CSC 290e Mobile Computing

Introduction to Distributed Computing (cont.)
17 September 2001

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Three basic ways to design distributed applications:

message passing - the programmer explicitly uses unicast/multicast to exchange
  information between hosts

remote procedure call - to programmer, it looks like making a normal
  procedure call, but the call is done remotely, and the answer shipped back.
  Java's RMI (Remote Method Invocation) is one example implementation.

shared memory - the programmer sees a large piece of memory which it and other
  processes/hosts have access to.  Lime (my middleware for mobile
  applications) is a shared memory model, and so it Interweave (a middleware
  designed by Michael Scott, Sandhya Dwarkadas and students).

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Some key concepts in distributed systems:

Event ordering
  Happened before A->B
    1. A executed before B in same process
    2. A sendX  B receive X
    3. A->B and B->C implies A->C

  may seem easy, but without synchronized clocks, this is difficult.  

Mutual Exclusion - ensure two processes have proper synchronization when
  accessing critical data or doing critical operations
  A simple solution uses a central coordinator
    - tell coordinator you want into the critical section
    - coordinator grants only one access at a time
    - what if coordinator dies?
    - what if process granted access dies?
  Another alternative is token-based.
    - Enter critical section when hold token
    - if no one wants into CS, still use bandwidth
    - what if holder of token dies?
  Dining Philosophers is distributed, pairwise mutex

Leader Election - elect one process/host to lead
  assume know id of all other hosts  <<<bad for mobile systems
  do not know which are up/down
  want to ensure that all processes elect the same leader
  Bully algorithm: 
    When a process comes up, it starts an election.  When a process finds it's
    leader is dead, it starts an election

    1. P sends ELECTION msg to all hosts with higher id
    2. if no one responds, P wins election and becomes coordinator
    3. If a higher-up answer, it takes over.  P's job is done

    When receive an ELECTION msg from lower, 
      send OK back to indicate alive and will take over

    Eventually all processes give up except one

    Eventually announce victory to all processes

    Processes 4 2 1 5 6 3 7 0
      initially 7 is leader, but 7 dies.
      4 detects 7's death, and starts election
        sends election msgs to 5 6 7
      5 and 6 send back OK.  
      4 realizes it is not the coordinator, but 5 and 6 start election
      5 sends election msgs to 6 and 7
      6 sends ok to 5 and 5 realizes it is not the coordinator
      6 sends election msg to 7
      when 6 does not receive reply from 7, it sends COORDINATOR msgs to all
          processes (except 7)

    Process with highest id always wins...hence "bully".