Undo, Redo and Collaboration in Web Applications
- 1. Miroslav Cupák, Hao Yan Undo, Redo and Collaboration in Web Applications Dec 10, 2012
- 2. Introduction ● idea: investigate the use of application-level undo-redo to achieve collaboration ● motivation - collaboration is a crucial feature ○ Google Docs, Zoho Writer, SubEthaEdit... ○ essential for web-based word processing and spreadsheet ● goal: enable collaboration for a web-based diagram drawing software
- 3. Background ● Brown, Patterson: Undo for operators: building an undoable e-mail store. ● Lowell, Chandra, Chen: Exploring failure transparency and the limits of generic recovery. ● Edwards, Mynatt: Timewarp: techniques for autonomous collaboration. ● Michael, Towndrow, Wiz: Conditions for successful online document collaboration.
- 4. Asciiflow
- 5. Asciiflow ● web application primarily used for diagrams ● based on Java + Google Web Toolkit ○ client-server model with RPC communication ● 20+ operations ● local physical undo and redo ○ maintains a set of pixel-based changes on the client ● no collaboration support ○ local save overwriting external changes
- 6. Goals ● allow multiple users to co-author a diagram smoothly ● changes merged and propagated to all the users ● each user is able to perform local undo and redo his own actions ● graceful resolution of conflicts without loss of data
- 7. Implementation ● modify RPC communication and persistence ● change model to enable state sharing ○ track the user issuing changes ○ extend information sent to the server ● add autosave capabilities ○ synchronize save after each operation ● adapt the concept to pixel-based operations ● enable manual undo-redo of local operations
- 8. Implementation ● load and save coordination on the server ○ check the owner of the snapshot before saving ○ if owner = current user, proceed with simple save ○ otherwise merge changes before saving ● merging of changes ○ undo local operations ○ load the foreign snapshot ○ redo local operations on the new snapshot ○ save the new state
- 9. Conflicts ● save merging might lead to conflicts ○ eg. two players draw two overlapping shapes simultaneously ● most modifications of the same region OK ○ the slightly late change will be applied on top of the earlier change ● conflicts become a problem when undo operation is involved
- 11. Resolving Conflicts ● all conflicts are associated with undo / redo actions ○ without knowledge of actions of other users, local physical undo / redo always cause consistency problems ● need to save a global log of user actions on server to fundamentally prevent conflicts from happening
- 12. Resolving Conflicts ● global action log stored as part of the state on server A B . . . undo redo undo redo TimeGlobal Log of User Actions Local Undo A Local Redo B
- 13. Evaluation ● performance overheads of the collaboration features compared to the original application (merge cost, amount of data being sent)
- 14. Conclusions ● successful implementation of features that allow collaboration for Asciiflow ● demonstration of application-level undo-redo to achieve collaboration ● may improve performance by sending only the updates to server
- 15. Questions?