High availability - fault tolerant

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  • using Redundancy and Replication to reach fault tolerant


  • key point
    • leader
      • control the whole process, interact with clients
    • candidate
      • if cannot receive heartbeat from leader, wait for a random timeout to be cadidate
    • follower

Leader election

  • two timeout

    • election timeout
      • is the amount of time a follower waits until becoming a candidate.
      • then it assumes there is no viable leader and begins an election to choose a new leader
      • randomized to be between 150ms and 300ms.
      • After the election timeout the follower becomes a candidate and starts a new election term
      • sends out Request Vote messages to other nodes.
    • heartbeat timeout
      • leader sends heartbeat messages to all of the other servers to establish its authority and prevent new elections.
  • Leader

    • once get majority votes, leader begins sending out Append Entries messages to its followers.
      • send in intervals specified by the heartbeat timeout
    • if node cannot receive heartbeat, then become a candidate and re-election
    • Requiring a majority of votes guarantees that only one leader can be elected per term.
  • split vote

    • if two or more candidates receive same votes
    • The nodes will wait for a new election and try again.

log replicate

  • This is done by using the same Append Entries message that was used for heartbeats.
  • committed
    • The leader decides when it is safe to apply a log entry to the state machines; such an entry is called committed.
    • Raft guarantees that committed entries are durable and will eventually be executed by all of the available state machines.
  • once new entry committed
    • leader executes command in its state machine ,return result to client
    • leader notifies followers of committed entries in subsequent AppendEntries RPCs
    • followers execute committed commands in their state machines

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