This paper discusses the synchronous viewing and editing of object-oriented
models based on the Unified Modeling Language (UML) by multiple users on t
he internet. Each user works on a copy of the model, which is continuously
synchronized with the master copy on a central server. A group of users can
work together in the common single writer-multiple reader mode, controlled
by a token that can be passed from user to user. Alternatively, different
users can lock different parts of the model for exclusive editing while the
changes are visible to all other users. One can improve on basic granular
locking by considering the semantics of the graphical notation. A semantics
-based locking mechanism has been designed and implemented for UML class di
agrams in order to allow users to collaborate more effectively. A prototype
collaborative UML (CUML) editing system has been implemented in Java to va
lidate the design.