ReferenceGlossary
Fan-out
One request that turns into many calls to other services or data stores. The more calls a request waits for, the more likely one of them is slow, so wide fan-out raises tail latency and load.
Performance
In Quorum#
A page view makes 2 read queries, and ORM frameworks (Laravel, Rails, Django) make 3 reads where 1 would do until Eager loading. The Activity feed Feature adds feeds: pulled, a feed makes 20 reads. With fan-out on write, a feed makes 1 read but every write fills 20 followers' timelines. Hybrid fan-out cuts that to 2 reads and 10 timeline writes.