Personal digital data is a critical asset, and governments worldwide have enforced laws and regulations to protect data privacy. Data users have been endowed with the right to be forgotten of their data. In the course of machine learning (ML), the forgotten right requires a model provider to delete user data and its subsequent impact on ML models upon user requests. Machine unlearning emerges to address this, which has garnered ever-increasing attention from both industry and academia. While the area has developed rapidly, there is a lack of comprehensive surveys to capture the latest advancements. Recognizing this shortage, we conduct an extensive exploration to map the landscape of machine unlearning including the (fine-grained) taxonomy of unlearning algorithms under centralized and distributed settings, debate on approximate unlearning, verification and evaluation metrics, challenges and solutions for unlearning under different applications, as well as attacks targeting machine unlearning. The survey concludes by outlining potential directions for future research, hoping to serve as a guide for interested scholars.

数字个人数据是一项重要的资产，机器遗忘权利要求模型提供者根据用户的请求删除用户数据，机器去学习涉及到中心化和分布式模型下的去学习算法、近似去学习、验证和评估指标、不同应用中的去学习挑战和解决方案、以及针对机器去学习的攻击，期望通过全面调研描绘机器去学习领域的最新进展，并为未来研究提供方向。

机器反学习：分类、度量、应用、挑战与前景