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The infrastructure available to large-scale and medium-scale web services now spans dozens of geographically dispersed datacenters. Deploying across many datacenters has the potential to significantly reduce end-user latency by serving users nearer their location. However, deploying across many datacenters requires the backend storage system be partially replicated. In turn, this can sacrifice the low latency benefits of many datacenters, especially when a storage system provides guarantees on what operations will observe. We present the K2 storage system that provides lower latency for large-scale and medium-scale web services using partial replication of data over many datacenters with strong guarantees: causal consistency, read-only transactions, and write-only transactions. K2 provides the best possible worst-case latency for partial replication, a single round trip to remote datacenters, and often avoids sending any requests to far away datacenters using a novel replication approach, write-only transaction algorithm, and read-only transaction algorithm. 

Replicated state machines are linearizable, fault-tolerant groups of replicas that are coordinated using a consensus algorithm. Copilot replication is the first 1-slowdown-tolerant consensus protocol: it delivers normal latency despite the slowdown of any 1 replica. Copilot uses two distinguished replicas—the pilot and copilot—to proactively add redundancy to all stages of processing a client’s command. Copilot uses dependencies and deduplication to resolve potentially differing orderings proposed by the pilots. To avoid dependencies leading to either pilot being able to slow down the group, Copilot uses fast takeovers that allow a fast pilot to complete the ongoing work of a slow pilot. Copilot includes two optimizations—ping-pong batching and null dependency elimination—that improve its performance when there are 0 and 1 slow pilots respectively. Our evaluation of Copilot shows its performance is lower but competitive with Multi-Paxos and EPaxos when no replicas are slow. When a replica is slow, Copilot is the only protocol that avoids high latencies.