Cold Start & the Serving Layer
Real-Time vs Batch: The Freshness Budget
Freshness is a per-signal purchase, not a platform
Real-time pipelines buy relevance where value decays in minutes: session context ('just watched a trailer'), trending items, breaking news, live inventory. They buy nothing for stable features — 'you like sci-fi' hasn't changed since lunch.
Price freshness per signal, not per system: user context justifies real-time (it's cheap and fast-decaying), item embeddings usually justify nightly reindexing (expensive, slower-moving), co-visit counts sit near-real-time. 'Real-time everything' is a tax on every request to benefit the 10% of features that needed it.
insight
Map each feature's value-decay curve against its update cost. Fast-decaying cheap features earn streaming; slow stable features stay batch. Freshness is a budget you spend per signal.