Refactor of caching & data storage part 1

core/live_data_cache.py

@@ -0,0 +1,72 @@
+import logging
+import threading
+import time
+
+import diskcache
+from cachetools import TTLCache
+
+
+class LiveDataCache:
+    """Cache for spots and alerts. Uses the faster in-memory TTLCache for normal data I/O, including the TTL to enforce
+    maximum lifetime, and adds a separate diskcache to which we can save and load the TTLCache to provide persistence.
+    Also adds thread safety which TTLCache doesn't do."""
+
+    def __init__(self, maxsize, ttl, snapshot_dir, snapshot_interval_sec):
+        self._cache = TTLCache(maxsize=maxsize, ttl=ttl)
+        self._lock = threading.Lock()
+        self._ttl = ttl
+        self._snapshot_dir = snapshot_dir
+        self._disk_cache = diskcache.Cache(str(snapshot_dir))
+        self._load_snapshot()
+        self._start_periodic_snapshot(snapshot_interval_sec)
+
+    def set(self, key, value):
+        with self._lock:
+            self._cache[key] = value
+
+    def get(self, key, default=None):
+        with self._lock:
+            return self._cache.get(key, default)
+
+    def delete(self, key):
+        with self._lock:
+            self._cache.pop(key, None)
+
+    def values(self):
+        with self._lock:
+            return list(self._cache.values())
+
+    def save_snapshot(self):
+        with self._lock:
+            # Store the time with the data so we can avoid loading anything nxt time that's older than TTL
+            data = [(k, v, time.time()) for k, v in self._cache.items()]
+        try:
+            self._disk_cache.set("snapshot", data)
+        except Exception as e:
+            logging.error("Failed to write snapshot to %s", self._snapshot_dir, e, exc_info=True)
+
+    def _load_snapshot(self):
+        data = self._disk_cache.get("snapshot")
+        if not data:
+            return
+
+        now = time.time()
+        with self._lock:
+            for key, value, saved_at in data:
+                # Only restore entries that would still be within TTL
+                if now - saved_at < self._ttl:
+                    self._cache[key] = value
+        logging.info("Loaded snapshot from %s", self._snapshot_dir)
+
+    def _start_periodic_snapshot(self, interval):
+        def loop():
+            while True:
+                time.sleep(interval)
+                self.save_snapshot()
+
+        t = threading.Thread(target=loop, daemon=True, name=f"snapshot-{self._snapshot_dir}")
+        t.start()
+
+    def close(self):
+        self.save_snapshot()
+        self._disk_cache.close()