Cluster Formation

A Raft node enters Learner state when it is created by Raft::new().

To form a cluster, application must call Raft::initialize(membership).

Raft::initialize()

This method will:

• Append one membership log at index 0, the log id has to be (leader_id=(0,0), index=0). The membership will take effect at once.

• Enter Candidate state and start vote to become leader.

• The leader will commit a blank log to commit all preceding logs.

Errors and failures

• Calling this method on an already initialized node just returns an error and is safe, i.e. last_log_id on this node is not None, or vote on this node is not (0,0).

• Calling this method on more than one node at the same time:

  • with the same membership, it is safe. Because voting protocol guarantees consistency.

  • with different membership it is ILLEGAL and will result in an undefined state, AKA the split-brain state.

Conditions for initialization

The conditions for a legal initialization is as the above because:

The first membership log with log id (vote, index=0) will be appended to initialize a node, without consensus. This has not to break the commit condition:

1. Log id (vote, index=0) must not be greater than any committed log id. if vote is not the smallest value, i.e. (term=0, node_id=0), it has chance to be greater than some committed log id. This is why the first log has to be the smallest: ((term=0, node_id=0), 0).

2. And a node should not append a log that is smaller than its vote. Otherwise, it is actually changing the history other nodes has seen. This has chance to (but not certainly will) break the consensus, depending on the protocol. E.g. if the cluster has been running a fast-paxos like protocol, appending a smaller log than vote is illegal. By not allowing to append a smaller log than vote, it will always be safe.

From these two reason, it is only allowed to append the first log if: vote==(0,0). And this is why the initial value of vote has to be (0,0).