We present QuiCK, a queuing system built for managing asynchronous tasks in CloudKit, Apple's storage backend service. QuiCK stores queued messages along with user data in CloudKit, and supports CloudKit's tenancy model including isolation, fair resource allocation, observability, and tenant migration. QuiCK is built on the FoundationDB Record Layer, an open source transactional DBMS. It employs massive two-level sharding, with tens of billions of queues on the first level (separately storing the queued items for each user of every CloudKit app), and hundreds of queues on a second level (one per FoundationDB cluster used by CloudKit). Our evaluation demonstrates that QuiCK scales linearly with additional consumer resources, effectively avoids contention, provides fairness across CloudKit tenants, and executes deferred tasks with low latency.

CloudKit is Apple's cloud backend service and application development framework that provides strongly-consistent storage for structured data and makes it easy to synchronize data across user devices or share it among multiple users. Launched more than 3 years ago, CloudKit forms the foundation for more than 50 Apple apps, including many of our most important and popular applications such as Photos, iCloud Drive, Notes, Keynote, and News, as well as many third-party apps. To deliver this at large scale, CloudKit explicitly leverages multi-tenancy at the application level as well as at the user level to guide efficient data placement and distribution. By using CloudKit application developers are free to focus on delivering the application front-end and logic while relying on CloudKit for scale, consistency, durability and security. CloudKit manages petabytes of data and handles hundreds of millions of users around the world on a daily basis.