The upgrade from jupyter-server v1 to v2 is a major release with significant new features and breaking changes, particularly for those who customize or extend the server.

Key Breaking Changes:

• New ServerKernelManager: If you have custom functionality that extends the default KernelManager, you must update your code to derive from the new jupyter_server.services.kernels.kernelmanager.ServerKernelManager class. This is a mandatory change to use new server-specific features. [2]
• New APIs for Identity and Authorization: Version 2.0 introduces new, pluggable APIs for identity and authorization. [2] While these are powerful new features, they represent a fundamental change that may require verification and configuration in multi-user or secured deployments.
• Terminals as an Extension: The terminals service is no longer a core feature and is now shipped as a default-enabled extension. [2] This should not affect most users but is a structural change to be aware of for custom deployments.
• Python Version Support: Support for Python 3.8 was dropped in v2.15.0, and support for Python 3.9 was dropped in v2.19.0. [1] Your environment must be using Python 3.10 or newer.

Recommendation:
This upgrade requires careful review, especially if you have custom extensions or configurations. Developers should:

1. Verify any custom KernelManager subclasses and update them to inherit from ServerKernelManager.
2. Review the new Identity and Authorization APIs if you have custom authentication or permission logic.
3. Ensure your deployment environment runs a supported Python version (3.10+).

Source: Jupyter Server 2.0 Release Announcement [2], Changelog [1]

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