This paper details and evaluates the use of optimistic replica consistency, automatic update conflict detection and repair, the peer-to-peer (as opposed to client-server) interaction model, and the stackable file system architecture in the design and construction of Ficus. The paper concludes with a number of lessons learned from the experience of designing, building, measuring, and living with an optimistcally replicated file system.

optimistic, replication, survey Replication is a key enabling technology in distributed data sharing systems for improving both availability and performance. This paper surveys optimistic replication algorithms, which allow replica contents to diverge in the short term, in order to support concurrent work and to tolerate failures in low-quality communication links. The importance of such techniques is increasing as collaboration through wide-area and mobile networks is becoming more popular. Optimistic replication algorithms employ techniques vastly different from those for traditional pessimistic algorithms. Whereas a pessimistic algorithm relies on synchronous

With web caching and cache-related services like CDNs and edge services playing an increasingly significant role in the modern Internet, the problem of the weak consistency and coherence provisions in current web protocols is drawing increasing attention. Toward this end, we differentiate definitions of consistency and coherence for web-like caching environments, and then present a novel web protocol we call "Basis Token Consistency" (BTC). This protocol allows compliant caches to guarantee strong consistency of content retrieved from supporting servers. We then compare the performance of BTC with the traditional TTL (Time To Live) algorithm under a range of synthetic workloads in order to illustrate its qualitative performance properties.

With web caching and cache-related services like CDNs and edge services playing an increasingly significant role in the modern internet, the problem of the weak consistency and coherence provisions in currently standardized web protocols is drawing greater attention. Toward this end, we propose definitions of consistency and coherence for web-like caching environments, and then present a novel web protocol we call "Basis Token Consistency" (BTC). This protocol allows compliant caches to guarantee strongly consistent views of content retrieved from supporting servers. We discuss this protocol and its extensions, and compare the performance of BTC with the traditional TTL (Time To Live) algorithm under a range of synthetic workloads.

Optimistic replication algorithms allow replica contents to be stale but in a controlled way. By propagating updates in background and allowing any replica to be read most of the time, they become more available and more efficient than traditional replication algorithms using unreliable network media and inexpensive computers. The use of optimistic replication has recently grown explosively due to the proliferation of the Internet and mobile computing devices, but its systematic study has begun only recently. We survey optimistic replication algorithms in this report. In particular, we focus on the update propagation mechanisms invented for optimistic replication and study how these mechanisms affect the functional characteristics of the systems, including data consistency guarantees and performance.

Optimistic replication algorithms allow replica contents to become stale but in a controlled way. In return, they become far more efficient and available than traditional replication algorithms that keep all the replicas consistent, especially when the network and computers are unreliable. The use of optimistic replication has grown explosively due to the proliferation of the use of the Internet and mobile computing devices, but its systematic study has begun only recently. This report surveys optimistic replication algorithms. In particular, it focuses on mechanisms for propagating updates among replicas and studies how they affect the functional characteristics of the systems, including data consistency guarantees and performance.

With web caching and cache-related services like CDNs and edge services playing an increasingly significant role in the modern internet, the problem of the weak consistency and coherence provisions in current web protocols is becoming increasingly significant and drawing the attention of the standards community [LCD01]. Toward this end, we present definitions of consistency and coherence for web-like environments, that is, distributed client-server information systems where the semantics of interactions with resources are more general than the read/write operations found in memory hierarchies and distributed file systems. We then present a brief review of proposed mechanisms which strengthen the consistency of caches in the web, focusing upon their conceptual contributions and their weaknesses in real-world practice. These insights motivate a new mechanism, which we call "Basis Token Consistency" or BTC; when implemented at the server, this mechanism allows any client (independent of the presence and conformity of any intermediaries) to maintain a self-consistent view of the server's state. This is accomplished by annotating responses with additional per-resource application information which allows client caches to recognize the obsolescence of currently cached entities and identify responses from other caches which are already stale in light of what has already been seen. The mechanism requires no deviation from the existing client-server communication model, and does not require servers to maintain any additional per-client state. We discuss how our mechanism could be integrated into a fragment-assembling Content Management System (CMS), and present a simulation-driven performance comparison between the BTC algorithm and the use of the Time-To-Live (TTL) heuristic.