  This paper presents an ecosystem for personal knowledge graphs (PKGs),
commonly defined as resources of structured information about entities related
to an individual, their attributes, and the relations between them. PKGs are a
key enabler of secure and sophisticated personal data management and
personalized services. However, there are challenges that need to be addressed
before PKGs can achieve widespread adoption. One of the fundamental challenges
is the very definition of what constitutes a PKG, as there are multiple
interpretations of the term. We propose our own definition of a PKG,
emphasizing the aspects of (1) data ownership by a single individual and (2)
the delivery of personalized services as the primary purpose. We further argue
that a holistic view of PKGs is needed to unlock their full potential, and
propose a unified framework for PKGs, where the PKG is a part of a larger
ecosystem with clear interfaces towards data services and data sources. A
comprehensive survey and synthesis of existing work is conducted, with a
mapping of the surveyed work into the proposed unified ecosystem. Finally, we
identify open challenges and research opportunities for the ecosystem as a
whole, as well as for the specific aspects of PKGs, which include population,
representation and management, and utilization.
