Add keepalive websocket ping

When nb2kg server is behind the proxy closing idle stream connection such as nginx and envoy websocket is closed and reconnected.
This behavior updates kernel `last_activity` so that it keeps `kernel_gateway` from culling kernels.

This change is from `WebsocketMixin` in jupyter/notebook.

Signed-off-by: Eunsoo Park <esevan.park@gmail.com>

nb2kg/handlers.py

@@ -11,7 +11,7 @@
 
 from tornado import gen, web
 from tornado.concurrent import Future
-from tornado.ioloop import IOLoop
+from tornado.ioloop import IOLoop, PeriodicCallback
 from tornado.websocket import WebSocketHandler, websocket_connect
 from tornado.httpclient import HTTPRequest
 from tornado.escape import url_escape, json_decode, utf8
@@ -40,6 +40,9 @@
 KG_CONNECT_TIMEOUT = float(os.getenv('KG_CONNECT_TIMEOUT', 20.0))
 KG_REQUEST_TIMEOUT = float(os.getenv('KG_REQUEST_TIMEOUT', 20.0))
 
+# Keepalive ping interval (30 seconds)
+WS_PING_INTERVAL_MS = 30000
+
 
 class WebSocketChannelsHandler(WebSocketHandler, IPythonHandler):
 
@@ -86,8 +89,23 @@ def get(self, kernel_id, *args, **kwargs):
         self.kernel_id = cast_unicode(kernel_id, 'ascii')
         yield gen.maybe_future(super(WebSocketChannelsHandler, self).get(kernel_id=kernel_id, *args, **kwargs))
 
+    def send_ping(self):
+        if self.stream.closed() and self.ping_callback is not None:
+            self.ping_callback.stop()
+            return
+
+        self.ping(b'')
+
+    def on_pong(self, data):
+        self.log.debug('Client has sent pong')
+
     def open(self, kernel_id, *args, **kwargs):
         """Handle web socket connection open to notebook server and delegate to gateway web socket handler """
+        loop = IOLoop.current()
+
+        self.ping_callback = PeriodicCallback(self.send_ping, WS_PING_INTERVAL_MS, io_loop=loop)
+        self.ping_callback.start()
+
         self.gateway.on_open(
             kernel_id=kernel_id,
             message_callback=self.write_message,