Raftkonsensusalgoritmit

Raft is a consensus algorithm designed to be understandable. It solves the problem of achieving consensus in a network of nodes, meaning that all nodes agree on the same sequence of operations even in the presence of failures. Raft divides the management of a replicated log into three distinct components: leader election, log replication, and safety.

Leader election is the primary mechanism through which Raft determines a single leader responsible for managing the replicated log. Nodes in the cluster can be in one of three states: follower, candidate, or leader. Followers passively respond to leader requests and timeouts. If a follower does not hear from a leader within a certain time, it transitions to candidate state and initiates an election. Candidates request votes from other nodes. The first candidate to receive votes from a majority of the servers becomes the leader. If multiple candidates receive votes and no single candidate achieves a majority, a new election is triggered.

Once a leader is elected, it is responsible for log replication. The leader accepts commands from clients, appends them to its log as new entries, and then replicates these entries to follower nodes. Followers acknowledge receipt of log entries. When an entry has been replicated to a majority of servers, the leader applies the entry to its state machine and responds to the client. The leader then informs followers which entries have been committed, and they also apply them to their state machines.

Safety is ensured through several mechanisms. Raft guarantees that if any two servers have the same log index, then they must have the same entry at that index. This is achieved by ensuring that leaders are always up-to-date. Specifically, an election restriction prevents a candidate from becoming leader if it has an outdated log. Furthermore, Raft ensures that once an entry is committed, it will be present in the state machines of all subsequent leaders. This prevents situations where a leader might lose committed entries.