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GeoQuorums

GeoQuorums. Implementing Atomic Memory in Mobile Ad Hoc Networks Seth Gilbert Shlomi Dolev, Nancy Lynch, Alex Shvartsman, Jennifer Welch DISC – October 3 rd 2003. Distributed Shared Memory. Read. Write(7). Write(0). Mobile Ad Hoc Networks. No fixed infrastructure e.g. ZebraNet

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GeoQuorums

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  1. GeoQuorums Implementing Atomic Memory in Mobile Ad Hoc Networks Seth Gilbert Shlomi Dolev, Nancy Lynch, Alex Shvartsman, Jennifer Welch DISC – October 3rd 2003

  2. Distributed Shared Memory Read Write(7) Write(0)

  3. Mobile Ad Hoc Networks • No fixed infrastructure • e.g. ZebraNet • GeoSensor • GPS • Synchronized Clock

  4. GeoCast • send(message, location) • See: • Navas and Imielinski (97) • Camp and Liu (2002)

  5. Read 7 Quorum Based Algorithms • See (for example): • Gifford (79) and Thomas (79) • Upfal and Wigderson (85) • Attiya, Bar-Noy, and Dolev (90/95) Write(7)

  6. Problems • Static • Potentially unbounded and dynamic universe of nodes? • Mobility • How do you find the nodes? • Location • What if the members of a quorum become dispersed?

  7. motion reconfiguration Reconfiguration?? • See (for example): • RAMBO (DISC 2002); RAMBO II (Lynch and Shvartsman; Gilbert, Lynch, and Shvartsman)

  8. Problems • Static • Potentially unbounded and dynamic universe of nodes? • Mobility • How do you find the nodes? • Location • What if the nodes are far apart? • Reconfiguration

  9. Focal Points • busy traffic intersection • water source • cafeteria • lounge

  10. Problems • Static • Potentially unbounded and dynamic universe of nodes • Mobility • How do you find the nodes? • Location • What if the nodes are far apart? • Reconfiguration

  11. New Problem • At most f focal points become empty • How do we respond to failed focal points?

  12. Replication read

  13. GeoQuorums Approach • Low Level: Focal Point Emulator • Implement focal points • Simulate data objects at fixed locations • High Level: Operation Manager • Use fixed data objects to implement atomic memory

  14. Focal Point Emulator • Operations: • put • updates tag/value • get • retrieves tag/value • confirm • confirms a write is complete • recon-done • reconfiguration is complete

  15. Focal Point Emulator • Participating mobile nodes are close together • Assume: • Local Totally Ordered Broadcast • Replicated State Machine • Tolerating joins/leaves

  16. Operation Manager • Write • Choose unique tag • use clock, pid • put tag/value to quorum • Read • get latest tag/value from quorum • (put tag/value to quorum) mobile host focal point data object

  17. Reconfiguration • Switch quorums of focal points • Limited to predetermined configurations • Primarily for performance • Also for fault tolerance • Simple • Two phase operation • get value from old configuration • put value in new configuration

  18. Geographic Quorums focalpoint cluster mobile host

  19. Safety and Performance • At most f focal points “fail” • Arbitrary message delays • We prove that GeoQuorums implements linearizable read/write objects • Good latency in partially synchronous executions

  20. Conclusions • Introduce a new algorithm for atomic memory in mobile, ad hoc networks • Develop notion of focal points • Bridge gap between static and mobile • Use static algorithms in mobile ad hoc setting

  21. The End (Questions?)

  22. Backup Slides

  23. GeoCast Service geo-send geo-rcv geo-rcv geo-send Focal Point Emulator Mobile Host Operation Manager GeoSensor read-ack write-ack recon-ack read/write recon lbcast-send lbcast-rcv LBCast Service Client

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