A vacuuming-enhanced transaction-time web server

Yingxia Wang

A transaction-time web server archive document versions during HTTP requests to create a complete history of the documents at a website. The server also processes transaction-time queries (TTQueries) to fetch requested versions from the archive. Internal to the server are an archive to store past versions and a history table to record information about the versions. Both the archive and the history table will grow over time as new versions are created, and eventually exceed the storage capacity. This thesis investigates a technique called vacuuming to control the size of the archive. Vacuuming removes selected versions from an archive. A vacuuming policy identifies which versions to vacuum. This thesis develops a wide range of vacuuming policies. Unfortunately, vacuuming controls the growth of the archive by sacrificing the completeness of the server’s history. Some TTQueries can no longer be satisfied, so query repair strategies are presented to tune those queries. Like the archive, the history table also grows over time. Two techniques for recouping space in the history table, coalescing and obliteration, are introduced. This thesis also explores resource migration, which migrates the history of a resource when the resource moves to a new location. All of the new functionalities were designed to be backwards compatible with existing protocols (e.g., HTTP) and standards (e.g., HTML), so a site can become a vacuum-enhanced, transaction-time web server at any time. This thesis presents a logical model for the design and URI-compatible syntax for supporting the new functionality. Transaction-time support, vacuuming, query repair, coalescing, obliteration, and resource migration were implemented as a modular extension of Apache v2.0. This thesis describes porting the implementation of transaction-time support in TTApache, which was based on Apache v1.3, to vTTApache (vacuum-enhanced, transaction-time Apache), based on Apache v2.0, as well as implementing the new functionality. Since the Apache internals changed significantly in v2.0, the port required a substantial redesign of the transaction-time support. The thesis presents extensive empirical measurements on the performance overhead of the transaction-time and vacuuming functionality in vTTApache. The empirical work suggests that the additional functionality has a marginal impact in most server environments.

A vacuuming-enhanced transaction-time web server

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