P2P Logging and Timestamping for Reconciliation
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P2P Logging and Timestamping for Reconciliation M. Tlili, W. Dedzoe, E. Pacitti, R. Akbarinia, P. Valduriez, P. Molli, G. Canals, S. Laurière. VLDB Auckland, New Zealand 2008. Outline. Motivations XWiki P2P Architecture P2P-LTR service Demonstration Conclusion. XWiki Application. XWIKI.

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Vldb auckland new zealand 2008

P2P Logging and Timestamping for ReconciliationM. Tlili, W. Dedzoe, E. Pacitti, R. Akbarinia, P. Valduriez, P. Molli, G. Canals, S. Laurière

VLDB Auckland, New Zealand 2008


  • Motivations

  • XWiki P2P Architecture

  • P2P-LTR service

  • Demonstration

  • Conclusion

Xwiki application
XWiki Application







  • Enterprise Wiki system in OSS (LGPL) from Xpertnet, Paris

    • Collaborative text editing among multiple users

    • Client-server

    • Wiki-page updating (last save wins)

XWiki Client server architecture


  • Problem: client-server architecture

    • Single point of failure

    • Hardly scalable

    • Low mobility

    • Etc.

  • Solution: pure P2P architecture

    • No central server

      • Scalable to millions of users

    • Multi-Master replication

    • Challenge: ensuring data consistency

      • Ensure convergence of copies

      • Support asynchronous updates, possibly disconnected

Xwiki p2p application
XWiki P2P Application

  • Optimistic replication

    • Asynchronous reconciliation

    • Yields eventual consistency

  • Use of a DHT

    • Efficient data location through put and get primitives

    • Scalability

    • Fault Tolerance through the DHT stabilization layer

    • Handles Dynamicity (mobility)

    • Simplify the construction of large-scale distributed applications

  • Minimal change to XWiki code base

    • All data remain in XWiki peers

    • Only action logs (called patchs) are published and applied by XWiki

Xwiki p2p replication
XWiki P2P Replication

  • We propose a new mechanism of optimistic replication

    • An algorithm for data reconciliation (So61,2)

      • Based on operational transformation approach (OT)

    • P2P-LTR, an extension of KTS3,4 service

      • Distributed continues timestamp generation

      • Publishing operations (patchs) in DHT network

1 Ecoo Team-Project, LORIA, http://dev.libresource.org

2 P. Molli, G. Oster, H. Skaf-Molli, A. Imine. Using the transformational approach to build a safe and generic

data synchronizer. ACM SIGGROUP Conference on Supporting Group Work, (GROUP),2003.

3 R. Akbarinia, E. Pacitti, P. Valduriez. Data Currency in replicated DHTs, SIGMOD2007.

4 M. Tlili, W. K. Dedzoe, E. Pacitti, R. Akbarinia P. Valduriez, et al. P2P Logging and Timestamping for

Reconciliation, Accepted demo paper in Int. Conf. on Very Large Databases (VLDB),2008.

Vldb auckland new zealand 2008

  • A configuration manager

    • Based on OT model

  • Uses a central timestamper

    • To totally order the operations

    • Disadvantages

      • Limited scalability

      • Bottleneck

      • Etc.

  • Solution : P2P-LTR

    • Generate timestamps

    • In a fully distributed fashion

Xwiki p2p architecture
Xwiki P2P Architecture

XWiki Application

XWiki Application

XWiki Doc

XWiki Doc

XWiki Doc

XWiki Application



P2P Network

Patch Service




ReconcileEngine (So6)




DHT (OpenChord)

Xwiki p2p services
XWiki P2P services

  • Patch Manager

    • Interfaces the XWiki application with replication Service

    • Generates operations (Patch)

  • Replication Service

    • Performs reconciliation using OT

      • By retrieving consistent patches from the DHT

    • Publishes consistent patches using P2P-LTR

  • P2P-LTR

    • Generates timestamps in integer serial order

    • Stores and replicates timestamped patches in the DHT

  • DHT

    • Implements basic DHT primitives, based on Chord

    • Network must be reliable with known bounds to assure consistency of routing tables1,2

1 P.Linga et. al. Guaranteeing Correctness and Availability in P2P Range Indices. SIGMOD 2005

2 M. Bawa, A. Gionis, H. Garcia-Molina, R. Motwani: The Price of Validity in Dynamic Networks, SIGMOD 2004

P2p ltr for p2p editing
P2P-LTR for P2P editing

Each XWiki document is identified by a key

Xwiki peer: application

Master-key: generates timestamps and publishes patches in log peers


Master-key succ: replaces Master-key

after crash

Log peer: stores timestamped patches

for a key




DHT = OpenChord

Reconciliation with p2p ltr
Reconciliation with P2P-LTR

Xwiki-Client: Post (key, patch, ts)

(3): get missing patches

and apply locally

Reconc. Engine

n Log-Peers and its successors


(1): put(hts(key), (patch+ts))

(4): put(hts(key), (patch,Last-ts))

(5): Put(h1(key,Last-ts),patch)

(2): ack / Last-ts

(5): Put(h2(key,Last-ts),patch)


Master of Key

(5): Put(succ,last-ts)

(5): Put(h3(key,Last-ts),patch)



DHT management

XWiki document management

Patch management

Patch & Timestamp informations

Network Monitoring

LTR & DHT configurations


  • XWiki P2P Architecture

    • Multi-Master replication

    • Eventual consistency

  • P2P-LTR

    • Continuous timestamp generation

    • Patch replication in DHT network

  • Demonstration

    • Concurrent document modification and patch publishing

Thank you
Thank you

  • Questions?