Zookeeper's election mechanism
Zookeeper's leader election mechanism is based on the ZAB (Zookeeper Atomic Broadcast) protocol, a variant based on the Paxos protocol, specially used for Zookeeper's distributed coordination services.
The election process is mainly divided into the following stages
1. Initialization phase
When a new Zookeeper server joins the cluster, it sends a LOOKING state message to other servers indicating that it is in a state of looking for a leader.
2. Voting process
- Voting initiation: Each server in LOOKING will vote for itself and start an election round (Election Round).
- Spread the voting results: The server will spread its own voting information to other servers in the cluster.
- Collect voting: Each server collects votes from other servers and calculates the server with the highest votes currently.
3. Election Round
If a server finds that the current vote is pointing to itself, it will continue to wait for a while to see if more votes will be received. If no more votes are received to other servers, it will declare itself a leader.
4. Become a leader
When a server receives more than half of the votes (i.e. Majority Quorum) and does not have a higher number of votes, it will become a leader. At this point, it will send a message of LEADING status to other servers, indicating that it has become a leader.
5. Follower confirmation
After receiving the message from the LEADING status, other servers confirm the leader, enter the FOLLOWING status, and start following the leader.
Reelection after leader failure
When the leader fails, other servers in the cluster detect this and restart the election process. Specifically:
- Leader failure detection: If a follower does not receive a leader's heartbeat message for a long time, it will think that the leader may have failed and switch to LOOKING state.
- Re-election: The server entering the LOOKING state will restart the voting process, spread its own voting information, and collect voting from other servers.
- New leaders emerge: After one or more rounds of voting, the cluster will select a new leader and repeat the above process.
Election mechanism
Voting rules
When comparing votes, the size of ZXID (Zookeeper Transaction ID) determines the priority. ZXID is a unique identifier for a transaction. It is a 64-bit integer composed of two parts: the high 32-bit represents epoch (period), and the low 32-bit represents counter (counter). Servers with larger ZXID are considered to have more information and are therefore more suitable as leaders. If the ZXID of both servers is the same, the server with a larger SID (server ID) wins. SID is an integer, usually specified in a configuration file, to distinguish different server instances.
Election Algorithm
Zookeeper uses a Leader election algorithm based on a variant of the Paxos algorithm, specifically, it uses an algorithm called Fast Leader Election (FLE), which is designed to minimize the time required for elections and ensure consistency in the election process. The FLE algorithm reaches a consensus by letting the servers send voting information to each other, and finally selects a leader supported by the majority.
Election Retry
In some cases, if the election is not successful, a re-election may be conducted until a new leader is elected. For example, if more than half of the members of the cluster fail to agree on a leader, the election may need to be re-run.
Election efficiency
To improve election efficiency, Zookeeper has taken some measures in design, such as heartbeat mechanism (leader regularly sends heartbeat messages to Follower to maintain its leadership), the half-principle (only if a member gets more than half of the votes, which ensures consensus among most members), and optimizes network communications (by optimizing network communication protocols and reducing unnecessary communications, speeding up elections).
Through the above mechanism, Zookeeper can achieve effective coordination in a distributed environment and quickly restore the normal operation of the cluster in the event of a failure. This mechanism is widely used in distributed systems that require high availability and consistency.
ZooKeeper's election mechanism is key to its high availability and fault tolerance. In the ZooKeeper cluster, there is a node that is elected as a leader, responsible for handling all write requests and most read requests. Other nodes act as followers or observers, responsible for processing read requests and receiving updates from leaders.
The general steps of the election process
- When the server starts: Election votes will be sent to other servers in the cluster.
- After the server receives the vote: Will check the validity of the vote. If the vote is valid, the server will add its own vote to the voting list.
- The server will send its own vote to other servers in the cluster。
- When the server receives enough valid votes: It will become a leader.
- This process will be repeated continuously: To ensure that new leaders can be selected in a timely manner when leaders collapse or other problems arise.
Voting rules
- ZXID priority: When comparing votes, the size of ZXID (ZooKeeper Transaction ID) determines the priority. ZXID is a unique identifier for a transaction. It is a 64-bit integer composed of two parts: the high 32-bit represents epoch (period), and the low 32-bit represents counter (counter). Servers with larger ZXID are considered to have more information and are therefore more suitable as leaders.
- SID second: If the ZXID of both servers is the same, the server with a larger SID (server ID) wins. SID is an integer, usually specified in a configuration file, to distinguish different server instances.
Election Algorithm
The Leader election algorithm used by ZooKeeper is a variant of the Paxos algorithm. Specifically, it uses an algorithm called Fast Leader Election (FLE), which is designed to minimize the time required for elections and ensure consistency in the election process. The FLE algorithm reaches a consensus by letting the servers send voting information to each other, and finally selects a leader supported by the majority.
Election Retry
In some cases, if the election is not successful, a re-election may be conducted until a new leader is elected.
For example, if more than half of the members of the cluster fail to agree on a leader, the election may need to be re-run.
Election efficiency
To improve election efficiency, ZooKeeper has taken some measures in design:
- Heartbeat mechanism: Leader regularly sends heartbeat messages to Follower to maintain its leadership.
- More than half principle: A member can only become a leader if he gets more than half of the votes, which ensures consensus among most members.
- Optimize network communication: Speed up elections by optimizing network communication protocols and reducing unnecessary communications.
Through the above mechanism, ZooKeeper can effectively elect a new leader in the cluster, thereby ensuring the normal operation of the cluster.
Summarize
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