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Abstract
With privacy legislation empowering the users with the right to be forgotten,
it has become essential to make a model amenable for forgetting some of its
training data. However, existing unlearning methods in the machine learning
context can not be directly applied in the context of distributed settings like
federated learning due to the differences in learning protocol and the presence
of multiple actors. In this paper, we tackle the problem of federated
unlearning for the case of erasing a client by removing the influence of their
entire local data from the trained global model. To erase a client, we propose
to first perform local unlearning at the client to be erased, and then use the
locally unlearned model as the initialization to run very few rounds of
federated learning between the server and the remaining clients to obtain the
unlearned global model. We empirically evaluate our unlearning method by
employing multiple performance measures on three datasets, and demonstrate that
our unlearning method achieves comparable performance as the gold standard
unlearning method of federated retraining from scratch, while being
significantly efficient. Unlike prior works, our unlearning method neither
requires global access to the data used for training nor the history of the
parameter updates to be stored by the server or any of the clients.