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Andrew ClarkandGitHub
203177bb75
`partialFallback` is an internal flag in the build output that controls
the behavior of ISR. When it's enabled, in a Cache Components app, pages
that are not generated at build time are upgraded to a full ISR entry
upon the first request.
This is a useful feature but there's a risk it could lead to an
explosion in ISR costs, because even a prefetch request is sufficient to
trigger an upgrade.
The desired behavior is that we only trigger an upgrade for an actual
navigation request, or if a link opts into prefetching with the
`prefetch` prop.
Currently there's no way for ISR to differentiate between a shell
prefetch request (no prefetch prop on the link) and an actual prefetch
(`prefetch={true}`). The plan is to introduce additional configuration
to the build output that specifies which headers should trigger which
operations.
So by default, this PR turns the `partialFallback` flag back off until
the new mechanism is available to us.
However, for apps that opt into Partial Prefetching, we will continue to
set `partialFallback` to true. That's because in a Partial Prefetching
app, per-link prefetch requests are always opt-in. So even without
additional support at the ISR behavior, we avoid an unexpected surge in
ISR costs by not sending the prefetching request in the first place.
There will still be a single request per distinct route (_not_ per
distinct URL!), which we will optimize later.