What is the Raft Consensus Algorithm? A Simple Guide

Consensus algorithms are crucial in distributed systems to ensure all the nodes agree on a common state, even when there are failures. One popular consensus algorithm is Raft. It is designed to be understandable and easy to implement. In this post, we'll break down how the Raft Consensus Algorithm works in simple terms.

What is Raft?

Raft is a consensus algorithm developed at Stanford University. It ensures that a group of computers (called nodes) can agree on a shared state, even if some of the nodes fail. Raft is typically used in distributed systems to manage replicated logs, which are essential for maintaining data consistency across multiple nodes. In a distributed system, it's crucial that all nodes have the same data, even if some of them fail or go offline temporarily. Raft achieves this by ensuring that any changes to the data are consistently replicated across all nodes in the system. This way, even if one or more nodes fail, the system can continue to operate correctly, and no data is lost.

Key Concepts in Raft

Before diving into how Raft works, let's understand some key concepts:

1. Cluster: A cluster is a group of nodes that work together to maintain a consensus. This group of nodes collaborates to ensure that they all agree on the same state of the system, even if some nodes fail or are temporarily offline.

2. Node: A node is an individual machine or server within the cluster. Each node plays a role in maintaining the overall state of the system and can take on different roles such as leader, follower, or candidate.

3. Log Entry: A log entry is a record of a change to the system state. These entries are crucial for maintaining consistency across the nodes. Each log entry contains information about a specific change, and these entries are replicated across all nodes to ensure they all have the same data.

4. Term: A term is a period during which a particular leader is in charge. Raft divides time into terms, and each term begins with an election to choose a new leader. Terms help in organizing the leadership and ensuring that there is a clear authority at any given time.

5. Leader: The leader is a node that has the authority to manage log entries and communicate with other nodes. The leader is responsible for accepting changes from clients, replicating these changes to followers, and ensuring that all nodes agree on the same log entries.

6. Follower: A follower is a node that accepts log entries from the leader. Followers do not accept changes directly from clients but instead rely on the leader to provide them with the log entries. Followers help in maintaining the replicated log and ensuring data consistency.

7. Candidate: A candidate is a node that is trying to become a leader. When a term begins, nodes can become candidates and participate in an election to be chosen as the leader. If a candidate receives a majority of votes from other nodes, it becomes the new leader for that term.

Understanding these key concepts is essential for grasping how the Raft Consensus Algorithm works. Each role and term plays a specific part in ensuring that the distributed system remains consistent and reliable, even in the face of failures.

Raft’s Phases

Raft operates in three main phases:

1. Leader Election

2. Log Replication

3. Safety

Phase 1: Leader Election

Raft ensures that there is a single leader at any given time. The leader is responsible for managing the replicated log and communicating with the other nodes (followers). The election process works as follows:

1. Election Timeout: Each follower node starts a timer. If a follower does not hear from the leader within the timeout period, it assumes that the leader has failed.

2. Becoming a Candidate: The follower node transitions to a candidate state and starts a new election term. It increments its term number and votes for itself.

3. Requesting Votes: The candidate sends vote requests to all other nodes in the cluster.

4. Voting: Other nodes can grant their vote to the candidate if they have not already voted in the current term and if the candidate’s log is as up-to-date as their own.

5. Majority Wins: If the candidate receives a majority of the votes, it becomes the new leader. If no candidate receives a majority, the election process is restarted.

Phase 2: Log Replication

Once a leader is elected, it starts managing the log entries. The process of log replication ensures that all nodes have the same log entries in the same order:

1. Client Requests: Clients send requests to the leader to make changes to the system state.

2. Appending Entries: The leader appends the client’s request as a new log entry and then sends these entries to its followers.

3. Commitment: Once the leader receives acknowledgments from a majority of the followers, it commits the entry to its log and applies the change to the system state.

4. Replicating Committed Entries: The leader informs the followers to commit the entry to their logs.

Phase 3: Safety

Raft ensures safety through a set of rules that guarantee consistency:

1. Leader Append-Only: Only the leader can append new entries to the log.

2. Term Matching: Followers only accept log entries from the current leader.

3. Log Matching: If two logs contain an entry with the same term and index, they are identical up to that entry.

4. Election Safety: At most one leader can be elected in a given term.

5. Commitment Rules: A log entry is considered committed if it is stored on a majority of nodes and the leader that created the entry is in its term.

Handling Failures

Raft is designed to handle various types of failures, ensuring the system remains robust and reliable even under adverse conditions:

1. Leader Failure: In the event that the leader node fails, the remaining follower nodes will detect this failure due to the election timeout mechanism. This timeout is a predefined period during which followers expect to receive heartbeats from the leader. If no heartbeat is received within this period, the followers assume the leader has failed and initiate a new election process to select a new leader. This ensures that the system can quickly recover from leader failures and continue to operate smoothly.

2. Follower Failure: When a follower node fails, the leader node continues to function normally, processing client requests and replicating log entries to the remaining active followers. The system remains operational, albeit with reduced redundancy. Once the failed follower node recovers and rejoins the cluster, the leader will bring this follower up-to-date by sending it all the log entries it missed during its downtime. This synchronization process ensures that the recovered follower is fully consistent with the rest of the cluster before it resumes normal operations.

3. Network Partitions: Network partitions can split the cluster into multiple isolated groups of nodes. In such cases, each partition will attempt to elect a leader within its group. The partition that contains a majority of the nodes (more than half of the total nodes in the cluster) will successfully elect a new leader and continue to process client requests. The minority partitions, lacking a majority, will fail to elect a leader and will remain in a follower state. These minority partitions will not process client requests until they can reconnect with the majority partition. Once the network partition is resolved and connectivity is restored, the minority partitions will synchronize with the majority partition to ensure consistency across the entire cluster.

Conclusion

The Raft Consensus Algorithm is a powerful tool for managing distributed systems. It ensures data consistency and fault tolerance through a clear and understandable process. By dividing the problem into leader election, log replication, and safety, Raft provides a robust solution that is easier to implement compared to other consensus algorithms.

Understanding Raft is essential for anyone working with distributed systems, as it forms the backbone of many modern data management systems. With its emphasis on simplicity and reliability, Raft continues to be a popular choice in the world of distributed computing.