Wednesday, February 21, 2007

On the Placement of Web Server Replicas

On the Placement of Web Server Replicas, L. Qiu, V. Padmanabhan, G. Voelker, IEEE Incofom 2001, 1587-96.

This paper is definitely the inspiration for this one or the other way around. One is a journal paper from 2002 and the other a conference paper from 2001, so I would have to see which one came out when.

Anyway, it covers much of the same ground: placing web server replicas to form a Content Distribution Network (CDN) is a problem which is hard, and the paper presents some heuristic algorithms to solve the problem using an AS topology (and other topologies).

The goal is to improve on the result of a paper which proves the optimal placement on a tree topology (B. Li, M. Golin, G. Ialiano, X. Deng, "on the optimal placement of web proxies on the internet", infocom 1999): the internet is a mesh, not a tree, so using the tree optimal placement result in a sub-optimal internet placement.

The heuristics are similar to here: random, greedy (pick the best spot for a replica that minimize the cost, then iterate taking into account the already place replicas), hot spot (put the replicas near the sites which generate the most traffic) and super-optimal algorithm (which is super-optimal not because it is really super-duper, but because it might not feasible, thus might perform better than the optimal; it is solved using some Lagrangian relaxation of the integer programming problem defined by the problem).

There is a neat evaluation section, over a lot of different topologies and which shows that the greedy algorithm behaves pretty good. One really cool link from the paper is the NP problem compendium web page. I mean, once you think to add the tilde to the reference provided in the paper.

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