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					                              Recap.
• Ubiquitous Computing Vision
  – The Computer for the Twenty-First Century, Mark Weiser
  – The Coming Age Of Calm Technology, Mark Weiser and
    John Seely Brown
  – People, Places, Things: Web Presence for the Real World
    Tim Kindberg, John Barton, Jeff Morgan, Gene Becker, Ilja
    Bedner, Debbie Caswell, Phillipe Debaty, Gita Gopal,
    Marcos Frid, Venky Krishnan, Howard Morris, Celine
    Pering, John Schettino, Bill Serra.
  – Next Century Challenges: Data-Centric Networking for
    Invisible Computing. Mike Esler, Jeffrey Hightower, Tom
    Anderson, and Gaetano Borriello


   Jan 31, 2001   CSCI {4,6}900: Ubiquitous Computing   1
                              Recap
• Distributed Systems Architecture
  – Intro. to Distributed system architecture (Domain Name
    Service (DNS), Gnutella, DNS round robin etc.)
  – Oceanstore: An Extremely Wide-Area Storage System
    David Bindel, Yan Chen, Patrick Eaton, Dennis Geels,
    Ramakrishna Gummadi, Sean Rhea, Haim
    Weatherspoon, Westley Weimer, Christopher Wells, Ben
    Zhao, and John Kubiatowicz
  – Feasibility of a Serverless Distributed File System
    Deployed on an Existing Set of Desktop PCs William J.
    Bolosky, John R. Douceur, David Ely, and Marvin Theimer



   Jan 31, 2001   CSCI {4,6}900: Ubiquitous Computing   2
                             Recap
• Location and Naming management
  – The Anatomy of a Context-Aware Application Andy Harter,
    Andy Hopper, Pete Steggles, Andy Ward, Paul Webster
  – Active Names: Flexible Location and Transport of Wide-
    Area Resources Amin Vahdat, Michael Dahlin, Thomas
    Anderson, and Amit Aggarwal




  Jan 31, 2001   CSCI {4,6}900: Ubiquitous Computing   3
                          Outline
1. The Dangers of Replication and a Solution, Jim Gray,
   Pat Helland, Patrick O'Neil, and Dennis Shasha. In
   Proceedings of the ACM SIGMOD international
   conference on Management of data, 1996




Jan 31, 2001   CSCI {4,6}900: Ubiquitous Computing    4
                    Replication – Intro.




        ….



• As systems grow, need to scale up


   Jan 31, 2001   CSCI {4,6}900: Ubiquitous Computing   5
                            Scale Up




        ….




• You can scale up by buying a bigger machine

   Jan 31, 2001   CSCI {4,6}900: Ubiquitous Computing   6
                            Partition




        ….




• You can scale up by partitioning the machines (e.g. service
  users in east coast from Atlanta and west coast from L.A.)
   Jan 31, 2001   CSCI {4,6}900: Ubiquitous Computing     7
                          Replication




        ….




• You can replicate
data
   Jan 31, 2001   CSCI {4,6}900: Ubiquitous Computing   8
                      Serializability – Intro.
             Node 1                   Node 2               Node 3
        X=1000                                           X=?
 time




                                X=200

         X=100
                                                         X=?



• What is the value of X in node 3?
• Causal ordering (Update x when you hear from
  Node 1 or Node 2)
    Jan 31, 2001   CSCI {4,6}900: Ubiquitous Computing         9
                      Serializability – Intro.
             Node 1                   Node 2                Node 3
        X=1000                                            X=?
 time




                                X=200

         X=100
                                                         X=200



• What is the value of X in node 3?
• Causal ordering (Update x when you hear from
  Node 1 or Node 2)
    Jan 31, 2001   CSCI {4,6}900: Ubiquitous Computing           10
                  Goals of replication
• Availability and scaleability
   Provide high availability and scaleability through replication


• Mobility
   Allow mobile nodes to read and update the database while
     disconnected from the network


• Serializability
   Provide single-copy serializable transaction execution

• Convergence
   Provide convergence to avoid system delusion

   Jan 31, 2001   CSCI {4,6}900: Ubiquitous Computing       11
                      Eager Replication

                           R                  R
           R




                                                         time
               R               R                  R

• All replicas synchronized to the same value
  immediately



   Jan 31, 2001    CSCI {4,6}900: Ubiquitous Computing          12
                      Eager Replication

                           R                  R
           R




                                                         time
               R               R                  R

• All replicas synchronized to the same value
• Lower update performance and response time


   Jan 31, 2001    CSCI {4,6}900: Ubiquitous Computing          13
                       Lazy Replication

                           R                  R
           R




                                                         time
               R               R                  R

• One replica is updated by the transaction
• Replicas synchronize asynchronously
• Multiple versions of data

   Jan 31, 2001    CSCI {4,6}900: Ubiquitous Computing          14
                  Single node Transaction

                       Checking –1000
                       Savings +500
                       CD       +500
                       Commit

• No conflicts




   Jan 31, 2001   CSCI {4,6}900: Ubiquitous Computing   15
                         Eager Transaction

   Checking –1000
                          Checking –1000
                                                Checking –1000
   Savings +500
                          Savings +500
                                                Savings +500
   CD             +500
                          CD          +500
                                                CD        +500
   Commit
                          Commit
                                                Commit
N nodes – N times as much work
   Jan 31, 2001     CSCI {4,6}900: Ubiquitous Computing          16
                      Lazy Transaction

   Checking –1000
   Savings +500
   CD       +500
   Commit                                      Checking –1000
                                               Savings +500
                       Checking –1000          CD       +500
                       Savings +500            Commit
                       CD       +500
                       Commit


• N nodes – N times as much work
• N transactions
   Jan 31, 2001   CSCI {4,6}900: Ubiquitous Computing           17
     Concurrency anomaly in Lazy Replication

                           R’                R```
           R




                                                         time
               R              R``                R`

• R` - Which version of data should it see?
• If committed transaction is ‘wrong’, conflict
• Conflicts have to be reconciled

   Jan 31, 2001    CSCI {4,6}900: Ubiquitous Computing          18
                         Scaleup pitfall

                           R’                R```
           R




                                                         time
               R              R``                R`

• When the nodes divulge hopelessly
• System delusion – database is inconsistent and no
  obvious way to repair it

   Jan 31, 2001    CSCI {4,6}900: Ubiquitous Computing          19
                  Regulate replica updates
• Group: Any node with a copy can update item
  – Update anywhere


• Master: Only a master can update the primary copy.
  All replicas are read-only. All update requests are
  sent to the master




   Jan 31, 2001    CSCI {4,6}900: Ubiquitous Computing   20
                  Replication strategies
   Propagation                    Lazy                    Eager
       Vs.
    Ownership

Group                  N transactions               1 transaction
                       N object owners              N object owners

Master                 N transactions               1 transaction
                       1 object owner               1 object owner

Two tier               N+1 transactions, 1 object owner
                       Tentative locate update, eager base update


   Jan 31, 2001   CSCI {4,6}900: Ubiquitous Computing           21

				
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Description: Pervasive computing, also known as general storage computing, pervasive computing (pervasive computing or Ubiquitous computing) and emphasized the concept of integrated computing environment, the computer itself, disappeared from sight. In the ubiquitous computing model, people can at any time, any place, any way for information access and processing.