FedINC: An Exemplar-Free Continual Federated Learning Framework with Small Labeled Data

Federated learning (FL) has shown great promise for privacy-preserving learning by enabling collaborative training on decentralized clients. However, in realistic FL scenarios, clients often collect new data continuously, join or exit learning dynamically. As a result, the global model tends to forget old knowledge while learning new knowledge. Meanwhile, labeling the continuously arriving data in real-time is usually challenging. Therefore, the catastrophic forgetting problem intertwined with the label deficiency issue poses significant challenges for both learning new knowledge and consolidating old knowledge. To address these challenges, we develop a novel exemplar-free continual federated learning framework named FedINC, to learn a global incremental model with limited labeled data. We begin by excavating the cause of catastrophic forgetting via in-depth empirical studies. Based on that, we introduce targeted mechanisms for FedINC, including a hybrid contrastive learning mechanism to efficiently learn new knowledge with limited labeled data, a plastic feature regularization mechanism to preserve old task's representation space, a prototype-guided regularization mechanism to mitigate feature overlap between old and new classes while aligning the features of non-iid clients, and a prototype evolution mechanism for flexible and efficient incremental classification. Extensive experiments demonstrate the superior performance of FedINC in terms of both convergence speed and accuracy of the global model.