This question evaluates competency in recommender systems and production machine learning engineering, covering collaborative filtering and embedding-based candidate generation, learning-to-rank, contextual personalization, constraint enforcement, and scalable serving architectures.

You are releasing a new recommendation feature that must generate and assign personalized, ranked product lists for each user at scale. Users may have different roles (e.g., buyer vs. seller), and recommendations must respect contextual signals (e.g., session/device/time) as well as business or policy constraints (e.g., eligibility, age/geography restrictions).
Discuss and justify choices across the typical recommendation stack, including:
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