Assignment 3 – Paxos
To gain an understanding of consensus and voting protocols in the presence of failures of one or
more of the participants.
Welcome to the Suburbs Council Election!
This year, Suburbs Council is holding elections for council president. Any member of its nine person
council is eligible to become council president.
Member M1 – M1 has wanted to be council president for a very long time. M1 is very chatty over
social media and responds to emails/texts/calls almost instantly. It is as if M1 has an in-brain
connection with their mobile phone!
Member M2 – M2 has also wanted to be council president for a very long time, except their very long
time is longer than everybody else’s. M2 lives in a remote part of the Suburbs and thus their internet
connection is really poor, almost non-existent. Responses to emails come in very late, and
sometimes only to one of the emails in the email thread, so it is unclear whether M2 has
read/understood them all. However, M2 sometimes likes to work at Café @ Bottom of the Hill. When
that happens, their responses are instant and M2 replies to all emails.
Member M3 – M3 has also wanted to be council president. M3 is not as responsive as M1, nor as
late as M2, however sometimes emails completely do not get to M3. The other councilors suspect
that it’s because sometimes M3 goes on retreats in the woods at the top of the Suburbs, completely
disconnected from the world.
Members M4-M9 have no particular ambitions about council presidency and no particular
preferences or animosities, so they will try to vote fairly. Their jobs keep them fairly busy and as such
their response times will vary.
How does voting happen: On the day of the vote, one of the councilors will send out an
email/message to all councilors with a proposal for a president. A majority (half+1) is required for
somebody to be elected president.
Write a program that implements a Paxos voting protocol for Suburbs Council President that is fault
tolerant and resilient to various failure types, some of which are shown in the above. Communication
happens strictly via sockets. You are responsible for the message design.
Your assignment will be marked out of 100 points, as following:
10 points – Paxos implementation works when two councillors send voting proposals at the same
30 points – Paxos implementation works in the case where all M1-M9 have immediate responses
to voting queries
30 points – Paxos implementation works when M1 – M9 have responses to voting queries
suggested by the profiles above, including when M2 or M3 propose and then go offline
20 points – Testing harness for the above scenarios + evidence that they work (in the form of
10 points for the quality of your code:
Code Quality Checklist
o write comments above the header of each of your methods, describing
o what the method is doing, what are its inputs and expected outputs
o describe in the comments any special cases
o create modular code, following cohesion and coupling principles
o use magic numbers
o use comments as structural elements (see video)
o mis-spell your comments
o use incomprehensible variable names
o have long methods (not more than 80 lines)
o allow TODO blocks
10 points – Paxos implementation works with a number ‘n’ of councilors with four profiles of
response times: immediate; medium; late; never
50 points – (you can use these points in this assignment, or in any other subsequent assignment)
– Fast Byzantine Paxos implementation that works when councilors lie, collude, or intentionally do
not participate in some voting queries but participate in others.
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