GMF Core: an open-source recommendation and decisioning engine for marketplaces

While building GILLZY , a digital goods marketplace, I ended up working on a problem that appears in almost every sufficiently large marketplace: How do you turn behavioral events and catalog candidates into explainable, safe, and reproducible recommendation decisions? At first, recommendation logic tends to be simple. Fetch some products. Calculate a score. Sort them. Return the top N. But as a marketplace grows, the pipeline quickly becomes more complicated: multiple candidate sources;…