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Consensus: An Introduction to Raft
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Patrick Van Stee
July 23, 2013
Programming
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Consensus: An Introduction to Raft
Patrick Van Stee
July 23, 2013
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Transcript
Consensus An Introduction to Raft
con·sen·sus /kənˈsensəs/ Agreeing upon state across distributed processes even in
the presence of failures.
Problem • Distributed System • Consistency • Partition tolerance
Solution • Quorum • Replicated State Machines
Consensus Data —
We are sacrificing Availability
Why not Paxos? • Difficult to understand • Not practical
enough to implement
Raft A Practical Paxos
Components • Consensus Module • State Machine • Log
Consensus Module • Roles: Leader, Follower, and Candidate • Time
is divided into Terms • Commands: RequestVote and AppendEntries
Leader Accept commands from clients, commit entries, and send heartbeats
Follower Replicate state from leaders and vote for candidates Candidate Start and handle leader elections
Follower Candidate Leader Times out, Starts election Times out, Restarts
election Wins election Discovers new leader, Steps down Discovers current leader or new leader, Steps down
Term Higher numbers are used to determine leaders and check
log entries. The term is incremented each time an election is started. Any command with an old term is ignored.
Example Happy Log Entry
A B C Role: Leader Term: 1 Commit Index: 0
Log: [] Role: Follower Term: 1 Commit Index: 0 Log: [] Role: Follower Term: 1 Commit Index: 0 Log: []
A B C Role: Leader Term: 1 Commit Index: 0
Log: [˒] Role: Follower Term: 1 Commit Index: 0 Log: [] Role: Follower Term: 1 Commit Index: 0 Log: [] Leader receives command ˒
A B C Role: Leader Term: 1 Commit Index: 0
Log: [˒] Role: Follower Term: 1 Commit Index: 0 Log: [˒] Role: Follower Term: 1 Commit Index: 0 Log: [˒] Leader sends log entries to followers
A B C Role: Leader Term: 1 Commit Index: 1
Log: [˒] Role: Follower Term: 1 Commit Index: 0 Log: [˒] Role: Follower Term: 1 Commit Index: 0 Log: [˒] Majority of followers respond with success
A B C Role: Leader Term: 1 Commit Index: 1
Log: [˒] Role: Follower Term: 1 Commit Index: 1 Log: [˒] Role: Follower Term: 1 Commit Index: 1 Log: [˒] Leader sends commit index to followers and responds to client
Example Sad Log Entry
A B C Role: Leader Term: 1 Commit Index: 0
Log: [] Role: Follower Term: 1 Commit Index: 0 Log: [] Role: Follower Term: 1 Commit Index: 0 Log: []
A B C Role: Leader Term: 1 Commit Index: 0
Log: [˒] Role: Follower Term: 1 Commit Index: 0 Log: [] Role: Follower Term: 1 Commit Index: 0 Log: [] Leader receives command ˒
A B C Role: Leader Term: 1 Commit Index: 0
Log: [˒] Role: Follower Term: 1 Commit Index: 0 Log: [˒] Role: Follower Term: 1 Commit Index: 0 Log: [] Leader sends log entries to followers
A B C Role: Leader Term: 1 Commit Index: 0
Log: [˒] Role: Follower Term: 1 Commit Index: 0 Log: [˒] Role: Follower Term: 1 Commit Index: 0 Log: [] Majority of followers do not respond
A B C Role: Leader Term: 1 Commit Index: 0
Log: [˒] Role: Follower Term: 1 Commit Index: 0 Log: [˒] Role: Follower Term: 1 Commit Index: 0 Log: [˒] Leader continues to retry log entry
Example Leader Failure
A B C Role: Leader Term: 1 Commit Index: 0
Log: [] Role: Follower Term: 1 Commit Index: 0 Log: [] Role: Follower Term: 1 Commit Index: 0 Log: [] D Role: Follower Term: 1 Commit Index: 0 Log: []
A B C Role: Leader Term: 1 Commit Index: 0
Log: [] Role: Follower Term: 1 Commit Index: 0 Log: [] Role: Follower Term: 1 Commit Index: 0 Log: [] Followers do not receive heartbeat D Role: Follower Term: 1 Commit Index: 0 Log: []
A B C Role: Leader Term: 1 Commit Index: 0
Log: [] Role: Candidate Term: 2 Commit Index: 0 Log: [] Role: Follower Term: 1 Commit Index: 0 Log: [] First follower to timeout becomes candidate D Role: Follower Term: 1 Commit Index: 0 Log: []
A B C Role: Leader Term: 1 Commit Index: 0
Log: [] Role: Candidate Term: 2 Commit Index: 0 Log: [] Role: Follower Term: 2 Commit Index: 0 Log: [] Candidate starts election and requests votes D Role: Follower Term: 2 Commit Index: 0 Log: []
A B C Role: Leader Term: 1 Commit Index: 0
Log: [] Role: Leader Term: 2 Commit Index: 0 Log: [] Role: Follower Term: 2 Commit Index: 0 Log: [] Followers respond with votes D Role: Follower Term: 2 Commit Index: 0 Log: []
Extras • Log safety and compaction • Cluster changes
Real-life Application • Distributed lock server • Configuration management •
Background job storage
Smart People • Raft Paper by Diego Ongaro and John
Ousterhout • Raft Implementation • ThinkDistributed