1. Consensus Algorithms
So let’s talk about consensus algorithms.

2. Administrivia HW1 due tonight!
HW2 out ~tonight (will be due in 2 weeks)
Spoilers: it’s about hashing
MUST be done in pairs
Tonight’s clinic will cover more hashing stuff
Quiz 2 also out today (will be due Friday night)
Covers Thurs lecture on hashing and Tues lecture on consensus algorithms (not today!)
Regular office hours schedule coming soon

3. Proof of Work (cont.)
So let’s talk about consensus algorithms.

4. Proof of Work
Miner

5. Proof of Work Also protection against fraud!
(So long as you don’t have 51% of computation power)
Miner
Evil
Miner

6. Proof of Work Is the Miner incentive actually all that incentivizing?
What’s the expected time for any individual Miner to win the race?
Alice owes Bob 100CC
Bob owes Charlie 80CC
Bob owes Deborah 30CC
Alice owes Charlie 500CC
Miner
Miner receives 50CC
+5CC
+2CC
+3CC

7. Proof of Work: Miner Pools
Alice owes Bob 100CC
SHA-256 ….
Bob owes Charlie 80CC
Bob owes Deborah 30CC
Alice owes Charlie 500CC
Miner
37212

8. Proof of Work: Miner Pools
Miner Pool Coordinator

9. Proof of Work: Miner Pools
Alice
Bob
Alice owes Bob 100CC
Bob owes Charlie 80CC
Bob owes Deborah 30CC
Miner Pool Coordinator
Alice owes Charlie 500CC
Miner
00021
Charlie
Deborah

10. Proof of Work: Miner Pools
Miners do work on behalf of the pool
As long as they can show that they’re helping, they get a cut
Very roughly like tip sharing at a diner
But… is there anything stopping you from being in multiple pools?
Potential for abuse: can broadcast near misses but save actual hits for your “real” pool

11. Paxos Citation: Google Tech Talks video on Paxos
The first consensus algorithm we’re going to look at is Paxos.
Citation: Google Tech Talks video on Paxos

12. Paxos What do we mean by consensus? Consensus is on one value.
Consensus is reached once a majority of participants agree.
Once a consensus is reached, everyone can eventually know the result.
Participants are happy to reach consensus on any result, not just the one they propose.
Communication channels are not perfect (messages may be lost).
So for our purposes, we’re going to be talking about a particular definition of consensus.
In particular, we’re just going to be talking about consensus on a single “thing.” So for instance, this could be a decision amongst you and your friends about what restaurant to go to this evening, but it doesn’t involved a decision about what to order when you get there (although you could do another “round” of the decision making process on that… more on that later).
We’re also not going to worry about uniform consensus (where ALL parties agree) and instead go for a “majority rules” consensus like in an election. Uniform consensus is necessarily harder (and less fault tolerant) since it has stronger requirements on a decision being made. If you’re interested it’s another area of study you can look into, but we’re not going to cover it in this course.
Now, even though we’re sticking with majority consensus, it’s important that eventually all participants can find out what the majority decided.
It’s also important that participants are happy to reach consensus on any value, not necessarily just the one they propose. In other words, no “rage quitting” :)
And lastly (and sort of the point of why this is hard), we have to assume that communication channels are imperfect. This could mean that messages are lost because a connection is bad, or one of the machines goes down, etc.

13. Paxos “Sure, Finding Nemo is great” “What about Mission Impossible”
“Ugh, OK fine”
Bob
Charlie
“Finding Nemooooooooo”
“Let’s see Finding Nemo”
Alice
Debbie
Erica
“OK, I guess it’s Finding Nemo”

14. Paxos “Hey guys, wanna watch the Star Wars Holiday Special” Bob
Charlie
Fred
“NO. We already decided Finding Nemo”
Alice
Debbie
Erica