Features flags for ISR'd pages using SvelteKit

This repository shows how to manually implement feature flags on Vercel for pages that are ISR'd using SvelteKit.

Deployed example: https://flags-precompute-manual-sveltekit.vercel.app/

Prerequisites:

This example uses Vercel flags to en/decrypt feature flags. Make sure to have FLAGS_SECRET setup in your Vercel project and pull it to an .env file so you can work with it locally aswell. (Alternatively if that's none of your concern, you can remove that logic and no longer need that env variable).

Flow:

1. Request comes to middleware.ts. A flag cookie is looked up to see what value it is. For first time visitors the value is computed randomly.
2. Based on the cookie value and the URL a rewrite is determined (in this example, / goes either to /home-a or /home-b)
3. Request comes to ISR'd SvelteKit page, serving either /home-a or /home-b
4. Flag cookie is sent along with the response (it's set in middleware.ts, so ISR/SvelteKit itself never sees it, which is important), so revisiting user gets the same flag value
5. On the client, we make use of async reroute to request a server endpoint we created (that is not prerendered or ISR'd) which determines the route for us (using similar logic to that in middleware.ts)