pbetkier / rafting

DIY Raft consensus algorithm

rafting

A playground for implementing Raft – an easy consensus algorithm.

Theory

• Raft paper – you should read this before diving into code, at least chapter 5: The Raft consensus algorithm (5.3 Log replication not required).
• Raft github site – short introduction and an interactive visualization.
• Raft visualization – another visualization, more guided and less interactive than the one above.

Contract

A node may be written in any technology as long as it conforms to the contract below:

• The transport protocol is JSON over HTTP.
• Node id is formed by it's IP and port, e.g. 192.168.0.12:5000.
• Time parameters:
  • Followers become candidates after 5 seconds without heartbeat.
  • Leaders send heartbeats every 3 seconds.
  • Request timeouts are set to 0.5 seconds.

Refer to condensed summary of the Raft algorithm on page 4 of the Raft paper for details on the specific fields and their meaning.

Request vote

Invoked by candidates to request votes. Corresponds to RequestVote RPC from the Raft paper.

Method	Path	Request fields	Response fields
POST	/raft/request-vote	term, candidateId	term, voteGranted

Example:

$ cat data.json
{
  "term": 1, 
  "candidateId": "192.168.0.12:5000"
}
$ curl -X POST -H Content-Type:application/json -d @data.json localhost:5000/raft/request-vote
{
  "term": 1,
  "voteGranted": true
}

Append entries

Invoked by leader to gather heartbeats. Corresponds to AppendEntries RPC from the Raft paper.

In real Raft implementation this is also used to replicate log entries, but we don't cover this feature in this exercise.

Method	Path	Request fields	Response fields
POST	/raft/append-entries	term	term, success

Example:

$ cat data.json
{
  "term": 1
}
$ curl -X POST -H Content-Type:application/json -d @data.json localhost:5000/raft/append-entries
{
  "term": 1,
  "success": true
}

State

Invoked by the rafting-dashboard to visualize the node state. For diagnostics purposes, not used in the leader election flow.

Method	Path	Response header	Response fields
GET	/raft/state	Access-Control-Allow-Origin: *	described below

Note the CORS header that the dashboard requires to obtain the results.

Fields:

• role (string): the current role: one of follower, candidate or leader.
• currentTerm (number): as in Raft paper.
• lastHeartbeat (number): last time a request vote or append entries was received, UTC timestamp in milliseconds.
• peers (array of strings): node ids of all known nodes, excluding self.
• nodeId (string): node id of this node.
• votedFor (string): node id of the last candidate this node voted for.
• votesGranted (number): how many votes this node received while being a candidate.

Example:

$ curl localhost:5000/raft/state
{
  "currentTerm": 1,
  "lastHeartbeat": 1539944386183,
  "peers": ["192.168.0.44:5000", "192.168.0.33:8080"],
  "role": "candidate",
  "nodeId": "192.168.0.12:5000",
  "votedFor": "192.168.0.44:5000",
  "votesGranted": 2
}

Dashboard

Consult rafting-dashboard readme.