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Pre-vote

Purpose​

The Pre-vote mechanism in Apache Ratis addresses the issue of unnecessary leader step-downs that can occur when a follower temporarily loses network connectivity and then recovers. Without Pre-vote, such a follower might initiate a new leader election upon recovery, even if a stable leader already exists, leading to disruptions in the cluster.

Mechanism​

The proposed solution involves a two-phase election process:

  1. Pre-vote Request: When a follower (e.g., s3) believes it needs to request a vote, it first sends a pre-vote request to other servers.
  2. Pre-vote Response: When the current leader (e.g., s1) and other followers (e.g., s2) receive the pre-vote request, they check their electionTimeout to determine if a leader election should be triggered.
  3. Preventing Unnecessary Elections: If s1 and s2 determine that a new leader election is not necessary (e.g., they are still receiving heartbeats from the current leader), they will refuse the pre-vote request. In this scenario, s3 remains a follower, preventing an unnecessary election and maintaining cluster stability.

Configuration​

To enable or disable the Pre-vote feature, the following property can be set in RaftServerConfigKeys:

PropertyDescriptionTypeDefault
raft.server.leaderelection.pre-voteEnable or disable the pre-vote mechanism.booleantrue

Methods to set/get this property:

static boolean preVote(RaftProperties properties);
static void setPreVote(RaftProperties properties, boolean preVote);

References​