JEP for Optional Features

jupyter-optional-features/jupyter-optional-features.md

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+---
+title: Jupyter Optional Features
+authors: Johan Mabille (@JohanMabille)
+issue-number: xxx
+pr-number: xxx
+date-started: 2023-01-16
+---
+
+# Summary
+
+This JEP introduces Jupyter optional features, which eases the way
+a kernel advertises which features it supports, without coupling it
+to the version of the protocol that it implements.
+
+# Motivation
+
+Some of the features that were added (the debugger) or proposed (the subshells)
+may require a lot of work from kernel authors to implement them. Besides, the
+changes they introduce on the protocol are self-contained; it is possible for a
+kernel to not support them without altering its exepected behavior regarding the
+rest of the protocol, and it is easy for a client to enable or disable such a
+feature as long as it knows whether the kernel supports it. By the way, the
+debugger is already optional, although it does not explicitly state it.
+
+The goal of this JEP is to introduce the notion of optional feature in the protocol
+and to have an explicit list of such features. This way, we do not prevent kernel
+authors from upgrading to a more recent version of the protocol when we introduce
+a new feature that may be complicated to implement and not mandatory for them.
+
+# Proposed Enhancement
+
+We propose to add a new `supportedFeatures` field to the `kernel_info_reply` message.
+This field is a list of optional features that the kernel supports. The boolean field
+`debugger` should be deprecated, as it would duplicate a possible value of the list.
+
+An optional feature can be a list of additional messages and/or a list of additional
+fields in different existing messages. When a feature introduces new messages, it is
+its responsibility to specify the order of these messages when it makes sense. Under
+no circumstances this feature should alter the order of already existing messages,
+nor interleave new messages between already existing messages.
+
+The documentation should indicate which optional feature a message (or a field of a
+message) is linked to when it is relevant. This would ease the implementation of
+new kernels and the upgrade to new versions of the protocol.