The Multiversion and Singledisplay Strategy in Undo Scheme

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The Multi-version and Single-display Strategy in
Undo Scheme

• Jiangming Yang
• Department of Computing and Information
  TechnologyFudan University220 Handan Rd,
  Shanghai 200433, P.R.China86 21 55664465

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CONTENT OUTLINE

• Introduction Motivation
• A generic Mark Retrace algorithm
• Achieve CCI model
• Intention violation puzzle
• Related works
• Conclusions

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CONTENT OUTLINE

• Introduction Motivation
• A generic Mark Retrace algorithm
• Achieve CCI model
• Intention violation puzzle
• Related works
• Conclusions

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INTRODUCTION

• Whats Group Editors?
• Group editors are one of the most commonly used
  groupware applications ?. It allows multiple
  users to participate in the editing and
  modification of shared data objects, such as
  word, graphics, and media documents, using a
  computer network.

?C. A. Ellis and S. J. Gibbs. Concurrency control
in groupware systems. In Proceedings of the ACM
SIGMOD 1989, pages 399407, Portland Oregon, May
1989.
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INTRODUCTION

• Whats Group Editors?
• Techniques for building group editors have been
  widely applied and practiced. Group editors have
  also been acknowledged as powerful research
  vehicles in exploring solution to consistency
  problems.

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INTRODUCTION

• Whats undo operation?
• Undo is a common operation ?, which can help user
  to quickly recover from the incorrect execution
  in document editor system and make the work more
  safe and efficient.
• Why we need undo operation in group editors?
• The effect of incorrect execution will be more
  extensive for it is done in distributed group
  editors, so the user will have more dependence on
  Undo

?G.D. Abowd and A.J. Dix. Giving undo attention.
Interacting with Computers, 4(3)317-342, 1992.
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MOTIVATION CHALLENGE

• Intention Maintenance
• The introduction of undo operation makes it more
  necessary to do the intention maintenance in
  group editors.
• Other undo puzzles
• Executing undo operation together with insert
  and delete will leads to some new puzzles.

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INTENTION MAINTENANCE

• Intention maintenance is becoming more urgent and
  harder to avoid and it especially concentrates on
  the concurrency intention-violation.
• Illustrated as figure, user U1 wants to delete
  a while user U2 want to reserve it. We cant
  execute all the operations for they are violated.
• We have to analyze the violated operations,
  separate them into two or more operation lists
  without internal violation and select a suitable
  one to execute.

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CONTENT OUTLINE

• Introduction Motivation
• A generic Mark Retrace algorithm
• Achieve CCI model
• Intention violation puzzle
• Related works
• Conclusions

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ADDRESS SPACE TRANSFORMATION

• The operation's intention is achieved by
  transforming the address space, rather than the
  operation itself.
• We shall re-trace the document's address space to
  the state at the time of operations generation.
  In this way, the operation can be executed
  directly in this address space.

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A Mark Retrace FRAMEWORK

• Any document is a liner structure composed by
  characters. Without loss of generality, assume
  each character is indivisible and each operation
  only aims at one character.
• We save the operation together with its
  timestamping to the characters liner node.
• we have also added the visible/invisible mark
  information on every liner structure of document,
  which shows whether the current node is visible
  or invisible.

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RETRACING PROCEDURE

• Scan all character nodes
• Set Counter0
• If a operation is effective
• Counter, when operation wants the node to be
  ineffective.
• Counter--, when operation wants the node to be
  effective.
• If the insert operation is effective and
  counter0, set node effective, otherwise set node
  ineffective.

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RETRACING PROCEDURE

• Delete and Undo operation toward Insert mean
  want the node to be ineffective
• Undo operation toward Delete means want the
  node to be effective.

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CONTROL ALGORITHM

• The definition of operation position is obtained
  by counting the number of effective character
  nodes on the left side of linear structure at the
  time of its generation. When the order of any two
  character nodes is certain, Retracing(DocS, SVO)
  ensures that the address space (effective
  character nodes) of the current document's liner
  structure is the same as the one at the time of
  the operation's generation.

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RANGE-SCAN

• Given two character nodes CNa and CNb. The
  attached operations are generated at site a and
  site b, and are timestamped by SVa and SVb.
  Consider
• We have TOrder(CNa) lt TOrder(CNb), iff
• sum(SVa) lt sum(SVb)
• a lt b when sum(SVa) sum(SVb)

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RANGE-SCAN

• Scan all ineffective character nodes in the
  insert range and save the insert position in
  variable P.
• When P is empty, a operation is causality before
  the new operation, return this position.
• When P is empty, a operation is concurrent with
  the new operation and the TOrder is bigger, save
  it in P.
• When a operation is causality before P and the
  TOrder is bigger then the new operation, empty
  the P.

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CONTENT OUTLINE

• Introduction Motivation
• A generic Mark Retrace algorithm
• Achieve CCI model
• Intention violation puzzle
• Related works
• Conclusions

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CAUSALITY PRESERVATION

• To achieve causality preservation, we only need
  to ensure that every remote operation is
  causally-ready before its execution, which we
  call execution condition.
• It should be noted that even if causality is
  preserved, the execution order of the operations
  would still be different because of their
  different orders of arrival at different sites.

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CONVERGENCE

• The Proof ?
• 2 operations
• Same Scan-Ranges
• Different Scan-Ranges
• n operations

?N. Gu, J. Yang, and Q. Zhang. Consistency
maintenance based on the mark retrace technique
in groupware systems. ACM GROUP 2005 Conference,
Sanibel Island, USA, Nov 2005.
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INTENTION PRESERVATION

• Since a Insert operation can find the correct
  scan-range and a Delete operation can find its
  corresponding character node at any sites and
  they do not interfere with other operations,
  those satisfy operations definition.
• However, there are still some arguments about the
  proof of achieving intention preservation, we
  plan to present more about intentions formal
  definition and the complete proof in the next
  future.

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CONTENT OUTLINE

• Introduction Motivation
• A generic Mark Retrace algorithm
• Achieve CCI model
• Intention violation puzzle
• Related works
• Conclusions

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INTENTION VIOLATION PUZZLE

• When there are several Delete operations in the
  algorithm, the character node is effective only
  when all the Delete operations are undone,
  otherwise it will keep ineffective.
• When there exists intention violation, the
  algorithm maintains the intention of users who
  want character node to be ineffective

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INTENTION VIOLATION PUZZLE

• Separate Process
• Scan operations list toward two violation
  operations.
• Two operations set R1 and R2. operations
  causality before O1 saved in R1 operations
  causality befor O2 saved in R2 operations
  concurrent with O1 and O2 saved in both R1 and R2.

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CONTENT OUTLINE

• Introduction Motivation
• A generic Mark Retrace algorithm
• Achieve CCI model
• Intention violation puzzle
• Related works
• Conclusions

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RELATED WORKS

• The initial concept of Mark Retrace was
  proposed early in ?? ? but was not able to solve
  the dOPT puzzle.

?X. Wu and N. GU. A concurrency control method
based on document marking. Journal of Computer
Research and Development, 39(12)16621667,
2002. ?H. He, Q. Wu, and L. Luo. Document
marking scheme for preserving intention of
operation in cooperative editing system. Journal
of Software, 10(2)160164, Feb 1999.
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RELATED WORKS

• dOPT, adOPTed and GOTO cannot fully satisfy TP2.
• NICE and SOCT3/4 is not able to resolve the
  false-tie (ERV) puzzle.
• GOT fail in the case of complicated false-tie
  (ERV) scenarios.
• Imine et al. tried to support TP2 by expanding IT
  function but fails in solving dOPT puzzle.
• A solution of false-tie (ERV) has been realized
  in Co-Word system, it is still not published.
• Li et al. also reported a solution and gave the
  proof.

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RELATED WORKS

• Only REDUCE can maintain convergence in the
  intention violation and obtain the effect of
  Multi-version and Single-display strategy, but it
  lacks related examining method and enough hints
  to users.

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CONTENT OUTLINE

• Introduction Motivation
• A generic Mark Retrace algorithm
• Achieve CCI model
• Intention violation puzzle
• Related works
• Conclusions

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CONCLUTIONS

• In this paper we propose a strategy which can
  maintain CCI model in fully replicated
  architecture, which is different from OT. It does
  not ensure the operations execution correctness
  by modifying the operation but by Retracing
  process.
• Mark Retrace can not only achieve CCI model,
  but can support any undo in replicated
  architecture as well.

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Thank You!