  Federated Learning (FL) is a machine learning paradigm that enables the
training of a shared global model across distributed clients while keeping the
training data local. While most prior work on designing systems for FL has
focused on using stateful always running components, recent work has shown that
components in an FL system can greatly benefit from the usage of serverless
computing and Function-as-a-Service technologies. To this end, distributed
training of models with serverless FL systems can be more resource-efficient
and cheaper than conventional FL systems. However, serverless FL systems still
suffer from the presence of stragglers, i.e., slow clients due to their
resource and statistical heterogeneity. While several strategies have been
proposed for mitigating stragglers in FL, most methodologies do not account for
the particular characteristics of serverless environments, i.e., cold-starts,
performance variations, and the ephemeral stateless nature of the function
instances. Towards this, we propose FedLesScan, a novel clustering-based
semi-asynchronous training strategy, specifically tailored for serverless FL.
FedLesScan dynamically adapts to the behaviour of clients and minimizes the
effect of stragglers on the overall system. We implement our strategy by
extending an open-source serverless FL system called FedLess. Moreover, we
comprehensively evaluate our strategy using the 2nd generation Google Cloud
Functions with four datasets and varying percentages of stragglers. Results
from our experiments show that compared to other approaches FedLesScan reduces
training time and cost by an average of 8% and 20% respectively while utilizing
clients better with an average increase in the effective update ratio of
17.75%.
