Mid-level interview questionAsked at Pinterest · interview write-up
Design user and visual-item embeddings for retrieval when a user's history can hold 160,000 events. Cover time-correct training data, cold start and freshness.
Model answer
A two-tower model. The item tower embeds image and text content, so new items get vectors immediately. The user tower runs a transformer over recent actions plus pooled summaries of older history, since 160,000 events won't fit attention.
Build each example as of its timestamp: only earlier events, split train and test by time. For freshness, refresh user embeddings daily and add recent actions at request time. New users fall back to onboarding interests.
Practise the rest
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