It is common to use weakly consistent replication to achieve high availability and low latency at a global scale. In this setting, concurrent updates may lead to states where application invariants do not hold. Some systems coordinate the execution of (conflicting) operations to avoid invariant violations, leading to high latency and reduced availability for those operations. This problem is worsened by the difficulty in identifying precisely which operations conflict. In this paper we propose a novel approach to preserve application invariants without coordinating the execution of operations. The approach consists of modifying operations in a way that application invariants are maintained in the presence of concurrent updates. When no conflicting updates occur, the modified operations present their original semantics. Otherwise, we use sensible and deterministic conflict resolution policies that preserve the invariants of the application. To implement this approach, we developed a static analysis, IPA, that identifies conflicting operations and proposes the necessary modifications to operations. Our analysis shows that IPA can avoid invariant violations in many applications, including typical database applications. Our evaluation reveals that the offline static analysis runs fast enough for being used with large applications. The overhead introduced in the modified operations is low and it leads to lower latency and higher throughput when compared with other approaches that enforce invariants.

Geo-replicated storage systems are at the core of current Internet services. The designers of the replication protocols used by these systems must choose between either supporting low-latency, eventually-consistent operations, or ensuring strong consistency to ease application correctness. We propose an alternative consistency model, Explicit Consistency, that strengthens eventual consistency with a guarantee to preserve specific invariants defined by the applications. Given these application-specific invariants, a system that supports Explicit Consistency identifies which operations would be unsafe under concurrent execution, and allows programmers to select either violation-avoidance or invariant-repair techniques. We show how to achieve the former, while allowing operations to complete locally in the common case, by relying on a reservation system that moves coordination off the critical path of operation execution. The latter, in turn, allows operations to execute without restriction, and restore invariants by applying a repair operation to the database state. We present the design and evaluation of Indigo, a middleware that provides Explicit Consistency on top of a causally-consistent data store. Indigo guarantees strong application invariants while providing similar latency to an eventually-consistent system in the common case.

Client-side apps (e.g., mobile or in-browser) need cloud data to be available in a local cache, for both reads and updates. For optimal user experience and developer support, the cache should be consistent and fault-tolerant. In order to scale to high numbers of unreliable and resource-poor clients, and large database, the system needs to use resources sparingly. The SwiftCloud distributed object database is the first to provide fast reads and writes via a causally-consistent client-side local cache backed by the cloud. It is thrifty in resources and scales well, thanks to consistent versioning provided by the cloud, using small and bounded metadata. It remains available during faults, switching to a different data centre when the current one is not responsive, while maintaining its consistency guarantees. This paper presents the SwiftCloud algorithms, design, and experimental evaluation. It shows that client-side apps enjoy the high performance and availability, under the same guarantees as a remote cloud data store, at a small cost.

Geo-replicated databases often offer high availability and low latency by relying on weak consistency models. The inability to enforce invariants across all replicas remains a key shortcoming that prevents the adoption of such databases in several applications. In this paper we show how to extend an eventually consistent cloud database for enforcing numeric invariants. Our approach builds on ideas from escrow transactions, but our novel design overcomes the limitations of previous works. First, by relying on a new replicated data type, our design has no central authority and uses pairwise asynchronous communication only. Second, by layering our design on top of a fault-tolerant database, our approach exhibits better availability during network partitions and data center faults. The evaluation of our prototype, built on top of Riak, shows much lower latency and better scalability than the traditional approach of using strong consistency to enforce numeric invariants.

Collaborative editing tools have become increasingly popular in the last decade, with some systems being used by massive numbers of users. While traditionally collaborative editing systems would either target synchronous or asynchronous collaboration settings, some recent systems support both types

It is typical in the domain of digital games to have many development problems due to its increasing complexity. Those difficulties include: i)little code reuse in order to develop a cross-platform game; and ii)performing game's verification through extensive and expensive tests. This of course results in low productivity in the development (evolution and maintenance) of game solutions.