Consensus: An Introduction to Raft

Transcript

1. 1.

   Consensus An Introduction to Raft

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   con·sen·sus /kənˈsensəs/ Agreeing upon state across distributed processes even in

   the presence of failures.
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   Problem • Distributed System • Consistency • Partition tolerance

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   Solution • Quorum • Replicated State Machines

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   Consensus Data —

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   We are sacrificing Availability

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   Why not Paxos? • Difficult to understand • Not practical

   enough to implement
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   Raft A Practical Paxos

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   Components • Consensus Module • State Machine • Log

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    Consensus Module • Roles: Leader, Follower, and Candidate • Time

    is divided into Terms • Commands: RequestVote and AppendEntries
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    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
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    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
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    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.
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    Example Happy Log Entry

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    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: []
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    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 ˒
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    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
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    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
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    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
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    Example Sad Log Entry

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    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: []
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    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 ˒
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    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
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    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
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    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
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    Example Leader Failure

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    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: []
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    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: []
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    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: []
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    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: []
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    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: []
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    Extras • Log safety and compaction • Cluster changes

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    Real-life Application • Distributed lock server • Configuration management •

    Background job storage
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    Smart People • Raft Paper by Diego Ongaro and John

    Ousterhout • Raft Implementation • ThinkDistributed