====== Raft Consensus Algorithm ======

===== What is Raft? =====

Raft is a consensus algorithm that is designed to be easy to understand.  It's equivalent to Paxos in fault-tolerance and performance.  The difference is that it's decomposed into relatively independent subproblems, and it cleanly addresses all major pieces needed for practical systems.  


===== What is consensus? =====

Consensus is a fundamental problem in fault-tolerant distributed systems.  Consensus involves multiple servers agreeing on values.  Once they reach a decision on a value, that decision is final.  Typical consensus algorithms make progress when any majority of their servers are available; for example, a cluster of 5 servers can continue to operate even if 2 servers fail.  If more servers fail, they stop making progress (but will never return an incorrect result).

Consensus typically arises in the context of replicated state machines, a general approach to building fault-tolerant systems.  Each server has a state machine and a log.  The state machine is the component that we want to make fault-tolerant, such as a hash table.  It will appear to clients that they are interacting with a single, reliable state machine, even if a minority of the servers in the cluster fail.  Each state machine takes as input commands from its log. In our hash table example, the log would include commands like set x to 3.  A consensus algorithm is used to agree on the commands in the servers' logs. The consensus algorithm must ensure that if any state machine applies set x to 3 as the nth command, no other state machine will ever apply a different nth command.  As a result, each state machine processes the same series of commands and thus produces the same series of results and arrives at the same series of states.


===== References =====

  * https://raft.github.io/