ALS Algorithm for Robust and Communication-Efficient Federated Learning

Federated learning is a distributed approach to machine learning in which a centralised server coordinates the learning task while training data is distributed among a potentially large set of clients. The focus of this paper is on top-N recommendations using a training set of implicit interactions between users and items. With this limited information, items with no user interaction must also be considered, to present accurate recommendations. In the past, federated recommender systems have been solved through communication of the local model updates using a Stochastic Gradient Descent (SGD) approach. However, SGD is unable to handle the full interaction dataset without the need for negative sampling. This poses a big strain in the setting of wireless networks, as negative sampling considerably increases the communication overhead. To overcome this obstacle we introduce the first federated learning matrix factorisation model fully based on Alternating Least Squares (ALS) computation. The ALS approach offers an efficient matrix factorisation solution with the ability to avoid negative sampling. We show that this novel approach can significantly reduce the communication overhead when compared to its SGD counterparts while maintaining high levels of accuracy.