Federated Learning. Yang Liu

      CHAPTER 1

       Introduction

       1.1 MOTIVATION

      We have witnessed the rapid growth of machine learning (ML) technologies in empowering diverse artificial intelligence (AI) applications, such as computer vision, automatic speech recognition, natural language processing, and recommender systems [Pouyanfar et al., 2019, Hatcher and Yu, 2018, Goodfellow et al., 2016]. The success of these ML technologies, in particular deep learning (DL), has been fueled by the availability of vast amounts of data (a.k.a. the big data) [Trask, 2019, Pouyanfar et al., 2019, Hatcher and Yu, 2018]. Using these data, DL systems can perform a variety of tasks that can sometimes exceed human performance; for example, DL empowered face-recognition systems can achieve commercially acceptable levels of performance given millions of training images. These systems typically require a huge amount of data to reach a satisfying level of performance. For example, the object detection system from Facebook has been reported to be trained on 3.5 billion images from Instagram [Hartmann, 2019].

      In general, the big data required to empower AI applications is often large in size. However, in many application domains, people have found that big data are hard to come by. What we have most of the time are “small data,” where either the data are of small sizes only, or they lack certain important information, such as missing values or missing labels. To provide sufficient labels for data often requires much effort from domain experts. For example, in medical image analysis, doctors are often employed to provide diagnosis based on scan images of patient organs, which is tedious and time consuming. As a result, high-quality and large-volume training data often cannot be obtained. Instead, we face silos of data that cannot be easily bridged.

      The modern society is increasingly made aware of issues regarding the data ownership: who has the right to use the data for building AI technologies? In an AI-driven product recommendation service, the service owner claims ownership over the data about the products and purchase transactions, but the ownership over the data about user purchasing behaviors and payment habits is unclear. Since data are generated and owned by different parties and organizations, a traditional and naive approach is to collect and transfer the data to one central location where powerful computers can train and build ML models. Today, this methodology is no longer valid.

      While AI is spreading into ever-widening application sectors, concerns regarding user privacy and data confidentiality expand. Users are increasingly concerned that their private information is being used (or even abused) by commercial and political purposes without their permission. Recently, several large Internet corporations have been fined heavily due to their leakage of users’ private data to commercial companies. Spammers and under-the-table data exchanges are often punished in court cases.

      In the legal front, law makers and regulatory bodies are coming up with new laws ruling how data should be managed and used. One prominent example is the adoption of the General Data Protection Regulation (GDPR) by the European Union (EU) in 2018 [GDPR website, 2018]. In the U.S., the California Consumer Privacy Act (CCPA) will be enacted in 2020 in the state of California [DLA Piper, 2019]. China’s Cyber Security Law and the General Provisions of Civil Law, implemented in 2017, also imposed strict controls on data collection and transactions. Appendix A provides more information about these new data protection laws and regulations.

      Under this new legislative landscape, collecting and sharing data among different organizations is becoming increasingly difficult, if not outright impossible, as time goes by. In addition, the sensitive nature of certain data (e.g., financial transactions and medical records) prohibits free data circulation and forces the data to exist in isolated data silos maintained by the data owners [Yang et al., 2019]. Due to industry competition, user privacy, data security, and complicated administrative procedures, even data integration between different departments of the same company faces heavy resistance. The prohibitively high cost makes it almost impossible to integrate data scattered in different institutions [WeBank AI, 2019]. Now that the old privacy-intrusive way of collecting and sharing data is outlawed, data consolidation involving different data owners is extremely challenging going forward.

      How to solve the problem of data fragmentation and isolation while complying with the new stricter privacy-protection laws is a major challenge for AI researchers and practitioners. Failure to adequately address this problem will likely lead to a new AI winter [Yang et al., 2019].

      Another reason why the AI industry is facing a data plight is that the benefit of collaborating over the sharing of the big data is not clear. Suppose that two organizations wish to collaborate on medical data in order to train a joint ML model. The traditional method of transferring the data from one organization to another will often mean that the original data owner will lose control over the data that they owned in the first place. The value of the data decreases as soon as the data leaves the door. Furthermore, when the better model as a result of integrating the data sources gained benefit, it is not clear how the benefit is fairly distributed among the participants. This fear of losing control and lack of transparency in determining the distribution of values is causing the so-called data fragmentation to intensify.

      With edge computing over the Internet of Things, the big data is often not a single monolithic entity but rather distributed among many parties. For example, satellites taking images of the Earth cannot expect to transmit all data to data centers on the ground, as the amount of transmission required will be too large. Likewise, with autonomous cars, each car must be able to process much information locally with ML models while collaborate globally with other cars and computing centers. How to enable the updating and sharing of models among the multiple sites in a secure and yet efficient way is a new challenge to the current computing methodologies.

1.2 FEDERATED LEARNING AS A SOLUTION

      As mentioned previously, multiple reasons make the problem of data silos become impediment to the big data needed to train ML models. It is thus natural to seek solutions to build ML models that do not rely on collecting all data to a centralized storage where model training can happen. An idea is to train a model at each location where a data source resides, and then let the sites communicate their respective models in order to reach a consensus for a global model. In order to ensure user privacy and data confidentiality, the communication process is carefully engineered so that no site can second-guess the private data of any other sites. At the same time, the model is built as if the data sources were combined. This is the idea behind “federated machine learning” or “federated learning” for short.

      Federated learning was first practiced in an edge-server architecture by McMahan et al. in the context of updating language models on mobile phones [McMahan et al., 2016a,b, Konecný et al., 2016a,b]. There are many mobile edge devices each holding private data. To update the prediction models in the Gboard system, which is the Google’s keyboard system for auto-completion of words, researchers at Google developed a federated learning system to update a collective model periodically. Users of the Gboard system gets a suggested query and whether the users clicked the suggested words. The word-prediction model in Gboard keeps improving based on not just a single mobile phone’s accumulated data but all phones via a technique known as federated averaging (FedAvg). Federated averaging does not require moving data from any edge device to one central location. Instead, with federated learning, the model on each