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.

Relevant Resources (GitHub repositories, Issues, PRs)#