Pre-vote protocol (experimental)

tinyraft.js

@@ -9,7 +9,8 @@
 // The only requirement is to guarantee preservation of entries confirmed by
 // all hosts participating in consensus.
 //
-// Supports leader expiration like in NuRaft:
+// Supports pre-vote protocol and leader expiration like in NuRaft:
+// https://github.com/eBay/NuRaft/blob/master/docs/prevote_protocol.md
 // https://github.com/eBay/NuRaft/blob/master/docs/leadership_expiration.md
 
 const EventEmitter = require('events');
@@ -18,6 +19,10 @@ const VOTE_REQUEST = 'vote_request';
 const VOTE = 'vote';
 const PING = 'ping';
 const PONG = 'pong';
+// Following 3 are required only for the "pre-vote" feature
+const PRE_VOTE_REQUEST = 'pre_vote_request';
+const PRE_VOTE = 'pre_vote';
+const FOLLOW = 'follow';
 
 const CANDIDATE = 'candidate';
 const LEADER = 'leader';
@@ -31,6 +36,7 @@ class TinyRaft extends EventEmitter
     //     electionTimeout?: number,
     //     heartbeatTimeout?: number,
     //     leadershipTimeout?: number,
+    //     enablePrevote?: bool,
     //     initialTerm?: number,
     // }
     constructor(config)
@@ -43,6 +49,7 @@ class TinyRaft extends EventEmitter
         this.randomTimeout = config.randomTimeout > 0 ? Number(config.randomTimeout) : this.electionTimeout;
         this.heartbeatTimeout = Number(config.heartbeatTimeout) || 1000;
         this.leadershipTimeout = Number(config.leadershipTimeout) || 0;
+        this.enablePrevote = config.enablePrevote ? true : false;
         if (!this.nodeId || this.nodeId instanceof Object ||
             !(this.nodes instanceof Array) || this.nodes.filter(n => !n || n instanceof Object).length > 0 ||
             !(this.send instanceof Function))
@@ -52,6 +59,7 @@ class TinyRaft extends EventEmitter
             this.term = 0;
         this.state = null;
         this.leader = null;
+        this.preVoting = false;
     }
 
     _nextTerm(after)
@@ -67,10 +75,32 @@ class TinyRaft extends EventEmitter
         }
     }
 
+    _prevote()
+    {
+        this.leaderTimedOut = true;
+        this.preVoting = true;
+        this.preVoteOk = this.preVoteFail = 0;
+        this.votes = { [this.nodeId]: [ this.nodeId ] };
+        for (const node of this.nodes)
+        {
+            if (node != this.nodeId)
+            {
+                this.send(node, { type: PRE_VOTE_REQUEST, term: this.term });
+            }
+        }
+        this._nextTerm(this.electionTimeout);
+    }
+
     start()
     {
         this._nextTerm(-1);
         this.electionTimer = null;
+        if (this.enablePrevote && !this.preVoting)
+        {
+            this._prevote();
+            return;
+        }
+        this.preVoting = false;
         this.term++;
         this.voted = 1;
         this.votes = { [this.nodeId]: [ this.nodeId ] };
@@ -123,7 +153,36 @@ class TinyRaft extends EventEmitter
 
     onReceive(from, msg)
     {
-        if (msg.type == VOTE_REQUEST)
+        if (msg.type == PRE_VOTE_REQUEST)
+        {
+            this.send(from, { type: PRE_VOTE, term: this.term, leader: this.leaderTimedOut ? null : this.leader });
+        }
+        else if (msg.type == PRE_VOTE && this.preVoting)
+        {
+            if (msg.term == this.term && msg.leader)
+            {
+                this.preVoteOk++;
+            }
+            else
+            {
+                this.preVoteFail++;
+            }
+            if (this.preVoteOk > this.nodes.length/2)
+            {
+                this.preVoting = false;
+                this.preVoteOk = 0;
+                this.preVoteFail = 0;
+                this._nextTerm(this.electionTimeout);
+            }
+            if (this.preVoteFail > this.nodes.length/2)
+            {
+                this._nextTerm(-1);
+                this.preVoteOk = 0;
+                this.preVoteFail = 0;
+                this.start();
+            }
+        }
+        else if (msg.type == VOTE_REQUEST)
         {
             if (msg.term > this.term && msg.leader)
             {
@@ -240,6 +299,8 @@ class TinyRaft extends EventEmitter
     }
 }
 
+TinyRaft.PRE_VOTE_REQUEST = PRE_VOTE_REQUEST;
+TinyRaft.PRE_VOTE = PRE_VOTE;
 TinyRaft.VOTE_REQUEST = VOTE_REQUEST;
 TinyRaft.VOTE = VOTE;
 TinyRaft.PING = PING;

tinyraft.spec.js

@@ -10,7 +10,7 @@ function newNode(id, nodes, partitions)
         electionTimeout: 500,
         send: function(to, msg)
         {
-            if (!partitions[n.nodeId+'-'+to] && !partitions[to+'-'+n.nodeId] && nodes[to])
+            if (!partitions[n.nodeId+'-'+to] && nodes[to])
             {
                 console.log('received from '+n.nodeId+' to '+to+': '+JSON.stringify(msg));
                 setImmediate(function() { nodes[to].onReceive(n.nodeId, msg); });
@@ -148,10 +148,62 @@ async function testAddNode()
     console.log('testAddNode: OK');
 }
 
+async function testPreVote()
+{
+    // The original example for pre-vote protocol from here:
+    // https://github.com/eBay/NuRaft/blob/master/docs/prevote_protocol.md
+    // only has partitions between nodes 1-5, 2-4, 3-4.
+    // But that setup has a problem of leader jumping between nodes 3 and 4
+    // which can't be prevented by prevotes because 4 only has connections
+    // to 2 nodes so they can't form a majority to reject the vote.
+    const partitions = {
+        '1-5': true,
+        '2-4': true,
+        '3-4': true,
+        '2-3': true,
+        '5-1': true,
+        '4-2': true,
+        '4-3': true,
+        '3-2': true,
+    };
+    const nodes = newNodes(5, partitions);
+    let leader = await new Promise((ok, no) =>
+    {
+        setTimeout(() => no(new Error('no leader in 500 ms')), 500);
+        const chg = (st) =>
+        {
+            if (st.leader)
+            {
+                nodes[1].off('change', chg);
+                if (st.leader != 1 && st.leader != 5)
+                    no('leader is not 1 or 5');
+                ok(st.leader);
+            }
+        };
+        nodes[1].on('change', chg);
+    });
+    console.log('OK: Got leader in 500 ms');
+    await new Promise((ok, no) =>
+    {
+        setTimeout(ok, 5000);
+        const chg = (st) =>
+        {
+            if (st.state != TinyRaft.FOLLOWER || st.leader != leader)
+            {
+                nodes[5].off('change', chg);
+                no(new Error('leader changed in 5000 ms'));
+            }
+        };
+        nodes[5].on('change', chg);
+    });
+    console.log('OK: No leader change in 5000 ms');
+}
+
 async function run()
 {
     await testStartThenRemoveNode();
     await testAddNode();
+    await testPreVote();
     process.exit(0);
 }