Direct communication with external API

The main idea is getting a shorter path to API from SSR and CSR without extra hops and proxies. For that, we should have different server URLs and client URLs for the same resource ID. Even now you can have different URLs for fetch between server and client but our serialization system detect resource by the whole URL and produce double request in that situation. This feature request is not something new and several times accrued in Sapper tracker for example https://github.com/sveltejs/sapper/issues/489 Also, I did some rouge PR for this here https://github.com/sveltejs/kit/pull/1406, and you can find some extra details there. It looks like we should more detail describe this feature and cases to make a clean solution.

It’s a little confusing theme, but I want to hear your opinions and ideas.

[Rich-Harris]

There’s an issue around browser-sent request headers here: https://github.com/sveltejs/kit/issues/696. I think what we’d need to do here is take the initial info, init and normalise them into a Request object that includes whichever request headers we decide to include.

So perhaps the interface is actually more like this, which conveniently deals with the footgun mentioned above:

/**
 * @param {Request} req
 * @returns {Response}
 */
export async function serverFetch(req) {
  const data = await get_data_somehow(req.url);
  return new Response(data); // https://developer.mozilla.org/en-US/docs/Web/API/Response/Response
}

[Rich-Harris]

Oh, the custom fetch replacement idea is really interesting. Lots of flexibility there. I would maybe suggest this interface:

/**
 * @param {RequestInfo} info
 * @param {RequestInit} init
 * @returns {Response}
 */
export async function serverFetch(info, init) {
  // potential footgun: RequestInfo can be a string or a Request object with a url property
  // — people would need to decide for themselves whether to accommodate the latter
  const data = await get_data_somehow(info);
  return new Response(data); // https://developer.mozilla.org/en-US/docs/Web/API/Response/Response
}

There’s no need to pass fetch in as it’s globally available inside a SvelteKit app (along with Response, Request and Headers).