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[Segment Cache] Implement behavior on cache miss #72841

Merged
merged 2 commits into from
Nov 15, 2024
Merged

[Segment Cache] Implement behavior on cache miss #72841

merged 2 commits into from
Nov 15, 2024

Commits on Nov 15, 2024

  1. [Segment Cache] Implement behavior on cache miss 

    In the current navigation implementation, a partially dynamic navigation
always does two separate requests: one for the static data, and one for
the dynamic data. Typically the static data is prefetched before the
navigation begins, but even in the case where it is not, the current
implementation will still fetch it first. It then wait to send a dynamic
request until the first chunk is received from the prefetch response,
leading to an unfortunate request waterfall.

In the Segment Cache implementation, our plan is to never block a
navigation on prefetch data that isn't already populated in the cache.
Instead, in the case of a cache miss, we'll immediately start a dynamic
navigation and rely on the fact that the first thing the dynamic
response sends is the static PPR shell of the target page.

Because we'll always have this as a fallback behavior for cache misses,
it's a good starting point for the Segment Cache implementation. Then
we can start incrementally adding more and more features until we've
eventually reached/surpassed parity with the current implementation.

---

To avoid duplication of logic, I've chosen to model cache misses as a
special case of the normal static + dynamic flow. We can pretend that
the route tree returned by the dynamic request is, in fact, the result
of a prefetch. Then we use that same server response to write data into
the CacheNode tree. So it's the same flow as the "happy path", except we
use a single server response for both stages.
    @acdlite
    acdlite committed Nov 15, 2024
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  2. Add basic navigation test for Segment Cache 

    Almost nothing in the Segment Cache is implemented yet — all
navigations result in a cache miss. This just confirms that when a
cache miss occurs, it successfully navigates to the next page.
    @acdlite
    acdlite committed Nov 15, 2024
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