A Survey of Federated Unlearning: A Taxonomy, Challenges and Future Directions

The evolution of privacy-preserving Federated Learning (FL) has led to an increasing demand for implementing the right to be forgotten. The implementation of selective forgetting is particularly challenging in FL due to its decentralized nature. This complexity has given rise to a new field, Federated Unlearning (FU). FU emerges as a strategic solution to address the increasing need for data privacy, including the implementation of the `right to be forgotten'. The primary challenge in developing FU approaches lies in balancing the trade-offs in privacy, security, utility, and efficiency, as these elements often have competing requirements. Achieving an optimal equilibrium among these facets is crucial for maintaining the effectiveness and usability of FL systems while adhering to privacy and security standards. This survey provides a comprehensive analysis of existing FU methods, incorporating a detailed review of the various evaluation metrics. Furthermore, we unify these diverse methods and metrics into an experimental framework. Additionally, the survey discusses potential future research directions in FU. Finally, a continually updated repository of related open-source materials is available at: https://github.com/abbottyanginchina/Awesome-Federated-Unlearning.