Refactor of caching & data storage part 2 #118

core/live_data_cache.py

@@ -7,14 +7,17 @@ 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
+    """Cache for spots and alerts. Uses the fast 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."""
+    Also adds thread safety so spots and alerts can come from any thread, and a listener mechanism so the web server
+    can get a callback when new spots/alerts are added, and send them to any SSE clients."""
 
     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._listeners = []
+        self._listeners_lock = threading.Lock()
         self._snapshot_dir = snapshot_dir
         self._disk_cache = diskcache.Cache(str(snapshot_dir))
         self._load_snapshot()
@@ -24,6 +27,16 @@ class LiveDataCache:
         with self._lock:
             self._cache[key] = value
 
+        # Notify listeners
+        with self._listeners_lock:
+            listeners = list(self._listeners)
+        for callback in listeners:
+            try:
+                callback(value)
+            except Exception:
+                logging.error("Listener raised an exception for key %s", key, exc_info=True)
+
+
     def get(self, key, default=None):
         with self._lock:
             return self._cache.get(key, default)
@@ -32,10 +45,25 @@ class LiveDataCache:
         with self._lock:
             self._cache.pop(key, None)
 
+    def keys(self):
+        with self._lock:
+            return list(self._cache.keys())
+
     def values(self):
         with self._lock:
             return list(self._cache.values())
 
+    def add_listener(self, callback):
+        """Register callback(value) which will be called whenever a new spot/alert item is added via set(). Used by the
+        web server (via SSEBroadcaster) to send SSE clients an update on every new spot."""
+
+        with self._listeners_lock:
+            self._listeners.append(callback)
+
+    def remove_listener(self, callback):
+        with self._listeners_lock:
+            self._listeners.remove(callback)
+
     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