Abstract: Ensuring the freshness of client data is a fundamental problem for applications that rely on cloud infrastructure to store data and mediate sharing. Thialfi is a notification service developed at Google to simplify this task. Thialfi supports applications written in multiple programming languages and running on multiple platforms, e.g., browsers, phones, and desktops. Applications register their interest in a set of shared objects and receive notifications when those objects change. Thialfi servers run in multiple Google data centers for availability and replicate their state asynchronously. Thialfi's approach to recovery emphasizes simplicity: all server state is soft, and clients drive recovery and assist in replication. A principal goal of our design is to provide a straightforward API and good semantics despite a variety of failures, including server crashes, communication failures, storage unavailability, and data center failures. Evaluation of live deployments confirms that Thialfi is scalable, efficient, and robust. In production use, Thialfi has scaled to millions of users and delivers notifications with an average delay of less than one second.