Add --reuse-sessions warm option (#8087)

* Add --reuse-sessions warm option

* Revert default

Author: philippjfr

doc/how_to/performance/reuse_sessions.md

@@ -16,6 +16,8 @@ panel serve app.py --reuse-sessions --global-loading-spinner
 
 :::{tip}
 If you want to warm up the server with the cached sessions you can also provide the `--warm` option.
+
+Optionally you can also configure `--reuse-sessions warm`, which will warm the session even before the Websocket request requesting it arrives.
 :::
 
 ## More complex cases

panel/command/serve.py

@@ -303,8 +303,11 @@ class Serve(_BkServe):
             type    = str
         )),
         ('--reuse-sessions', Argument(
-            action  = 'store_true',
+            action  = 'store',
             help    = "Whether to reuse sessions when serving the initial request.",
+            default = False,
+            const   = True,
+            nargs   = "?"
         )),
         ('--global-loading-spinner', Argument(
             action  = 'store_true',

panel/config.py

@@ -189,13 +189,15 @@ class _config(_base_config):
         The notification to display when the application is ready and
         fully loaded.""")
 
-    reuse_sessions = param.Boolean(default=False, doc="""
+    reuse_sessions = param.Selector(default=False, objects=[True, False, 'warm'], doc="""
         Whether to reuse a session for the initial request to speed up
         the initial page render. Note that if the initial page differs
         between sessions, e.g. because it uses query parameters to modify
         the rendered content, then this option will result in the wrong
         content being rendered. Define a session_key_func to ensure that
-        reused sessions are only reused when appropriate.""")
+        reused sessions are only reused when appropriate. If set to 'warm'
+        session reuse is enabled and the session is warmed up as soon as
+        the initial request arrives.""")
 
     session_key_func = param.Callable(default=None, doc="""
         Used in conjunction with the reuse_sessions option, the

panel/io/server.py

@@ -516,6 +516,15 @@ async def get(self, *args, **kwargs):
                 expiration=app.session_token_expiration,
                 extra_payload=extra_payload
             )
+            if config.reuse_sessions == 'warm':
+                state.execute(
+                    partial(
+                        self.application_context.create_session_if_needed,
+                        session_id,
+                        self.request,
+                        token
+                    )
+                )
         else:
             token = session.token
         logger.info(LOG_SESSION_CREATED, id(session.document))