ReferenceGlossary
Fan-out on write
Building each user's feed when a post is written: the post is copied into the timeline of every follower, so reading a feed is one cheap lookup. It moves work from reads to writes, and it is expensive for accounts with many followers. Many systems use a hybrid: push for normal accounts and pull for celebrities at read time.
Data
In Quorum#
From Series C, the Activity feed Feature adds the feed kind, and each App Server card offers Feeds: Pull or Push. With Pull, a feed reads 20 posts. With Push, a feed reads one precomputed timeline, but every write makes 20 timeline writes. The Hybrid fan-out Research cuts that to 10 writes and 2 reads per feed, and lets a Cache or Wide-column Store split a Hot key across its shards.