document: Managing versions of web documents in a transaction-time web server This paper presents a transaction-time HTTP server , called TTApache that supports document versioning . A document often consists of a main file formatted in HTML or XML and several included files such as images and stylesheets . A change to any of the files associated with a document creates a new version of that document . To construct a document version history , snapshots of the document 's files are obtained over time . Transaction times are associated with each file version to record the version 's lifetime . The transaction time is the system time of the edit that created the version . Accounting for transaction time is essential to supporting audit queries that delve into past document versions and differential queries that pinpoint differences between two versions . TTApache performs automatic versioning when a document is read thereby removing the burden of versioning from document authors . Since some versions may be created but never read , TTApache distinguishes between known and assumed versions of a document . TTApache has a simple query language to retrieve desired versions . A browser can request a specific version , or the entire history of a document . Queries can also rewrite links and references to point to current or past versions . Over time , the version history of a document continually grows . To free space , some versions can be vacuumed . Vacuuming a version however changes the semantics of requests for that version . This paper presents several policies for vacuuming versions and strategies for accounting for vacuumed versions in queries .
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