NSDI '06 Abstract
Pp. 381394 of the Proceedings
Pastwatch: A Distributed Version Control System
Alexander Yip, Benjie Chen, and Robert Morris, MIT Computer Science and AI Laboratory
Pastwatch is a version control system that acts like a
traditional client-server system when users are connected to the
users can see each other's changes immediately after the changes are
When a user is not connected, Pastwatch
also allows users to read revisions
from the repository, commit new revisions and share
modifications directly between users, all without access to
the central repository. In
contrast, most existing version control systems require connectivity to
a centralized server in order to read or update the repository.
Each Pastwatch user's host keeps its own writable replica
of the repository, including historical revisions. Users can
synchronize their local replicas with each other or with one or more
servers. Synchronization must handle inconsistency between
replicas because users may commit concurrent and conflicting changes
to their local replicas. Pastwatch represents its repository as a
"revtree" data structure which tracks the relationships among these
conflicting changes, including any reconciliation.
The revtree also ensures that the replicas eventually converge to
identical images after sufficient synchronization.
We have implemented Pastwatch and evaluate it in a setting distributed
over North America. We have been using it actively for more than a
year. We show that the system is scalable beyond 190 users per
project and that commit and update operations only take 2-4 seconds.
Currently, five users and six different projects regularly use the
system; they find that the system is easy to use and that the system's
replication has masked several network and storage failures.
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