Distributed computing


WHY DOES DISTANCE DECREASE?



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distcomp

WHY DOES DISTANCE DECREASE?

  • Well, when you readjust the mean, it decreases for that set.
  • When you reassign, every distance gets smaller still.
  • So every step readjusts the total distance.
  • How do we do this for a distributed asynchronous system?
  • What if you have rounds?
  • What if you don’t?

SETI-at-Home Style Projects?

  • SETI stands for search for extra-terrestrial intelligence. It consists of testing radio signal receptions for some regularity.
  • Ideal distributed system project: master sends out work. Servers do work.
  • Servers may crash. What to do? Work queue trick: send work description to server and put job at bottom of queue. If server completes, then remove work description from queue.
  • Servers may be dishonest. What to do?

BROADCAST PROTOCOLS

  • Often it is important to send a message to a group of processes in an all-or-nothing manner. That is, either all non-failing processes should receive the message or none should.
  • This is called atomic broadcast
  • Assumptions:
    • fail-stop processes
    • messages are received from one process to another in the order they are sent

ATOMIC (UNORDERED) BROADCAST PROTOCOL

  • Application: Update all copies of a replicated data item
  • Initiator: Send message m to all destination processes
  • Destination process: When receiving m for the first time, send it to all other destinations
  • Initiator
  • Initiator

Fault-Tolerant Broadcasts

  • Reference: “A Modular Approach to Fault-Tolerant Broadcasts and Related Problems” Vassos Hadzilacos and Sam Toueg.
  • Describes reliable broadcast, FIFO broadcast, causal broadcast and ordered broadcast.

Stronger Broadcasts

  • FIFO broadcast: Reliable broadcast that guarantees that messages broadcast by the same sender are received in the order they were broadcast.
  • A bit more precise: If a process broadcasts a message m before it broadcasts a message m’, then no correct process accepts m’ unless it has previously accepted m. (Might buffer a message before accepting it.)

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