomic: concurrent readers see whole files only
+        except OSError:
+            # An unwritable cache (bad volume ownership, full disk) must never
+            # fail the hot path — serve from memory; the cache is an
+            # accelerator, not a dependency.
+            buf = io.BytesIO()
+            df.write_parquet(buf)
+            return Response(buf.getvalue(),
+                            media_type="application/vnd.apache.parquet",
+                            headers={"X-Lake-Point": f"{i},{j}"})
     return FileResponse(path, media_type="application/vnd.apache.parquet",
                         headers={"X-Lake-Point": f"{i},{j}"})
 

backend/tests/web/test_lake_app.py

@@ -69,6 +69,20 @@ def test_history_serves_parquet_and_caches(lake, tmp_path):
                     params={"lat": 51.5074, "lon": -0.1278}).content == r.content
 
 
+def test_history_serves_from_memory_when_cache_unwritable(lake, tmp_path, monkeypatch):
+    """An unwritable cache dir (bad volume ownership — seen live) degrades to
+    serving from memory, never a 500."""
+    import lake_app
+    ro = tmp_path / "ro-cache" / "nested"   # parent made read-only below
+    (tmp_path / "ro-cache").mkdir()
+    (tmp_path / "ro-cache").chmod(0o555)
+    monkeypatch.setattr(lake_app, "CACHE_DIR", str(ro))
+    r = lake.get("/history", params={"lat": 51.5074, "lon": -0.1278})
+    (tmp_path / "ro-cache").chmod(0o755)
+    assert r.status_code == 200
+    assert r.headers["x-lake-point"] == "154,1439"
+
+
 def test_history_404_for_point_outside_lake(lake):
     r = lake.get("/history", params={"lat": -45.0, "lon": 170.0})
     assert r.status_code == 404