Nowadays, much of the data is born decentralized. For example, billions of phones and IoT devices constantly generate data, and the data can be used to enable better products. Informally, instead of bringing all data into a centralized server and train a model, FL brings the model to where the data lives, train it locally, and only upload the update to the server. Local data storing and processing with global coordination is made possible by the emerging technology of mobile edge computing(MEC), where edge nodes, such as sensors, home gateways, micro servers, and small cells, are equipped with storage and computation capability.