13- consensus1

13- consensus1 - CSE 486/586 Distributed Systems Consensus...

Info iconThis preview shows pages 1–8. Sign up to view the full content.

View Full Document Right Arrow Icon
CSE 486/586, Spring 2012 CSE 486/586 Distributed Systems Consensus --- 1 Steve Ko Computer Sciences and Engineering University at Buffalo
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
CSE 486/586, Spring 2012 Recap: Reliable and Ordered Multicast How do a group of processes communicate? R-Multicast Properties: integrity, agreement, validity Ordering FIFO Total Causal Ordered multicast algorithms FIFO order: maintains a per-process clock similar to a vector clock Total order: sequencer-assigned, ISIS (sender-assigned) Causal order: uses a vector clock 2
Background image of page 2
CSE 486/586, Spring 2012 Let’s Consider This… 3
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
CSE 486/586, Spring 2012 One Reason: Impossibility of Consensus Q: should Steve give an A to everybody taking CSE 486/586? Input: everyone says either yes/no. Output: an agreement of yes or no. Bad news Asynchronous systems cannot guarantee that they will reach consensus even with one faulty process. Many consensus problems Reliable, totally-ordered multicast (what we saw already) Mutual exclusion, leader election, etc. (what we will see) Cannot reach consensus. 4
Background image of page 4
CSE 486/586, Spring 2012 The Consensus Problem N processes Each process p has input variable xp : initially either 0 or 1 output variable yp : initially b (b=undecided) – can be changed only once Consensus problem: design a protocol so that either all non-faulty processes set their output variables to 0 Or all non-faulty processes set their output variables to 1 There is at least one initial state that leads to each outcomes 1 and 2 above 5
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
CSE 486/586, Spring 2012 Assumptions (System Model) Processes fail only by crash-stop ping Synchronous system: bounds on Message delays Max time for each process step e.g., multiprocessor (common clock across processors) Asynchronous system: no such bounds E.g., the Internet 6
Background image of page 6
CSE 486/586, Spring 2012 First: Synchronous Systems Every process starts with
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 8
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 21

13- consensus1 - CSE 486/586 Distributed Systems Consensus...

This preview shows document pages 1 - 8. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online