Building replicated Internet services using TACT: a toolkit for tunable availability and consistency tradeoffs

An ultimate goal for modern Internet services is the development of scalable, high-performance, highly-available and fault-tolerant systems. Replication is an important approach to achieve this goal. However replication introduces the issue of consistency among replicas, which is further complicated by network partitions. Generally, higher consistency levels result in lower system availability in the presence of network partitions. Thus, there is a fundamental tradeoff between consistency and availability in building replicated Internet services. We argue that Internet services can benefit from dynamically choosing availability/consistency tradeoffs. With three consistency metrics, Unseen Writes, Uncommitted Writes and Staleness, we show how consistency can be meaningfully quantified for many Internet services. We present the design of the TACT (Tunable Availability and Consistency Tradeoffs) toolkit that allows Internet services to flexibly and dynamically choose their own availability/consistency, tradeoffs, enabling differentiated availability/consistency quality of service. Further TACT makes it possible for Internet services to dynamically trade consistency for performance.