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Why it is shaped this way

The problem​

An extension is the unit of delivery — "what would I uninstall to lose this?". In an application with two tiers, the honest answer to that question usually spans both: the checkout view and the endpoint that prices a cart are one capability, and nobody wants to install them separately or discover they are at different versions.

Today they are two installs. In the music example that is five pip install -e lines and an npm install, and the two halves are kept in step by hand.

What already pointed the right way​

  • The Python tier already discovers plugins from distributions. platform.discover(group) registers whatever is advertised under an entry-point group, so installing a distribution publishes its plugins and nothing is hardcoded in the host. That is precisely the mechanism a packaged extension would extend.
  • The manifest already spans the wire. A Python PluginManifest already declares frontend_dependencies and the same four presentation fields as its TypeScript counterpart. A distribution that carried both halves would not need a new vocabulary — it would need somewhere to put the built JavaScript.
  • Jupyter already proves the pattern. A JupyterLab extension is a Python distribution with a labextension shipped inside it; that is the shape being aimed at here.

How the open questions were answered​

  • Where the frontend build lives in the wheel — under share/datalayer/reactor/extensions/<name>/, declared as shared-data; the entry-point group datalayer.reactor.extensions advertises it.
  • How the shell finds it at runtime — GET /plugins/frontend-extensions lists it with its manifests, GET /reactor-extensions/{name}/{path} serves it, and the browser loads it through the same seam as any remote plugin — as a plain module or as a Module Federation container.
  • Version coupling — the container is built straight into share/, so the two halves cannot ship at different versions.
  • Development ergonomics — the Python half stays pip install -e; the frontend runs on a dev server and is hot-updated into the running host by name.