docs: add migration guides for prerender and rendertron - #2
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Both upstream projects are gone: prerender/prerender returns a 404, and GoogleChrome/rendertron has been archived since 2022-10-06 with no commits since. There is no other maintained self-hosted prerendering server, and anyone still running either is on unmaintained software. migrating-from-prerender.md maps the compatible surface directly: the readiness flag and meta tags renamed but kept identical semantics, plugins folded into config flags and hooks, and the env vars that moved. migrating-from-rendertron.md is the more consequential change -- the URL shape moves from path-embedded (GET /render/<url>) to query-encoded (GET /render?url=), and Rendertron's built-in cache backends have no renderready equivalent by design. The one genuinely good finding: Rendertron never had a page-side readiness signal at all, just page-load plus network-quiet -- which is exactly what renderready's fallback already does, so no application changes are required to get equivalent behavior. Verified before writing: prerender/prerender still 404s, Rendertron's last push date, and Rendertron's actual config.json/API shape via its README rather than from memory. The injected x-renderready-render-id / x-renderready-render-at header names are checked against src/render/htmlTransforms.ts rather than asserted. Linked from the README's Compatibility section and table of contents.
The intro implied the old server was disappearing -- it isn't. The npm package still resolves and installs regardless of the source repo's status, and npm doesn't unpublish a package because its GitHub repo goes away. Existing deployments and fresh `npm install prerender` both keep working indefinitely. What's actually lost is a path to a fix, not availability. Re-read lib/server.js and lib/plugins/browserForceRestart.js rather than relying on an earlier summary before writing the replacement, which turned up two things I'd have gotten wrong: the old server does retry once immediately on an unexpected browser crash (not zero retries), and only exits the whole process if two crashes land within a second of each other; and its forced- restart plugin does drain in-flight requests before restarting, just with no bound if one hangs. Recycling itself is reasonable -- idle-restart always, scheduled restart too if you use the documented entry point or register the plugin on a custom build.
- CHROME_LOCATION and PRERENDER_READY_DELAY were never env vars in the old server, only JS constructor options — table and checklist now reflect that instead of implying a rename - note that Rendertron's ?mobile also swapped the User-Agent, not just viewport size - remove the fabricated "in-memory / S3 cache" bundled plugin — the old server never shipped one
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Summary
docs/migrating-from-prerender.mdanddocs/migrating-from-rendertron.mdBoth upstream projects are gone (prerender/prerender 404s; Rendertron archived since 2022-10-06),
so these are aimed at the searches people are already making rather than a launch audience.
Test plan
x-renderready-render-*header names against sourceprettier --checkclean