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dOPT

dOPT. Kevin Barnes kbarnes3@uicu.edu. dOPT. Proposed by C. A. Ellis in 1989 dOPT makes locking unnecessary, Ellis claims Others dispute this, but this is covered later dOPT allows a user’s changes to be seen immediately while maintaining consistency

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dOPT

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  1. dOPT Kevin Barnes kbarnes3@uicu.edu

  2. dOPT • Proposed by C. A. Ellis in 1989 • dOPT makes locking unnecessary, Ellis claims • Others dispute this, but this is covered later • dOPT allows a user’s changes to be seen immediately while maintaining consistency • dOPT is fully distributed and resilient to site failure

  3. How it works • Each machine has an identifier i, which could be an IP or MAC • Each machine also maintains a state vector s with n components where n is the number of machines • The i component store the number of changes recorded by the ith machine

  4. Requests • Requests are of the form: <i, s, o, p> • i - the identifier • s - the state vector of the sending machine • o - the operation to be performed • p - the priority, ignore this because it is wrong

  5. Handling Requests • Action is based on comparison between s_i and s_x the states of the sending and receiving machines. • If s_i = s_x: execute o immediately • If s_i > s_x: queue o and execute later • If s_i < s_x: transform o and execute

  6. Why It Doesn’t Work • The priority suggested doesn’t work for two clients, suggest a different model for priority • This suggested model require point to point communication, but this can be expanded using a tree. • Implies a client-server model • Load-Balancing also seems possible using this model.

  7. Locking • Ellis claimed locking was unnecessary, but others argue otherwise • Chengzheng Sun argues that locking and operational transforms solve different problems

  8. An Example • Consider the following text with a grammar error: • “The dog eat dog food” • Two users noticed this mistake and correct in simultaneously. • One changes it to: “The dog eats dog food” • The other changes it to: “The dog does eat dog food”

  9. An Example, cont’d. • dOPT would then consolidate these changes to: • “The dog does eats dog food” • This is not what either user intended, nor is it proper English.

  10. Conclusion • Locking would have prevented both users from changing this phrase simultaneously. • These sort of problems would be even harder to detect in code than in English, so we need to consider this

  11. Sources • C.A. Ellis: Concurrency Control in Groupware Systems • http://portal.acm.org/citation.cfm?id=66963&coll=GUIDE&dl=ACM&CFID=56244673&CFTOKEN=48388425 • Gordon V. Cormack: A Counterexample to the Distributed Operational Transform and a Corrected Algorithm for Point-to-point Communication • http://www.cs.uwaterloo.ca/research/tr/1995/08/dopt.pdf • Chengzheng Sun: Optional Locking Integrated with Operational Transformation in Distributed Real-Time-Group Editors • http://portal.acm.org/citation.cfm?id=301322&coll=portal&dl=ACM&CFID=56244468&CFTOKEN=72761822&ret=1#Fulltext

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