A serverless, wide-area version control system

This thesis describes Pastwatch, a distributed version control system. Pastwatch maintains versions of users' shared files. Each version is immutable: to make changes, a user checks out a version onto the user's computer, edits the files locally, then commits the changes to create a new version. The motivation behind Pastwatch is to support wide-area read/write file sharing. An example of this type of sharing is when loosely affiliated programmers from different parts of the world collaborate to work on open-source software projects. To support such users, Pastwatch offers three properties. First, it allows users who travel frequently or whose network connections fail from time to time to access historical versions of the shared files or make new versions while disconnected. Second, Pastwatch makes the current and historical versions of the shared files highly available. For example, even when their office building experiences a power failure, users can still create new versions and retrieve other users' changes from other locations. Supporting disconnected operation is not adequate by itself in these cases; users also want to see others' changes. Third, Pastwatch avoids using dedicated servers. Running a dedicated server requires high administrative costs, expertise, and expensive equipment. Pastwatch achieves its goals using two interacting approaches. First, it maintains a local branch tree of versions on each user's computer. A user can check out versions from the local tree and commit changes into the local tree. Second, Pastwatch uses a shared branch tree in a DHT to publish users' new versions. It contacts the tree to keep a user's local branch tree up-to-date. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)