Cold Start & the Serving Layer
Cold Start: Users, Items, Catalogs
Three cold starts, three different toolkits
New user (no history): first-session context signals — device, locale, referrer, time — plus onboarding questions and contextual bandits that learn within the visit.
New item (no interactions): content features must carry it — the item tower takes metadata, text, and image embeddings so a usable vector exists before the first click, plus a deliberate exploration budget.
New system (no data): content-based heuristics and hand rules until interactions accumulate — you can't factorize an empty matrix.
Conflating them wastes months: fixing user cold start with better item features, or item cold start with better onboarding, solves a problem nobody has.
insight
Cold start never ends — new items and churned-back users arrive every day. Systems that treat it as an edge case build it badly; systems that treat it as a traffic class build it once, correctly.