We consider the following conflicts: one user modifies a slide while another user deletes it, two users move a slide to different positions in the presentation, or two users concurrently modify the same slide. We refer to these conflicts as edit-delete, move-move, and edit-edit, respectively. For simplicity, for the rest of this section, we refer to the copy of the presentation available at the remote and local proxies as the remote and local copies, respectively.
Our PowerPoint policy resolves edit-delete and move-move conflicts automatically. For edit-delete conflicts, our policy prioritizes editing over deletion, recreating the slide in the replica where it was deleted. For move-move conflicts, our policy gives priority to the local copy, moving the slide in the remote copy to reflect its position in the local copy. Finally, we resolve edit-edit conflicts by using the CoFi PowerPoint driver to present the two conflicting slides to the user, and prompting the user to resolve the conflict by either choosing one of the slides or by merging their content. The above policy is, however, just one of the possible ways to resolve conflicts, and we can easily envision variations or extensions to this simple policy.
The cost of conflict detection is highly dependent on the size of the documents. The bulk of the cost stems from writing out the document (see Section 4.2.2) and from transmitting the data over the network. All other aspects, including the execution of the conflict detection algorithm and the use of the APIs to display conflicts to the user, are insignificant.