1. Semantic Concurrency Control for Real-Time Diagramming
Group 99 Workshop
Jeffrey Campbell
University of Pittsburgh

2. Introduction
Diagrams
Background
Current Work

3. Diagrams of Interest Focus of activity, final product (deliverable)
Object oriented graphics
Synchronous usage
Multi-object actions
Domain constraints on correctness

7. Background Database Concurrency Control CSCW Control Approaches
Problems

8. Concurrency Control DBMS Concurrency Control Mechanism
Supports multiple simultaneous transactions
Isolates transactions from each other
Maintains database consistency

9. Transactions Transaction ACID Properties Logical Unit of Work
Atomic: All or nothing
Consistent: DB correct before and after
Isolated: No interference between trans
Durable: Protected from system failure

10. Transactions Correctness Two Phase Locking Serializability Semantic
Holds locked resources to end of transaction

11. Extended Transactions
Many Approaches (Elmagarmid, 1992)
Split Transactions (Kaiser)
Release resources before end of transaction
Serializability correctness criterion
If resources are used by others, transaction may have to be aborted if later action aborts

12. Collaborative Diagramming Locking Problems
Difficult to identify beginning and end of transactions
Actions take a long time, blocking items for a long time
Focus is on transaction identification

13. Transaction Identification Semantic Approach
Identify logical units of work in diagrams
Domain constraints define such units
Diagram correctness is preserved
partial / incorrect diagrams may be saved
transaction re-starts when diagram re-opened

14. Long Transactions User action blocked for long time
2PL holds locks until end of transaction
Possible Improvements
Reduce length of transaction
Allow users to work by blocking access, not all other work
Release locks before end of transaction

15. Long Transaction Reduce Length
Better transaction identification results in shorter transactions than the work session
Does not require separate user action to indicate transaction begin or end point

16. Long Transaction Action Not Blocked
DB transaction requesting locked item waits for that item
CSCW users can
Work on something else
Wait for release of item needed
Request the item from another user

17. Long Transaction Release Locks
Tickle locks (Greif, et al)
Release if requested and inactive
Split Transaction (Kaiser)
Divide transaction into two transactions
Release resources before end of transaction
Serializability correctness criterion
If resources are read by others, they will have to be aborted if source transaction aborts

18. Long Transactions Enhanced Split Transactions
Domain criteria define correctness
Any diagramming transaction may be split
Users control request and granting of split

19. Current Work
CoDiagram System
Usability Evaluation

20. CoDiagram
Collaboration Transparent user interface (Visio) with central server
Implementation lessons:
Difficult to divide graphic event stream into user actions
Local versus remote origin
Redundant events for multi-shape actions
Sequence issues for local undo

21. CoDiagram Supports several locking approaches
Adjustable awareness features
Varied levels of syntax support

22. Usability Evaluation How groups develop diagrams
Types of conflicting actions
Frequency of conflicts with no system controls
Currently in progress

23. Summary
Domain constraints define transactions that support automatic acquisition and release of locks
Locks may be transferred between people by implementing split transactions
Not a complete solution for all applications, but another technique to use where appropriate