Abstract System designers wishing to build fast and highly available Internet services must contend with network congestion, latency, and unpredictable failures.
Replication is a key approach for improving service performance and availability.
Unfortunately, the benefits of replication are limited by the overhead of maintaining consistency across the wide area. The overhead of strong consistency protocols limits practical wide-area replication to a handful of sites. While many Internet services do not require strong consistency, existing optimistic consistency models allow replicas to become arbitrarily stale. This talk describes TACT, a continuous consistency model that allows replicas to bound their divergence from strong consistency. Using these metrics, applications can dynamically trade consistency for performance and availability based on changing client, network, and service characteristics.