Lock-free Asynchronously Distributed Linked Lists

Abstract

Modern databases use dynamic search structures that store an enormous amount of data, and often serve them using multi-threaded algorithms to support the ever-increasing throughput needs. When this throughput need exceeds the capacity of the machine hosting the structure, one either needs to replace the underlying hardware (an option that is typically not viable and introduces a long down time) or make the data structure distributed. Static partitioning of the data structure for distribution is not desirable, as it is prone to uneven load distribution over time, and having to change the partitioning scheme later will require downtime. The goal of this paper is to extend a concurrent data structure to distributed data structures that provide dynamic load balancing while preserving important properties such as lock freedom. With this intuition, first, we introduce the notion of conditional lock-freedom which extends the notion of lock-free computation with reasonable assumptions about communication between processes. Then, we present DiLi, a conditional lock-free, linearizable, and distributed linked list that can be asynchronously and dynamically (1) partitioned into multiple sublists and (2) load balanced by distributing sublists across multiple machines. DiLi contains primitives for these that also maintain the lock-free property of the underlying search structure that supports find, remove, and insert of a key as the client operations. We show that DiLi bridges the gap between concurrent data structures and distributed data structures. Specifically, DiLi provides comparable (and better in write-intensive workloads) performance to skip lists (which are typically the fastest data structures for search in a concurrent environment). In addition, it provides horizontal scaling with dynamic load balancing in a distributed environment.