  Federated learning shows promise as a privacy-preserving collaborative
learning technique. Existing heterogeneous federated learning mainly focuses on
skewing the label distribution across clients. However, most approaches suffer
from catastrophic forgetting and concept drift, mainly when the global
distribution of all classes is extremely unbalanced and the data distribution
of the client dynamically evolves over time. In this paper, we study the new
task, i.e., Dynamic Heterogeneous Federated Learning (DHFL), which addresses
the practical scenario where heterogeneous data distributions exist among
different clients and dynamic tasks within the client. Accordingly, we propose
a novel federated learning framework named Federated Multi-Level Prototypes
(FedMLP) and design federated multi-level regularizations. To mitigate concept
drift, we construct prototypes and semantic prototypes to provide fruitful
generalization knowledge and ensure the continuity of prototype spaces. To
maintain the model stability and consistency of convergence, three
regularizations are introduced as training losses, i.e., prototype-based
regularization, semantic prototype-based regularization, and federated
inter-task regularization. Extensive experiments show that the proposed method
achieves state-of-the-art performance in various settings.
