高可用性

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High availability - fault tolerant

Useful link

• Raft algorthem
• visualization of Raft
• original paper
• zh_cn version

work

• using Redundancy and Replication to reach fault tolerant

Raft

• 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