[video] https://www.youtube.com/watch?v=6Kwx8zfGW0Y In this talk, we will discuss Raft and its TLA+ spec. Raft is a consensus algorithm for managing a replicated log. It produces a result equivalent to (multi-)Paxos. The design of Raft separates key elements of consensus algorithm, such as leader election, log replication, etc.., which results in the Raft more understandable and implementable. Raft has been widely taught and implemented, with a partial list of implementation available at https://raft.github.io/. Paper and Spec (not required, but helpful to take a look before the lecture) - In Search of an Understandable Consensus Algorithm (https://raft.github.io/raft.pdf) - TLA+ specification for the Raft consensus algorithm (https://github.com/ongardie/raft.tla)

4. RAFT Paxos
Concept
Replicated Log for
Replicated State Machine
Consensus Algorithm
Leader Election
Key (First elected leader,
then normal operation)
Optional (A form of
operation)
Log maintenance
Leader’s Log is replicated to
all Followers
Node may log
consensus operation,
but it may have holes
TLA+ Spec
Close to developer’s
implementation & usage
More abstract
State Space Large Small

5. a
b
c
d
e
f
a b c e f g
h i
g

6. unique

7. S1: 1
S2: 1
S3: 1

8. S1: 2
S2: 1
S3: 1

9. S1: 2
S2: 1
S3: 1
RequestVoteRequest

10. S1: 2
S2: 2
S3: 2
RequestVoteRespose

11. S1: 2
S2: 2
S3: 2
1
2
a
2
2
b
3
2
c

12. S1: 2
S2: 2
S3: 2
1
2
a
2
2
b
3
2
c
4
2
d
5
2
e
6
2
f
1
2
a
2
2
b
1
2
a
Committed
Index

13. S1: 2
S2: 2
S3: 2
1
2
a
2
2
b
3
2
c
4
2
d
5
2
e
6
2
f
1
2
a
2
2
b
3
2
c
1
2
a
2
2
b
3
2
c
4
2
d
Committed
Index

15. one server per term

16. persisted

17. AppendEntries
AppendEntries

18. 1
2
a
2
2
b
1
2
a
2
2
b
1
2
a
2
2
a
1
2
a
2
2
b
1
2
a
2
2
b
Committed
Index
Leader S1: 2
Follower S2: 2
Follower S3: 2
Follower S4: 2
Follower S5: 2

19. 1
2
a
2
2
b
1
2
a
2
2
b
1
2
a
2
2
a
1
2
a
2
2
b
1
2
a
2
2
b
Committed
Index
Crash S1: 2
Crash S2: 2
Follower S3: 3
Leader S4: 3
Follower S5: 3
3
3
c
4
3
d
3
3
c
4
3
d

20. 1
2
a
2
2
b
1
2
a
2
2
b
1
2
a
2
2
a
1
2
a
2
2
b
1
2
a
2
2
b
Committed
Index
Follower S1: 4
Crash S2: 2
Crash S3: 3
Follower S4: 4
Leader S5: 4
3
3
c
4
3
d
3
3
c
4
3
d
5
4
e
6
4
f
7
4
g

21. 1
2
a
2
2
b
1
2
a
2
2
b
1
2
a
2
2
a
1
2
a
2
2
b
1
2
a
2
2
b
Committed
Index
Follower S1: 5
Follower S2: 5
Leader S3: 5
Crash S4: 4
Crash S5: 4
3
3
c
4
3
d
3
3
c
4
3
d
5
4
e
6
4
f
7
4
g
3
5
h
3
5
h
3
5
h
4
5
i

22. 1
2
a
2
2
b
1
2
a
2
2
b
1
2
a
2
2
a
1
2
a
2
2
b
1
2
a
2
2
b
Committed
Index
Leader S1: 6
Follower S2: 6
Crash S3: 5
Crash S4: 4
Follower S5: 6
3
3
c
4
3
d
3
3
c
4
3
d
5
4
e
6
4
f
7
4
g
3
5
h
3
5
h
3
5
h
4
5
i
4
6
j
5
6
k
6
6
l
4
6
j

23. 1
2
a
2
2
b
1
2
a
2
2
b
1
2
a
2
2
a
1
2
a
2
2
b
1
2
a
2
2
b
Committed
Index
Crash S1: 6
Follower S2: 6
Follower S3: 5
Follower S4: 4
Follower S5: 6
3
3
c
4
3
d
3
3
c
4
3
d
5
4
e
6
4
f
7
4
g
3
5
h
3
5
h
3
5
h
4
5
i
4
6
j
5
6
k
6
6
l
4
6
j

24. 1
2
a
2
2
b
1
2
a
2
2
b
1
2
a
2
2
a
1
2
a
2
2
b
1
2
a
2
2
b
Committed
Index
Crash S1: 6
Crash S2: 6
Follower S3: 5
Follower S4: 4
Follower S5: 6
3
3
c
4
3
d
3
3
c
4
3
d
5
4
e
6
4
f
7
4
g
3
5
h
3
5
h
3
5
h
4
5
i
4
6
j
5
6
k
6
6
l
4
6
j

25. 1
2
a
2
2
b
1
2
a
2
2
b
1
2
a
2
2
a
1
2
a
2
2
b
1
2
a
2
2
b
Committed
Index
Crash S1: 6
Crash S2: 6
Leader S3: 8
Follower S4: 8
Follower S5: 8
3
3
c
4
3
d
3
3
c
4
3
d
5
4
e
6
4
f
7
4
g
3
5
h
3
5
h
3
5
h
4
5
i
4
6
j
5
6
k
6
6
l
4
6
j

27. 1
2
a
2
2
b
1
2
a
2
2
b
1
2
a
2
2
a
1
2
a
2
2
b
1
2
a
2
2
b
Committed
Index
Crash S1: 6
Leader S2: 7
Follower S3: 7
Follower S4: 7
Follower S5: 7
3
3
c
4
3
d
3
3
c
4
3
d
5
4
e
6
4
f
7
4
g
3
5
h
3
5
h
3
5
h
4
5
i
4
6
j
5
6
k
6
6
l
4
6
j
5
7
m

28. 5
7
m
1
2
a
2
2
b
3
5
h
4
6
j
5
7
m
4
6
1
2
a
2
2
b
Follower S5: 7
3
3
c
4
3
d
5
4
e
6
4
f
7
4
g
Leader S2: 7

29. 5
7
m
1
2
a
2
2
b
3
5
h
4
6
j
5
7
m
1
2
a
2
2
b
Follower S5: 7
3
3
c
4
3
d
5
4
e
6
4
f
7
4
g
Leader S2: 7
4
6
j
3
5

30. 5
7
m
1
2
a
2
2
b
3
5
h
4
6
j
5
7
m
1
2
a
2
2
b
Follower S5: 7
3
3
c
4
3
d
5
4
e
6
4
f
7
4
g
Leader S2: 7 4
6
j
3
5
h
2
2

31. 5
7
m
1
2
a
2
2
b
3
5
h
4
6
j
5
7
m
1
2
a
2
2
b
Follower S5: 7
Leader S2: 7 4
6
j
3
5
h
2
2
3
5
h
4
6
j
5
7
m

32. unique id

34. S1
S2
S3
S4
S5
S3, S4, S4 can be
elected as new
Leader
S1, S2 can be elected
as old Leader

36. S1
S2
S3
S4
S5
Configuration
Change to 3+5
Change
configuration to 5

37. S1
S2
S3
S4
S5
Change
Configuration 3+5
Change
Configuration to 3
Step Down

39. • Server Variable
currentTerm: (persisted)
state: { Follower, Leader, Candidate}
votedFor: (persisted)
log: (persisted)
commitIndex
• Candidate Variable
votesResponded
votesGranted
• Leader Variable
nextIndex
matchIndex
1
1
x  3
2
1
y  1
3
1
y  9
4
2
x  2
5
3
x  0
6
3
y  7
7
3
x  5
8
3
x  4

40. Follower
Leader
Candidate
Init
Timeout,
Start election
Timeout,
New election
BecomeLeaderDiscover leader with
Higher term
Discover leader with
Higher term
Restart
RequestVote
ClientRequest
AdvanceCommitIndex
AppendEntriesHandleRequestVoteRequest
HandleRequestVoteResponse
HandleAppendEntriesRequest
HandleAppendEntriesResponse
DropStaleResponse
DuplicateMessage
DropMessage