Raft is a consensus algorithm designed to manage replicated logs in distributed systems. Developed as an alternative to Paxos, Raft prioritizes understandability and simplicity, making it an attractive choice for building fault-tolerant distributed systems.
## Concepts
### Leader Election
Raft employs a leader election mechanism where nodes contend to become the leader.
Nodes transition through follower, candidate, and leader states based on timeouts and votes.
## Log Replication
The leader manages log replication by sending heartbeat messages and append entries to followers.
Followers replicate the leader's log, ensuring consistency across the system.
## Term and Commit Index
Raft introduces the concept of terms to track changes in leadership.
Each leader's term is associated with a commit index, indicating the highest log entry replicated to a majority of nodes.
## How Raft Works
- Follower State:
Nodes start as followers, waiting for incoming leader messages.
If a follower doesn't receive a message within a specified timeout, it transitions to the candidate state.
- Candidate State:
In the candidate state, a node requests votes from other nodes in the system.
If a candidate receives votes from a majority, it transitions to the leader state.
- Leader State:
The leader manages log replication by sending heartbeat messages to maintain its leadership.
It appends new log entries and sends append entries messages to followers for replication.
- Log Replication:
Followers replicate the leader's log, responding with acknowledgments to confirm successful replication.
Log entries are committed when they have been replicated to a majority of nodes.
- Handling Failures:
Raft ensures fault tolerance by allowing nodes to detect leader failures and initiate new elections.
Only the most up-to-date log entries are committed, preventing inconsistencies.
## Use Cases
- Distributed Databases: Raft is widely used in distributed databases to ensure data consistency and reliability.
- Distributed Systems: Applications requiring consensus for critical decision-making leverage Raft for its simplicity and reliability.
