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.

FoundationDB is an open source transactional key value store created more than ten years ago. It is one of the first systems to combine the flexibility and scalability of NoSQL architectures with the power of ACID transactions (a.k.a. NewSQL). FoundationDB adopts an unbundled architecture that decouples an in-memory transaction management system, a distributed storage system, and a built-in distributed configuration system. Each sub-system can be independently provisioned and configured to achieve the desired scalability, high-availability and fault tolerance properties. FoundationDB uniquely integrates a deterministic simulation framework, used to test every new feature of the system under a myriad of possible faults. This rigorous testing makes FoundationDB extremely stable and allows developers to introduce and release new features in a rapid cadence. FoundationDB offers a minimal and carefully chosen feature set, which has enabled a range of disparate systems (from semi-relational databases, document and object stores, to graph databases and more) to be built as layers on top. FoundationDB is the underpinning of cloud infrastructure at Apple, Snowflake and other companies, due to its consistency, robustness and availability for storing user data, system metadata and configuration, and other critical information.

The FoundationDB Record Layer is an open source library that provides a record-oriented data store with semantics similar to a relational database implemented on top of FoundationDB, an ordered, transactional key-value store. The Record Layer provides a lightweight, highly extensible way to store structured data. It offers schema management and a rich set of query and indexing facilities, some of which are not usually found in traditional relational databases, such as nested record types, indexes on commit versions, and indexes that span multiple record types. The Record Layer is stateless and built for massive multi-tenancy, encapsulating and isolating all of a tenant's state, including indexes, into a separate logical database. We demonstrate how the Record Layer is used by CloudKit, Apple's cloud backend service, to provide powerful abstractions to applications serving hundreds of millions of users. CloudKit uses the Record Layer to host billions of independent databases, many with a common schema. Features provided by the Record Layer enable CloudKit to provide richer APIs and stronger semantics with reduced maintenance overhead and improved scalability.