Design a Similar Items System

Problem Formulation#

Before you write a single line of model code, you need to nail down what the system is actually doing. Interviewers will probe this early, and candidates who skip straight to "train a two-tower model" without clarifying the product surface almost always back themselves into a corner.

Start by asking: is this a "customers who viewed this also viewed" widget, or a "similar items" widget on the product detail page? They sound identical but they're not. The first is collaborative by nature, built on session co-occurrence. The second can be answered purely from item content, which means it works even for brand-new listings. That distinction drives your entire model architecture.