  Vizier is the de-facto blackbox and hyperparameter optimization service
across Google, having optimized some of Google's largest products and research
efforts. To operate at the scale of tuning thousands of users' critical
systems, Google Vizier solved key design challenges in providing multiple
different features, while remaining fully fault-tolerant. In this paper, we
introduce Open Source (OSS) Vizier, a standalone Python-based interface for
blackbox optimization and research, based on the Google-internal Vizier
infrastructure and framework. OSS Vizier provides an API capable of defining
and solving a wide variety of optimization problems, including multi-metric,
early stopping, transfer learning, and conditional search. Furthermore, it is
designed to be a distributed system that assures reliability, and allows
multiple parallel evaluations of the user's objective function. The flexible
RPC-based infrastructure allows users to access OSS Vizier from binaries
written in any language. OSS Vizier also provides a back-end ("Pythia") API
that gives algorithm authors a way to interface new algorithms with the core
OSS Vizier system. OSS Vizier is available at https://github.com/google/vizier.
