Loss of functionality: Ability to set SSR cache-control based on API response

[steve-taylor]

In the old router, getServerSideProps can be used to set Cache-Control headers based on an API response. It's mentioned in the docs here.

It would look something like this:

export async function getServerSideProps({ req, res }) {
  const response = await callSomeApi();
  const maxAge = response.isLive ? 30 : 1800;

  res.setHeader(
    'Cache-Control',
    `public, s-maxage=${maxAge}, stale-while-revalidate=59`
  )

  return {
    props: response,
  }
}

The important thing here is that we don't know what s-maxage will be until we get a response from the API.

In the new app router, we can't do this. We need to set revalidate ahead of time:

fetch('https://api.example.com/api/...', { next: { revalidate: maxAge } } });

And then that revalidate value is used as the TTL of the SSR page.

Unless there is something I'm missing here, this is functionality that has gone missing in the new app router and the revalidate thing on fetch needs to be thought through properly. Alternatively, there needs to be a dynamic alternative to export const revalidate = <whatever>;, e.g. export const async getRevalidate() { /* ... */ }.