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Advances and Open Problems in Federated Learning
Peter Kairouz
7∗
H. Brendan McMahan
7∗
Brendan Avent
21
Aur
´
elien Bellet
9
Mehdi Bennis
19
Arjun Nitin Bhagoji
13
Keith Bonawitz
7
Zachary Charles
7
Graham Cormode
23
Rachel Cummings
6
Rafael G.L. D’Oliveira
14
Salim El Rouayheb
14
David Evans
22
Josh Gardner
24
Zachary Garrett
7
Adri
`
a Gasc
´
on
7
Badih Ghazi
7
Phillip B. Gibbons
2
Marco Gruteser
7,14
Zaid Harchaoui
24
Chaoyang He
21
Lie He
4
Zhouyuan Huo
20
Ben Hutchinson
7
Justin Hsu
25
Martin Jaggi
4
Tara Javidi
17
Gauri Joshi
2
Mikhail Khodak
2
Jakub Kone
ˇ
cn
´
y
7
Aleksandra Korolova
21
Farinaz Koushanfar
17
Sanmi Koyejo
7,18
Tancr
`
ede Lepoint
7
Yang Liu
12
Prateek Mittal
13
Mehryar Mohri
7
Richard Nock
1
Ayfer
¨
Ozg
¨
ur
15
Rasmus Pagh
7,10
Mariana Raykova
7
Hang Qi
7
Daniel Ramage
7
Ramesh Raskar
11
Dawn Song
16
Weikang Song
7
Sebastian U. Stich
4
Ziteng Sun
3
Ananda Theertha Suresh
7
Florian Tram
`
er
15
Praneeth Vepakomma
11
Jianyu Wang
2
Li Xiong
5
Zheng Xu
7
Qiang Yang
8
Felix X. Yu
7
Han Yu
12
Sen Zhao
7
1
Australian National University,
2
Carnegie Mellon University,
3
Cornell University,
4
´
Ecole Polytechnique F
´
ed
´
erale de Lausanne,
5
Emory University,
6
Georgia Institute of Technology,
7
Google Research,
8
Hong Kong University of Science and Technology,
9
INRIA,
10
IT University of Copenhagen,
11
Massachusetts Institute of Technology,
12
Nanyang Technological University,
13
Princeton University,
14
Rutgers University,
15
Stanford University,
16
University of California Berkeley,
17
University of California San Diego,
18
University of Illinois Urbana-Champaign,
19
University of Oulu,
20
University of Pittsburgh,
21
University of Southern California,
22
University of Virginia,
23
University of Warwick,
24
University of Washington,
25
University of Wisconsin–Madison
Abstract
Federated learning (FL) is a machine learning setting where many clients (e.g. mobile devices or
whole organizations) collaboratively train a model under the orchestration of a central server (e.g. service
provider), while keeping the training data decentralized. FL embodies the principles of focused data
collection and minimization, and can mitigate many of the systemic privacy risks and costs resulting
from traditional, centralized machine learning and data science approaches. Motivated by the explosive
growth in FL research, this paper discusses recent advances and presents an extensive collection of open
problems and challenges.
∗
Peter Kairouz and H. Brendan McMahan conceived, coordinated, and edited this work. Correspondence to kairouz@
google.com and mcmahan@google.com.
1
arXiv:1912.04977v1 [cs.LG] 10 Dec 2019

Contents
1 Introduction 4
1.1 The Cross-Device Federated Learning Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.1 The Lifecycle of a Model in Federated Learning . . . . . . . . . . . . . . . . . . . . . . . . 7
1.1.2 A Typical Federated Training Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.2 Federated Learning Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.3 Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2 Relaxing the Core FL Assumptions: Applications to Emerging Settings and Scenarios 11
2.1 Fully Decentralized / Peer-to-Peer Distributed Learning . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.1 Algorithmic Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1.2 Practical Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2 Cross-Silo Federated Learning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3 Split Learning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3 Improving Efficiency and Effectiveness 18
3.1 Non-IID Data in Federated Learning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.1.1 Strategies for Dealing with Non-IID Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2 Optimization Algorithms for Federated Learning . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.2.1 Optimization Algorithms and Convergence Rates for IID Datasets . . . . . . . . . . . . . . . 21
3.2.2 Optimization Algorithms and Convergence Rates for Non-IID Datasets . . . . . . . . . . . . 25
3.3 Multi-Task Learning, Personalization, and Meta-Learning . . . . . . . . . . . . . . . . . . . . . . . . 28
3.3.1 Personalization via Featurization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.3.2 Multi-Task Learning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.3.3 Local Fine Tuning and Meta-Learning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.3.4 When is a Global FL-trained Model Better? . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.4 Adapting ML Workflows for Federated Learning . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.4.1 Hyperparameter Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.4.2 Neural Architecture Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.4.3 Debugging and Interpretability for FL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.5 Communication and Compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.6 Application To More Types of Machine Learning Problems and Models . . . . . . . . . . . . . . . . 34
4 Preserving the Privacy of User Data 35
4.1 Actors, Threat Models, and Privacy in Depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.2 Tools and Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.2.1 Secure Computations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.2.2 Privacy-Preserving Disclosures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.2.3 Verifiability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.3 Protections Against External Malicious Actors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.3.1 Auditing the Iterates and Final Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.3.2 Training with Central Differential Privacy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.3.3 Concealing the Iterates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.3.4 Repeated Analyses over Evolving Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.3.5 Preventing Model Theft and Misuse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.4 Protections Against an Adversarial Server . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.4.1 Challenges: Communication Channels, Sybil Attacks, and Selection . . . . . . . . . . . . . . 52
4.4.2 Limitations of Existing Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.4.3 Training with Distributed Differential Privacy . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.4.4 Preserving Privacy While Training Sub-Models . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.5 User Perception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.5.1 Understanding Privacy Needs for Particular Analysis Tasks . . . . . . . . . . . . . . . . . . . 58
2

4.5.2 Behavioral Research to Elicit Privacy Preferences . . . . . . . . . . . . . . . . . . . . . . . . 58
5 Robustness to Attacks and Failures 59
5.1 Adversarial Attacks on Model Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.1.1 Goals and Capabilities of an Adversary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.1.2 Model Update Poisoning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.1.3 Data Poisoning Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.1.4 Inference-Time Evasion Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5.1.5 Defensive Capabilities from Privacy Guarantees . . . . . . . . . . . . . . . . . . . . . . . . . 68
5.2 Non-Malicious Failure Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.3 Exploring the Tension between Privacy and Robustness . . . . . . . . . . . . . . . . . . . . . . . . . 71
6 Ensuring Fairness and Addressing Sources of Bias 72
6.1 Bias in Training Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
6.2 Fairness Without Access to Sensitive Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.3 Fairness, Privacy, and Robustness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
6.4 Leveraging Federation to Improve Model Diversity . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
6.5 Federated Fairness: New Opportunities and Challenges . . . . . . . . . . . . . . . . . . . . . . . . . 76
7 Concluding Remarks 77
A Software and Datasets for Federated Learning 103
3

1 Introduction
Federated learning (FL) is a machine learning setting where many clients (e.g. mobile devices or whole or-
ganizations) collaboratively train a model under the orchestration of a central server (e.g. service provider),
while keeping the training data decentralized. It embodies the principles of focused collection and data
minimization, and can mitigate many of the systemic privacy risks and costs resulting from traditional, cen-
tralized machine learning. This area has received significant interest recently, both from research and applied
perspectives. This paper describes the defining characteristics and challenges of the federated learning set-
ting, highlights important practical constraints and considerations, and then enumerates a range of valuable
research directions. The goals of this work are to highlight research problems that are of significant theo-
retical and practical interest, and to encourage research on problems that could have significant real-world
impact.
The term federated learning was introduced in 2016 by McMahan et al. [289]: “We term our approach
Federated Learning, since the learning task is solved by a loose federation of participating devices (which
we refer to as clients) which are coordinated by a central server.” An unbalanced and non-IID (identically
and independently distributed) data partitioning across a massive number of unreliable devices with limited
communication bandwidth was introduced as the defining set of challenges.
Significant related work predates the introduction of the term federated learning. A longstanding goal
pursued by many research communities (including cryptography, databases, and machine learning) is to ana-
lyze and learn from data distributed among many owners without exposing that data. Cryptographic methods
for computing on encrypted data were developed starting in the early 1980s [340, 421], and Agrawal and
Srikant [15] and Vaidya et al. [390] are early examples of work that sought to learn from local data using
a centralized server while preserving privacy. Conversely, even since the introduction of the term federated
learning, we are aware of no single work that directly addresses the full set of FL challenges. Thus, the term
federated learning provides a convenient shorthand for a set of characteristics, constraints, and challenges
that often co-occur in applied ML problems on decentralized data where privacy is paramount.
This paper originated at the Workshop on Federated Learning and Analytics held June 17–18th, 2019,
hosted at Google’s Seattle office. During the course of this two-day event, the need for a broad paper
surveying the many open challenges in the area of federated learning became clear.
1
A key property of many of the problems discussed is that they are inherently interdisciplinary — solving
them likely requires not just machine learning, but techniques from distributed optimization, cryptography,
security, differential privacy, fairness, compressed sensing, systems, information theory, statistics, and more.
Many of the hardest problems are at the intersections of these areas, and so we believe collaboration will be
essential to ongoing progress. One of the goals of this work is to highlight the ways in which techniques from
these fields can potentially be combined, raising both interesting possibilities as well as new challenges.
Since the term federated learning was initially introduced with an emphasis on mobile and edge device
applications [289, 287], interest in applying FL to other applications has greatly increased, including some
which might involve only a small number of relatively reliable clients, for example multiple organizations
collaborating to train a model. We term these two federated learning settings “cross-device” and “cross-silo”
respectively. Given these variations, we propose a somewhat broader definition of federated learning:
Federated learning is a machine learning setting where multiple entities (clients) collaborate
in solving a machine learning problem, under the coordination of a central server or service
provider. Each client’s raw data is stored locally and not exchanged or transferred; instead,
1
During the preparation of this work, Li et al. [262] independently released an excellent but less comprehensive survey.
4

focused updates intended for immediate aggregation are used to achieve the learning objective.
Focused updates are updates narrowly scoped to contain the minimum information necessary for the specific
learning task at hand; aggregation is performed as earlier as possible in the service of data minimization.
We note that this definition distinguishes federated learning from fully decentralized (peer-to-peer) learning
techniques as discussed in Section 2.1.
Although privacy-preserving data analysis has been studied for more than 50 years, only in the past
decade have solutions been widely deployed at scale (e.g. [156, 135]). Cross-device federated learning and
federated data analysis are now being applied in consumer digital products. Google makes extensive use of
federated learning in the Gboard mobile keyboard [323, 196, 420, 98, 329], as well as in features on Pixel
phones [18] and in Android Messages [375]. While Google has pioneered cross-device FL, interest in this
setting is now much broader, for example: Apple is using cross-device FL in iOS 13 [27], for applications
like the QuickType keyboard and the vocal classifier for “Hey Siri” [28]; doc.ai is developing cross-device
FL solutions for medical research [130], and Snips has explored cross-device FL for hotword detection
[259].
Cross-silo applications have also been proposed or described in myriad domains including finance risk
prediction for reinsurance [407], pharmaceuticals discovery [158], electronic health records mining [162],
medical data segmentation [19, 121], and smart manufacturing [305].
The growing demand for federated learning technology has resulted in a number of tools and frameworks
becoming available. These include TensorFlow Federated [38], Federated AI Technology Enabler [34],
PySyft [342], Leaf [35], PaddleFL [36] and Clara Training Framework [33]; more details in Appendix A.
Commercial data platforms incorporating federated learning are in development from established technology
companies as well as smaller start-ups.
Table 1 contrasts both cross-device and cross-silo federated learning with traditional single-datacenter
distributed learning across a range of axes. These characteristics establish many of the constraints that
practical federated learning systems must typically satisfy, and hence serve to both motivate and inform the
open challenges in federated learning. They will be discussed at length in the sections that follow.
These two FL variants are called out as representative and important examples, but different FL settings
may have different combinations of these characteristics. For the remainder of this paper, we consider the
cross-device FL setting unless otherwise noted, though many of the problems apply to other FL settings as
well. Section 2 specifically addresses some of the many other variations and applications.
Next, we consider cross-device federated learning in more detail, focusing on practical aspects common
to a typical large-scale deployment of the technology; Bonawitz et al. [74] provides even more detail for a
particular production system, including a discussion of specific architectural choices and considerations.
1.1 The Cross-Device Federated Learning Setting
This section takes an applied perspective, and unlike the previous section, does not attempt to be definitional.
Rather, the goal is to describe some of the practical issues in cross-device FL and how they might fit into a
broader machine learning development and deployment ecosystem. The hope is to provide useful context
and motivation for the open problems that follow, as well as to aid researchers in estimating how straight-
forward it would be to deploy a particular new approach in a real-world system. We begin by sketching the
lifecycle of a model before considering a FL training process.
5
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