
Co-design the Modeling Process for Eicient VFL via Visualization Chinese CHI 2022, October 22–23, 2022, Guangzhou, China and Online, China
condence in their labels is a desirable capability for real-world
VFL deployments.
In this study, we co-design the modeling process to help VFL
practitioners improve the eciency of VFL modeling from the
perspective of visualization. We rst conduct an observational study
of the current practices of collaborating domain experts to identify
their main needs and concerns regarding VFL applications. Then,
we streamline the analysis pipeline of feature and sample spaces and
propose an interactive visualization system called VFLens.VFLens
helps domain experts to interactively participate in feature selection,
assessment, and sample data iteration processes before the VFL
model training phase, in feature interpretation after the VFL model
training phase, and in data sample selection during the VFL model
inference phase. A case study and expert feedback conrm the
ecacy of VFLens. Our main contributions are summarized below.
•
We describe the problem in the VFL context from the perspec-
tive of feature and sample space through an observational
study and in-depth discussions of design requirements with
VFL domain experts.
•
We co-design the VFL modeling process to support domain
experts to interactively participate in the data iteration, fea-
ture selection and assessment, and sample prediction pro-
cesses. To the best of our knowledge, VFLens is the rst such
eort in the VFL scenario.
•
We evaluate VFLens through a usage scenario, a quantitative
experiment and expert interviews.
2 RELATED WORK
The literature that overlaps this work can be categorized into four
groups, namely, federated learning,visualizations for federated learn-
ing,feature selection and assessment, and sample selection in machine
learning.
2.1 Federated Learning
Federated learning was rst proposed by Google, which prevents
data from being transmitted by distributing model training to each
mobile terminal [
5
]. Later, they released the rst commercial FL
application, GBoard [
17
], which uses a recursive neural language
model to predict the next word in a keyboard application. GBoard
allows each local mobile device to train the model using local data
from the same distributed ML model. The global model can be
updated by averaging the model parameters collected over all lo-
cal models. Along the same lines, many studies have reshaped
dierent ML models into a federated framework, including deci-
sion trees [
31
,
58
], linear/logistic regression [
32
,
36
], and neural
networks [
46
,
56
]. These works are categorized as HFL because
the clients share the same feature space but dier in the sample
space. Unlike HFL, VFL is applicable to scenarios where we have
many overlapping instances but few overlapping features [
51
]. For
example, an insurance company and an online retailer in a local
city have many overlapping users, but each has its own feature
space. VFL “merges” features and uses homomorphic encryption to
protect the data privacy of the participating parties, and requires
a more sophisticated mechanism to decompose the loss function
of each party. This study focuses on VFL, “virtually aggregation”
of dierent features to compute training losses and gradients in
a privacy-preserving manner, and jointly build an ML model [
11
]
with data from both parties.
2.2 Visualizations for Federated Learning
Researchers from academia and industry are using visualizations to
demonstrate, explain, and monitor the process of federated learn-
ing. For example, in industry, Lenovo has simulated the industrial
revolution in factories by demonstrating the process of horizontal
federated learning to predict the internal pressure of hardware [
38
].
Similarly, Cloudera Fast Forward Labs released an interactive sim-
ulation prototype, Turbofan Tycoon, which takes advantage of vi-
sualization to examine the federated model and predict when a
turbofan will fail [
35
]. FATEBoard
1
utilizes dashboard visualizations
to display modeling logs, metrics, and evaluation results, including
information on data sets, job status, computational plots, and model
output [
12
]. While FATEBoard can help domain experts understand
the ranking of features and the performance of models, it does
not support detailed and interactive inspection of the sample and
feature spaces. On the other hand, in academia, Wei et al. [
47
] de-
veloped a game to demonstrate the superiority of HFL and built a
visualization prototype to help understand the operation of HFL.
However, this work assumes that client-side data can be witnessed
by the server-side. Li et al. [
30
] proposed HFLens, which strictly
follows a data privacy-preserving design and supports comparative
visual interpretation at the overview, communication round, and
client instance levels. HFLens facilitates the investigation of the
overall HFL process involving all clients, the correlation analysis
of client information in one or dierent communication rounds,
the identication of potential anomalies, and the evaluation of the
contribution of each HFL client. However, the pain point for VFL
is not the anomaly detection like HFLens, because for VFL there
are generally not as many data collaborators as for HFL, and the
collaborators partnerships with common interests. In this work, we
do not focus on the operational process of FL, but rather improve
the eciency of VFL modeling by involving domain experts in the
sample and feature space.
2.3 Feature Selection and Assessment
There is a large amount of existing work related to feature selec-
tion [
4
,
6
], which has two main diculties. First, a large number
of features are used in the process of building machine learning
models; however, if several features are linearly correlated with
each other, many of them will be redundant, which adds additional
computational eort and leads to more complex parameters. Second,
common feature analysis methods use feature correlation metrics,
but correlation metrics cannot measure nonlinear relationships.
Isabelle et al. [
15
] performed a survey of automatic feature selec-
tion methods. The authors abstracted the core problem of feature
selection, which is to nd a minimal subset of features from a large
number of features. The authors also argued that there are many
options for feature selection and that there is no one universal and
unique solution. There are other types of feature selection methods,
such as wrappers [
25
], which iteratively eliminate features by re-
gression or classication models to nd the ideal subset of features.
There are also metric-based methods [
2
,
14
], where users pick the
1https://fate.fedai.org/