[carbocation]

In short, the feeds are heavily denormalized and are constructed when new photos are added by people you follow, rather than at request time.

They seem to favor using disk space and saving on processing time and memory.

[mattdw]

Which was where Twitter had to go in their first major refactor, as well. Originally timeline views would be built on-demand, but that approach proved unscalable very quickly, so they moved to preemptively building timelines by copying tweets everywhere they needed to go at publish-time rather than request-time.

It's basically just an inversion; instead of the cache being a read-through layer over the database, the database becomes a write-through layer over the cache.