Add kernel_ws_protocol configuration option

jupyter_server/base/zmqhandlers.py

@@ -272,11 +272,12 @@ def _reserialize_reply(self, msg_or_list, channel=None):
             return cast_unicode(smsg)
 
     def select_subprotocol(self, subprotocols):
-        selected_subprotocol = (
-            "v1.kernel.websocket.jupyter.org"
-            if "v1.kernel.websocket.jupyter.org" in subprotocols
-            else None
-        )
+        preferred_protocol = self.settings.get("kernel_ws_protocol")
+        if preferred_protocol is None:
+            preferred_protocol = "v1.kernel.websocket.jupyter.org"
+        elif preferred_protocol == "":
+            preferred_protocol = None
+        selected_subprotocol = preferred_protocol if preferred_protocol in subprotocols else None
         # None is the default, "legacy" protocol
         return selected_subprotocol
 

jupyter_server/serverapp.py

@@ -314,6 +314,8 @@ def init_settings(
                 "no_cache_paths": [url_path_join(base_url, "static", "custom")],
             },
             version_hash=version_hash,
+            # kernel message protocol over websoclet
+            kernel_ws_protocol=jupyter_app.kernel_ws_protocol,
             # rate limits
             limit_rate=jupyter_app.limit_rate,
             iopub_msg_rate_limit=jupyter_app.iopub_msg_rate_limit,
@@ -1613,6 +1615,19 @@ def _update_server_extensions(self, change):
         help=_i18n("Reraise exceptions encountered loading server extensions?"),
     )
 
+    kernel_ws_protocol = Unicode(
+        None,
+        allow_none=True,
+        config=True,
+        help=_i18n(
+            "Preferred kernel message protocol over websocket to use (default: None). "
+            "If an empty string is passed, select the legacy protocol. If None, "
+            "the selected protocol will depend on what the front-end supports "
+            "(usually the most recent protocol supported by the back-end and the "
+            "front-end)."
+        ),
+    )
+
     limit_rate = Bool(
         False,
         config=True,