Merge pull request #82 from JohanMabille/kernel_info

Support kernel_info request on the control channel

Author: JohanMabille

80-kernel-info/kernel-info.md

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+# Support kernel\_info request on the control channel
+
+## Problem
+
+When connecting a new websocket to an existing kernel via the Jupyter server, if the kernel execution is
+busy (e.g. long running cell) or stopped (e.g. on a breakpoint), the messages sent over the websocket get no reply. This is because when
+establishing a new websocket connection, the Jupyter server will send `kernel_info` requests to the kernel
+and will prevent sending any other request until it receives a `kernel_info` reply. Since the `kernel_info`
+request is sent on the shell channel and the kernel execution is stopped, it cannot reply to that request.
+
+## Proposed enhancement
+
+We propose to state in the Jupyter Messaging Protocol that the `kernel_info` request can be sent on both the shell and the control channels. Although both channels supports the `kernel_info` message, clients are encouraged to send it on the control channel as it will always be able to handle it, while the shell channel may be stopped.
+
+### Impact on existing implementations
+
+This JEP impacts kernels since it requires them to support receiving 'kernel\_info\_request' on the control channel in addition to receiving them on the shell channel.
+
+It also has an impact on the Jupyter Server. For example, the reference implementation of Jupyter Server will attempt to send a a `kernel_info` request on both channels and listen for a response from _either_ channel. Any response informs the UI that the kernel is connected. 
+
+## Relevant Resources (GitHub repositories, Issues, PRs)
+
+### GitHub repositories
+
+- [Jupyter server](https://github.com/jupyter-server/jupyter_server): the backend to Jupyter web applications
+- [Jupyter client](https://github.com/jupyter/jupyter_client):  Jupyter protocol client APIs
+- [JupyterLab](https://github.com/jupyterlab/jupyterlab): JupyterLab computational environment
+
+### GitHub Issues
+
+- New websocket unable to communicate with kernel paused in debugging [#622](https://github.com/jupyter-server/jupyter_server/issues/622)
+- [Debugger] Active sessions cause the kernel to deadlock on page refresh [#10174](https://github.com/jupyterlab/jupyterlab/issues/10174)
+
+### GitHub Pull Requests
+
+Nudge kernel with info request until we receive IOPub messages [#361](https://github.com/jupyter-server/jupyter_server/pull/361)
+