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ACM Group 2005 Conference
Consistency Maintenance Based on the Mark
& Retrace Technique in Groupware Systems
Ning Gu, Jiang-Ming Yang and Qi-Wei ZhangFudan University
Jiang-Ming Yang
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 2
OUTLINE
Introduction & Motivation
Address Space Transformation
Mark & Retrace Technique
Efficiency Analysis
Undo Supported
Related Works & Conclusions
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 3
OUTLINE
Introduction & Motivation
Address Space Transformation
Mark & Retrace Technique
Efficiency Analysis
Undo Supported
Related Works & Conclusions
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 4
INTRODUCTION What’s Group Editors?
Most commonly used groupware applications. Multiple users’ participationUser can edit and modify shared data objects,
such as word, graphics, and media documents, using a computer network.
Techniques for building group editors have been widely applied and practiced.
A Powerful research vehicles in exploring solution to consistency problems.
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 5
MOTIVATION & CHALLENGE In the past decade, OT method has been
gradually improved.
Control AlgorithmsOT Functions
An OT algorithm which can maintains the CCI model and solved TP2 puzzle and false-tie (ERV) puzzle will be very intricate.
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 6
OUTLINE
Introduction & Motivation
Address Space Transformation
Mark & Retrace Technique
Efficiency Analysis
Undo Supported
Related Works & Conclusions
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 7
ADDRESS SPACE TRANSFORMATION
Example 1:
Initial document is “abd”.
O4 and O5 are two concurrent operations.
In OT Technique, due to the existence of concurrent operation O4, O5 shall be transformed to Insert [“c”, 2] before execution on site U1.
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 8
ADDRESS SPACE TRANSFORMATION
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 9
OUTLINE
Introduction & Motivation
Address Space Transformation
Mark & Retrace Technique
Efficiency Analysis
Undo Supported
Related Works & Conclusions
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 10
THE STRUCTURE OF MARK & RETRACE A document is a liner
structure composed of characters.
Each character may have several operations targeting itself while each operation targets only one character.
Add the effective / ineffective mark information to every node in the liner structure.
The address space can be expressed as the effective character nodes from left to right.
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 11
THE RETRACING PROCEDURE The goal of the Retracing procedure is to retrace
the address space to the state at a given timestamp SV.
Consider the following four situations. if SVOins ≥ SV , then this node is marked ineffective;
if SVOins ≤ SV and there is no Delete targeting this character node, then this node is marked effective;
if SVOins ≤ SV and SVOdel ≥ SV , then this node is marked effective;
if SVOins ≤ SV but SVOdel ≤ SV , then this node is marked ineffective.
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 12
THE CONTROL ALGORITHM Control-Algorithm (DocS, O), execute O on
DocS.
Retracing(DocS, SVO)
Execute the operation O, and attach the operation with its timestamp to the character node.
SVS[R] ⇐ SVS[R] + 1
Retracing(DocS, SVS)
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 13
EXAMPLE 1: Effective node & ineffective
node
User U1 intends to execute operation Insert[“c”, 3] with timestamp <1,1,2>.
User’s view is “ad”.
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 14
EXAMPLE 1: First, retrace the
document to the state at the time of timestamp <1,1,2>.
Conceal the effect of operation Delete[“b”, 2].
Character node “b” become “Effective”.
User’s view is “abd”.
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 15
EXAMPLE 1: The document’s state at the time of timestamp
<1,1,2> is the state at the time of operation’s ( Insert [“c”, 3] ) generation.
Operation Insert [“c”, 3] can be executed directly in this new address space.
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 16
EXAMPLE 1: The State Vector of the local site has been adjusted to
<2,1,2> after the execution is finished.
And then it retraces to the view “acd” of new State Vector by restoring the effect of operation Delete[“b”, 2] and adding the execution effect of the new operation Insert[“c”, 3].
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 17
OPERATION EXECUTION The Delete operation’s execution is simple. The process is to find the
corresponding character node, attach the operation on the character node and then perform the Retracing procedure to give this node a correct effective/ineffective mark.
The Insert is to search for an Inserting position in the address space, which is corresponding to a range in document between two effective character nodes. There may exist some other ineffective character nodes in this range.
A Range-Scan process to determine the right-and-left relation between the new Insert and these ineffective character nodes and then find an accurate position.
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 18
OPERATION EXECUTION To realize the comparability between two Insert
operations, define a TOrder function:
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.
TOrder(CNa) < TOrder(CNb), iff
sum(SVa) < sum(SVb)
a < b, when sum(SVa) = sum(SVb)
Where sum(SV) =
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 19
OPERATION EXECUTION INSERT RULES:
1) The new character node should be inserted in the range.
2) If there is a existed character node CN, the insert operation of CN is causally before CNnew, CNnew should be inserted on the left of CN.
3) If there is a existed character node CN1, TOrder(CNnew)<TOrder(CN1) and CN1 do not meets the rule 4, CNnew should be insert on the left of CN1.
4) Otherwise, if there is two existed character node CN1 and CN2, CN2 is on the right of CN1, CNnew should be inserted on the right of CN2 when :
TOrder(CNnew)<TOrder(CN1)
TOrder(CNnew)>TOrder(CN2)
CN2 is causally before CN1
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 20
OPERATION EXECUTION INSERT EXECUTION:
Scan the range from left to right. If there is a existed character node meets the rule 2,
return this position immediately. If there is a existed character node meets the rule 3,
record this position into a variable P. If the record meets the rule 4, clean the record.
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 21
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.
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 22
CONVERGENCE
The Proof:
2 operations
Same Scan-Ranges Different Scan-Ranges
n operations
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 23
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 intention’s formal definition and the complete proof in the next future.
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 24
OUTLINE
Introduction & Motivation
Address Space Transformation
Mark & Retrace Technique
Efficiency Analysis
Undo Supported
Related Works & Conclusions
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 25
OPERATION EXECUTION EFFICIENCY Reorganize the character nodes to a Red-Black Tree and
ensure the result of pre-order traversal of the Black-Red tree is the original linear order.
Every node of the tree saves two additional data —the two numbers of the effective character nodes of left sub-tree and right sub-tree.
The amortized costs of Insert and Delete are both O(logn).
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 26
EFFICIENCY OPTIMIZATION OF RETRACING Use the “Garbage Collection” method to handle a
operation list containing the operations which haven’t been executed at some sites.
Suppose the length of operation list is h which means we need to check and change at most h character nodes’ effective/ineffective mark during Retracting process.
In the second Retracing process, we restore the changed nodes instead of the Retracing process. After then, we only need to deal with the targeting character node of the new operation.
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 27
OUTLINE
Introduction & Motivation
Address Space Transformation
Mark & Retrace Technique
Efficiency Analysis
Undo Supported
Related Works & Conclusions
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 28
UNDO SUPPORTED Given an Undo operation O2, Undo[O1], in which O1’s
target position is P1 and timestamped by SVO1. The execution process can be performed as follow.
Retrace the address space to the state at the time SVO1; count to the P1th effective character node and attach O2
on it. Then the second Retracing process takes this new Undo
into consideration and gives this node the correct effective / ineffective mark.
Append a Delete counter to every character node to realize the Multi-version / Single-display strategy.
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 29
OUTLINE
Introduction & Motivation
Address Space Transformation
Mark & Retrace Technique
Efficiency Analysis
Undo Supported
Related Works & Conclusions
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 30
RELATED WORKS The initial concept of Mark & Retrace was proposed early in
Document Mark Technique but was not able to solve the dOPT puzzle.
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.
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 31
CONCLUSIONS In this paper we propose a strategy which can
maintain CCI model and is different from OT. It does not ensure the operation’s execution correctness by modifying the operation’s address but by transforming the address space of the document.
Since the Mark & Retrace processes do not affect the relative position (left or right) between characters, it is easier to maintain operations’ order. It can not only achieve all the consistency criteria, but can support any Undo as well.
ACM Group 2005 Conference
April 10, 2023 Fudan University, Cooperative Information and Systems Laboratory 32
Thanks!