What is the Leader election mechanism in Zookeeper? What are the election process and rules?

Answer

Zookeeper's Leader election mechanism is the core of ensuring cluster high availability, implemented based on the ZAB protocol.

Election Trigger Timing

1. During cluster startup: All nodes participate in election to elect a Leader
2. When Leader fails: Followers detect Leader failure and trigger re-election
3. When Leader exits voluntarily: Leader shuts down normally, triggering election

Election Algorithm

Zookeeper uses the Fast Leader Election algorithm:

Voting Structure:

• sid: Server ID, specified in configuration file
• zxid: Transaction ID, indicating data update count
• epoch: Election cycle, increments with each election

Election Rules:

1. Compare zxid first: Larger zxid means newer data, priority for election
2. Then compare sid: When zxid is the same, larger sid has priority

Election Process

1. Initialize voting:

   • Each node votes for itself first
   • Voting information: (epoch, zxid, sid)

2. Vote exchange:

   • Nodes exchange voting information with each other
   • Update their own voting status

3. Vote counting:

   • Count votes for each candidate
   • Candidate supported by more than half of nodes wins

4. Election complete:

   • Winner becomes Leader
   • Other nodes become Followers
   • Leader starts processing requests

Election States

Nodes have the following states during election:

1. LOOKING: Looking for Leader, participating in election
2. FOLLOWING: Found Leader, running as Follower
3. LEADING: Running as Leader
4. OBSERVING: Running as Observer

Election Optimization

Fast Election:

• Nodes prioritize voting for the node with most data updates
• Reduce voting rounds, speed up election

Vote Validation:

• Validate legitimacy of voting information
• Prevent invalid votes from interfering with election

Timeout Mechanism:

• Set reasonable election timeout
• Avoid long-term election blocking

Cluster Scale Impact

• 3-node cluster: 2 nodes agreeing is sufficient for election
• 5-node cluster: 3 nodes agreeing is sufficient for election
• 7-node cluster: 4 nodes agreeing is sufficient for election

Considerations

1. Split-brain problem: Avoided through majority mechanism
2. Network partition: Cannot elect Leader after partition
3. Election time: Usually completes within a few seconds
4. Data consistency: No write requests processed during election