Cache web

Résumé

La mise en cache de document Web (ex : page web, images) est utilisée afin de réduire la consommation de bande passante, la charge du serveur web (les tâches qu'il effectue), ou améliorer la rapidité de consultation lors de l'utilisation d'un navigateur web. Un cache Web conserve des copies de documents transitant par son biais. Le cache peut, dans certaines conditions, répondre aux requêtes ultérieures à partir de ses copies, sans recourir au serveur Web d'origine. Plusieurs équipements gèrent le cache de pages web :

L'application qui génère les pages du site web (ex : fichiers de cache d'un système de gestion de contenu).
Les serveurs qui hébergent le site web (ex : cache Apache).
Les proxys (ex : Varnish).
Le microordinateur client avec la gestion d'un cache local à partir de l'historique des connexions (ex : cache de navigateur).
Le réseau Internet lui-même par les opérateurs Internet ou des sociétés spécialisées dans la gestion de cache ; ce mode de cache est très ut

Source officielle

https://fr.wikipedia.org/wiki/Cache_web

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Cours associés (1)

This advanced course will provide students with the knowledge to tackle the design of privacy-preserving ICT systems. Students will learn about existing technologies to prect privacy, and how to evaluate the protection they provide.

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We present a consistent and transparent caching system for dynamic web pages produced by a server-side application using a back-end database. Cached pages always reflect current database values. No intervention from the programmer is necessary to implement caching. The system is an improvement on earlier methods that either did not guarantee consistency and/or relied on substantial programmer intervention. The novel idea is that a compiler analyzes and transforms the server-side application code to include cache checks, inserts, and invalidations. In order to provide precise invalidations and attendant good hit ratios, we check the intersection of the database table columns used by the read and the write queries, augmented by uniqueness information from the database schema and comparison of the query selection predicates against values inserted in the database. We use Java bytecode rewriting to implement the transformation of the server-side application. Using the Rubis benchmark, we demonstrate that transparent and consistent caching achieves substantial improvements in response time and throughput.

2003

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Caching Dynamic Web Content in J2EE Applications: An Automated Solution Using Aspect Programming

Sumit Mittal, Willy Zwaenepoel

Caching is an effective means for reducing load on web servers, especially for those that dynamically generate web documents in J2EE applications. While adding caching to the application server can greatly reduce response times for requests, the logic to ensure consistency with the backend database requires considerable effort to develop. Much of the complexity is in minimizing unnecessary page invalidations, a key goal for improving the cache hit rate and response times. In this paper we explore a range of invalidation policies that are progressively more precise. A contribution of this work is in achieving precise invalidations at the application server layer automatically. To explore these issues, we introduce AutoWebCache, a system for adding caching for dynamic content automatically to J2EE server-side applications having a back-end database. It uses aspect oriented programming for injecting the cache code automatically into an application, but this method requires the SQL query analysis-- needed for invalidation-- be performed at run-time, which is costly. We demonstrate how to eliminate this run-time analysis overhead through effective caching of intermediate analysis results. In two e-commerce benchmarks, RUBiS and TPC-W, we show our method can be highly effective, reducing the response times, 63% and 97%, respectively.

2004

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Modern large-scale data platforms manage colossal amount of data, generated by the ever-increasing number of concurrent users. Geo-replicated and sharded key-value data stores play a central role when building such platforms. As the strongest consistency model proven not to compromise availability, causal consistency (CC) is perfectly positioned to address the needs of such large-scale systems, while preventing some ordering anomalies. Transactional Causal Consistency (TCC) augments CC by providing richer transactional semantics, simplifying the development of distributed applications. However, achieving CC/TCC in an efficient manner is very challenging. In this thesis we introduce several protocols and designs for high performance causally consistent geo-replicated key-value data stores in several different settings. First, we present a new approach to implementing CC in geo-replicated data stores, called Optimistic Causal Consistency (OCC). By introducing a technique that we call client-assisted lazy dependency resolution, OCC makes it possible for updates replicated to a remote data center to become visible immediately, without checking if their causal dependencies have been received. We further propose a recovery mechanism that allows an OCC system to fall back on a pessimistic protocol to continue operating during network partitions. We show that OCC improves data freshness, while offering performance that is comparable or better than its pessimistic counterpart. Next, we address the problem of providing low-latency TCC reads under full replication. We present Wren, the first TCC system that at the same time achieves low latency by implementing nonblocking read operations and efficiently scales out by sharding. Wren introduces new protocols for transaction execution, dependency tracking and stabilization. The transaction protocol supports nonblocking reads by providing a transaction snapshot as a union of a fresh causal snapshot installed by every partition in the local data center and a client-side cache for writes that are not yet included in the snapshot. The dependency tracking and stabilization protocols require only two scalar timestamps, resulting in efficient resource utilization and providing scalability in terms of shards and replication sites, under a full replication setting. In return for these benefits, Wren slightly increases the visibility latency of updates. Finally, we present PaRiS, the first TCC system that supports partial replication and provides low latency by implementing non-blocking parallel read operations. PaRiS relies on a novel protocol to track dependencies, called Universal Stable Time (UST). By means of a lightweight background gossip process, UST identifies a snapshot of the data that has been installed by every data center in the system. PaRiS equips clients with a private cache, in which they store their own updates that are not yet reflected in the snapshot. The combination of the UST-defined snapshot with client-side cache enables interactive transactions that can consistently read from any replication site without blocking. Moreover, PaRiS requires only one timestamp to track dependencies and define transactional snapshots, thereby achieving resource efficiency and scalability in terms of shards and replication sites, in a partial replication setting.

EPFL2020

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EPFL2020