Raft is a consensus algorithm for managing a replicated log. Consensus algorithms allow a collection of machines to work as a coherent group that can survive the failures of some of its members.

Consensus algorithms

They ensure safety (never returning an incorrect result) under all non-Byzantine conditions, including network delays, partitions, and packet loss, duplication, and reordering.
They are fully functional (available) as long as any majority of the servers are operational and can communicate with each other and with clients. Thus, a typical cluster of five servers can tolerate the failure of any two servers. Servers are assumed to fail by stopping; they may later recover from state on stable storage and rejoin the cluster.
They do not depend on timing to ensure the consistency of the logs: faulty clocks and extreme message delays can, at worst, cause availability problems.
In the common case, a command can complete as soon as a majority of the cluster has responded to a single round of remote procedure calls, a minority of slow servers need not impact overall system performance.

Novel Raft Features

Strong leader: Raft uses a stronger form of leadership than other consensus algorithms. For example, log entries only flow from the leader to other servers. This simplifies the management of the replicated log and makes Raft easier to understand.
Leader election: Raft uses randomized timers to elect leaders. This adds only a small amount of mechanism to the heartbeats already required for any consensus algorithm, while resolving conflicts simply and rapidly.
Membership changes: Raft’s mechanism for changing the set of servers in the cluster uses a new joint consensus approach where the majorities of two different configurations overlap during transitions. This allows the cluster to continue operating normally during configuration changes.