Refactor of caching & data storage part 2 #118

server/sse_broadcaster.py

@@ -0,0 +1,36 @@
+import logging
+import threading
+
+from tornado.ioloop import IOLoop
+
+
+class SSEBroadcaster:
+    """Bridge between DataStore listener callbacks (which fire on provider threads) to Tornado's async SSE handlers
+    (which live on the IOLoop thread) to avoid any interdependency between them."""
+
+    def __init__(self):
+        self._handlers = set()
+        self._lock = threading.Lock()
+        self._loop = IOLoop.current()
+
+    def register(self, handler):
+        with self._lock:
+            self._handlers.add(handler)
+
+    def unregister(self, handler):
+        with self._lock:
+            self._handlers.discard(handler)
+
+    def publish(self, value):
+        self._loop.add_callback(self._fan_out, value)
+
+    def _fan_out(self, value):
+        with self._lock:
+            handlers = list(self._handlers)
+        for handler in handlers:
+            try:
+                handler.callback(value)
+            except Exception:
+                # Connection probably dropped, ignore and de-register the handler to stop getting future items.
+                logging.debug("Failed to push to an SSE client; dropping it")
+                self.unregister(handler)