When deploying a barebones Next.js App Router project, I am surprised to see that despite the entire application consisting of static assets, Functions (and ISR Functions) are still generated:
(This was created via pnpm create next-app --example reproduction-template app-name. The app is almost entirely empty; it does nothing and renders nothing.)
My intuition is: since the app is entirely static, a function is unnecessary, as the app can be entirely represented with a directory of files.
However, I eventually stumbled upon this page: Vercel Data Cache, which states:
When a page contains entirely static data , Vercel will use ISR to generate the whole page. However, when a page contains a mix of static and dynamic data , the dynamic data needs to be re-fetched when rendering the page. In this scenario, Vercel Data Cache is used to cache the static data to avoid slow origin fetches.
I remain curious on why ISR is deemed necessary, because there are no fetch calls present anywhere; the app does not require any dynamic data.
This has also led me to conclude that an entirely static Next.js App Router app incurs both Fast Data Transfer and Fast Origin Transfer usage; the latter occurs due to the (ISR) Functions.
One possible reason is that the ISR functions “live” on the Edge, and cache the static data such that it only needs to reach out to the origin to transfer it once (which would explain why my FOT is significantly lower than my FDT). These ISR functions never actually regenerate, and effectively have an invalidation time of ∞. Then, when users request my website (incurring FDT, but no FOT when cached), the ISR Function can immediately respond with the cached static content, without having to hop back to the origin. In this way, it is effectively identical to propagating the static content to the Edge Network (billed as FOT once per deployment), with future request bandwidth incurring FDT. Is this correct?
An underlying concern that may frame this question better is that I am worried that the FOT limit of 100 GB would easily be surpassed, even on a static site, because all routes are ISR Functions. However, if it’s the case that the FOT transfer definitely only occurs once per deployment, then that’s much less of a concern.
