In this paper, we argue that weak cache consistency mechanisms supported by existing web proxy caches must be augmented by strong consistency mechanisms to support the growing diversity in application requirements. Existing strong consistency mechanisms are not appealing for web environments due to their large state space or control message overhead. We focus on the lease approach that balances these tradeoffs and present analytical models and policies for determining the optimal lease duration. We present extensions to the http protocol to incorporate leases and then implement our techniques in the Squid proxy cache and the Apache web server. Our experimental evaluation of the leases approach shows that: (i) our techniques impose modest overheads even for long leases (a lease duration of 1 hour requires state to be maintained for 1030 leases and imposes an per-object overhead of a control message every 33 minutes); (ii) leases yields a 138-425% improvement over existing strong consist...

Web content caches are often placed between end-users and origin servers as a mean to reduce server load, network usage, and ultimately, user-perceived latency. Cached objects typically have associated expiration times, after which they are considered stale and must be validated with a remote server (origin or another cache) before they can be sent to a client. A considerable fraction of cache "hits" involve stale copies that turned out to be current. These validations of current objects have small message size, but nonetheless, often induce latency comparable to full-edged cache misses. Thus, the functionality of caches as a latency-reducing mechanism highly depends not only on content availability but also on its freshness. We propose policies for caches to proactively validate selected objects as they become stale, and thus allow for more client requests to be processed locally. Our policies operate within the existing protocols and exploit natural properties of request patterns such as frequency and recency. We evaluated and compared different policies using trace-based simulations.

In this paper, we argue that cache consistency mechanisms designed for stand-alone proxies do not scale to the large number of proxies in a content distribution network and are not flexible enough to allow consistency guarantees to be tailored to object needs. To meet the twin challenges of scalability and flexibility, we introduce the notion of cooperative consistency along with a mechanism, called cooperative leases, to achieve it. By supporting -consistency semantics and by using a single lease for multiple proxies, cooperative leases allows the notion of leases to be applied in a flexible, scalable manner to CDNs. Further, the approach employs application-level multicast to propagate server notifications to proxies in a scalable manner. We implement our approach in the Apache web server and the Squid proxy cache and demonstrate its efficacy using a detailed experimental evaluation. Our results show a factor of 2.5 reduction in server message overhead and a 20% reduction in server state space overhead when compared to original leases albeit at an increased inter-proxy communication overhead.

This paper provides an overview of techniques for improving Web performance. For improving server performance, multiple Web servers can be used in combination with efficient load balancing techniques. We also discuss how the choice of server architecture affects performance. We examine content distribution networks (CDN's) and the routing techniques that they use. While Web performance can be improved using caching, a key problem with caching is consistency. We present different techniques for achieving varying forms of cache consistency.

This paper describes an adaptive protocol that manages storage for base stations. The protocol integrates leasing storage management with a time-based coordinated checkpointing mechanism. The leasing enables storage managers to effectively control disk space. Leasing prevents hanged processes from indefinitely retaining storage and, in addition, garbage collection is simple. Time-based 1

In this paper, we argue that cache consistency mechanisms designed for stand-alone proxies do not scale to the large number of proxies in a content distribution network and are not flexible enough to allow consistency guarantees to be tailored to object needs. To meet the twin challenges of scalability and flexibility, we introduce the notion of cooperative consistency along with a mechanism, called cooperative leases, to achieve it. By supporting #-consistency semantics and by using a single lease for multiple proxies, cooperative leases allows the notion of leases to be applied in a flexible, scalable manner to CDNs. Further, the approach employs application-level multicast to propagate server notifications to proxies in a scalable manner. We implement our approach in the Apache web server and the Squid proxy cache and demonstrate its efficacy using a detailed experimental evaluation. Our results show a factor of 2.5 reduction in server message overhead and a 20% reduction in server state space overhead when compared to original leases albeit at an increased inter-proxy communication overhead.

In this paper, we examine the potential benefits of web proxy caches in improving the effective capacity of servers and networks. Since networks and servers are typically provisioned based on a high percentile of the load, we focus on the effects of proxy caching on the tail of the load distribution. We find that, unlike their substantial impact on the average load, proxies have a diminished impact on the tail of the load distribution. The exact reduction in the tail and the corresponding capacity savings depend on the percentile of the load distribution chosen for provisioning networks and servers---the higher the percentile, the smaller the savings. In particular, compared to over a 50% reduction in the average load, the savings in network and server capacity is only 2035 % for the 99 th percentile of the load distribution. We also find that while proxies can be somewhat useful in smoothing out some of the burstiness in web workloads; the resulting workload continues, however, to exhibit substantial burstiness and a heavy-tailed nature. We identify the poor locality of large objects to be the limiting factor that diminishes the impact of proxies on the tail of load distribution. We conclude that, while proxies are immensely useful to users due to the reduction in the average load, they are less effective in improving the capacities of networks and servers. 1

Since CDN simulations are known to be highly memory-intensive, in this paper, we argue the need for reducing the memory requirements of such simulations. We propose a novel memory-efficient data structure that stores cache state for a small subset of popular objects accurately and uses approximations for storing the state for the remaining objects. Since popular objects receive a large fraction of the requests while less frequently accessed objects consume much of the memory space, this approach yields large memory savings and reduces errors. We use bloom filters to store approximate state and show that careful choice of parameters can substantially reduce the probability of errors due to approximations.

Continuous queries are used to monitor changes to time varying data and to provide results useful for online decision making. Typically a user desires to obtain the value of some function over distributed data items, for example, to determine when and whether (a) the traffic entering a highway from multiple feed roads will result in congestion in a thoroughfare or (b) the value of a stock portfolio exceeds a threshold. Using the standard Web infrastructure for these applications will increase the reach of the underlying information. But, since these queries involve data from multiple sources, with sources supporting standard HTTP (pullbased) interfaces, special query processing techniques are needed. Also, these applications often have the flexibility to tolerate some incoherency, i.e., some differences between the results reported to the user and that produced from the virtual database made up of the distributed data sources.

Mobility is a critical feature of computer systems, and while wireless networks are common, most applications that run on mobile hosts lack flexible mechanisms for data access in an environment with large and frequent variations in network connectivity. Such conditions arise, for example, in collaborative work applications, particularly when wireless and wired users share files or databases. In this paper, we describe some techniques for adapting data access to network variability in the context of MFS, a client cache manager for a distributed file system. We show how MFS is able to adapt to widely varying bandwidth levels through the use of modeless adaptation, and evaluate the benefit of mechanisms for improving file system performance and cache consistency using microbenchmarks and file system traces.