Don't Contrast the Impossible: Region-Constrained Batching for Contrastive User Modeling on a Local Community Platform

Abstract:Contrastive learning is widely used for user modeling in large-scale recommender systems, where standard in-batch negatives implicitly assume universal exposure that any user can be shown any item. On local community platforms such as Karrot, however, exposure is geographically constrained; many user-item pairs are impossible by design yet still treated as negatives during training, diluting the contrastive learning signal. We address this impossible negatives problem and propose Region-Constrained Batch Sampling (RCBS), a simple yet effective batching method that constructs region-homogeneous mini-batches so that users are contrasted primarily against items they could feasibly see. By replacing impossible negatives with feasible ones, RCBS naturally introduces harder and more informative negatives under realistic exposure constraints. With offline evaluations and online A/B tests, we show that RCBS consistently improves user representation quality and consequently enhances home feed ranking, retrieval, and display ads ranking. The resulting user embeddings have been deployed in production across various applications.

Information Retrieval (cs.IR); Machine Learning (cs.LG)

arXiv:2607.28971 [cs.IR]

(or arXiv:2607.28971v1 [cs.IR] for this version)

https://doi.org/10.48550/arXiv.2607.28971 Focus to learn more arXiv-issued DOI via DataCite (pending registration)

https://doi.org/10.1145/3805712.3808467 Focus to learn more DOI(s) linking to related resources

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