logging: configure default logging config

* Bump Traitlets to >=5.2.1 (needed for logging_config) trait.
* Add default logging config to write to ~/.cylc/uiserver/uiserver.log
  at the INFO level.
* Document in the readme for now.

README.md

@@ -170,6 +170,13 @@ Jupyter Server can run multiple extensions. To control the extensions that
 are run use the `ServerApp.jpserver_extensions` configuration, see the
 [Jupyter Server configuration documentation](https://jupyter-server.readthedocs.io/en/latest/other/full-config.html#other-full-config).
 
+By default the Cylc part of the UI Server log is written to
+`~/.cylc/uiserver/uiserver.log`.
+
+<!--
+TODO: Link to Jupyter Server logging_config docs when published
+-->
+
 ### UI
 
 The UI can be configured via the "Settings" option in the Dashboard.

cylc/uiserver/jupyter_config.py

@@ -57,8 +57,38 @@
     )
 ]
 
-# store the JupyterHub runtime files in ~/.cylc/hub
+# store JupyterHub runtime files in the user config directory
 USER_CONF_ROOT.mkdir(parents=True, exist_ok=True)
 c.JupyterHub.cookie_secret_file = f'{USER_CONF_ROOT / "cookie_secret"}'
 c.JupyterHub.db_url = f'{USER_CONF_ROOT / "jupyterhub.sqlite"}'
 c.ConfigurableHTTPProxy.pid_file = f'{USER_CONF_ROOT / "jupyterhub-proxy.pid"}'
+
+# write Cylc logging to the user config directory
+# NOTE: Parallel UIS instances will conflict over this file.
+#       See https://github.com/cylc/cylc-uiserver/issues/240
+c.CylcUIServer.logging_config = {
+    'version': 1,
+    'handlers': {
+        'file': {
+            'class': 'logging.handlers.RotatingFileHandler',
+            'level': 'INFO',
+            'filename': str(USER_CONF_ROOT / 'uiserver.log'),
+            'mode': 'a',
+            'backupCount': 5,
+            'maxBytes': 10485760,
+            'formatter': 'file_fmt',
+        },
+    },
+    'loggers': {
+        'CylcUIServer': {
+            'level': 'INFO',
+            'handlers': ['console', 'file'],
+        },
+    },
+    'formatters': {
+        'file_fmt': {
+            'format': '%(asctime)s %(levelname)-8s %(message)s',
+            'datefmt': '%Y-%m-%dT%H:%M:%S',
+        }
+    },
+}

setup.cfg

@@ -56,7 +56,7 @@ install_requires =
     graphql-ws==0.4.4
     jupyter_server>=1.10.2
     tornado>=6.1.0  # matches jupyter_server value
-    traitlets>=5
+    traitlets>=5.2.1  # required for logging_config (5.2.0 had bugs)
 
     # Fix lack of upper pin for rx in graphql-core<2.3.0 (remove when
     # upgrading graphene to 3.0):