Client-serveur

Résumé

Le protocole ou environnement client–serveur désigne un mode de transmission d'information (souvent à travers un réseau) entre plusieurs programmes ou processus : l'un, qualifié de client, envoie des requêtes ; l'autre, qualifié de serveur, attend les requêtes des clients et y répond. Le serveur offre ici un service au client. Par extension, le client désigne souvent l'ordinateur sur lequel est exécuté le logiciel client, et le serveur, l'ordinateur sur lequel est exécuté le logiciel serveur. Les machines serveurs sont généralement dotées de capacités supérieures à celles des ordinateurs personnels en ce qui concerne la puissance de calcul, les entrées-sorties et les connexions réseau, afin de pouvoir répondre de manière efficace à un grand nombre de clients. Les clients sont souvent des ordinateurs personnels ou terminaux individuels (téléphone, tablette), mais pas systématiquement. Un serveur peut répondre aux requêtes de plusieurs clients. Parfois le client et le serveur peuvent êt

Source officielle

https://fr.wikipedia.org/wiki/Client-serveur

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Technologies d'interface client

Ahmed Al-Muslemani, Omar Kallel, Mahdi Sajjadpour, Conail Stewart, Luis Terán Tamayo

2007

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Causal consistency is an attractive consistency model for geo-replicated data stores. It is provably the strongest model that tolerates network partitions. It avoids the long latencies associated with strong consistency, and, especially when using read-only transactions (ROTs), it prevents many of the anomalies of weaker consistency models. Recent work has shown that causal consistency allows "latency-optimal" ROTs, that are nonblocking, single-round and single-version in terms of communication. On the surface, this latency optimality is very appealing, as the vast majority of applications are assumed to have read-dominated workloads. In this paper, we show that such "latency-optimal" ROTs induce an extra overhead on writes that is so high that it actually jeopardizes performance even in read-dominated workloads. We show this result from a practical as well as from a theoretical angle. We present the Contrarian protocol that implements "almost latency-optimal" ROTs, but that does not impose on the writes any of the overheads incurred by latency-optimal protocols. In Contrarian, ROTs are nonblocking and single-version, but they require two rounds of client-server communication. We experimentally show that this protocol not only achieves higher throughput, but, surprisingly, also provides better latencies for all but the lowest loads and the most read-heavy workloads. We furthermore prove that the extra overhead imposed on writes by latency-optimal ROTs is inherent, i.e., it is not an artifact of the design we consider, and cannot be avoided by any implementation of latency-optimal ROTs. We show in particular that this overhead grows linearly with the number of clients.

ASSOC COMPUTING MACHINERY2018

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Mutable Checkpoint-Restart: Automating Live Update for Generic Server Programs

Calin Iorgulescu

The pressing demand to deploy software updates without stopping running programs has fostered much research on live update systems in the past decades. Prior solutions, however, either make strong assumptions on the nature of the update or require extensive and error-prone manual effort, factors which discourage live update adoption. This paper presents Mutable Checkpoint-Restart (MCR), a new live update solution for generic (multiprocess and multithreaded) server programs written in C. Compared to prior solutions, MCR can support arbitrary software updates and automate most of the common live update operations. The key idea is to allow the new version to restart as similarly to a fresh program initialization as possible, relying on existing code paths to automatically restore the old program threads and reinitialize a relevant portion of the program data structures. To transfer the remaining data structures, MCR relies on a combination of precise and conservative garbage collection techniques to trace all the global pointers and apply the required state transformations on the fly. Experimental results on popular server programs (Apache httpd, nginx, OpenSSH and vsftpd) confirm that our techniques can effectively automate problems previously deemed difficult at the cost of negligible run-time performance overhead (2% on average) and moderate memory overhead (3.9x on average).

ACM New York2014

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