feat: implement raft election and term state

src/NATS.Server/Raft/RaftNode.cs

@@ -0,0 +1,56 @@
+namespace NATS.Server.Raft;
+
+public sealed class RaftNode
+{
+    private int _votesReceived;
+
+    public string Id { get; }
+    public int Term => TermState.CurrentTerm;
+    public RaftRole Role { get; private set; } = RaftRole.Follower;
+    public RaftTermState TermState { get; } = new();
+    public long AppliedIndex { get; set; }
+
+    public RaftNode(string id)
+    {
+        Id = id;
+    }
+
+    public void StartElection(int clusterSize)
+    {
+        Role = RaftRole.Candidate;
+        TermState.CurrentTerm++;
+        TermState.VotedFor = Id;
+        _votesReceived = 1;
+        TryBecomeLeader(clusterSize);
+    }
+
+    public VoteResponse GrantVote(int term)
+    {
+        if (term < TermState.CurrentTerm)
+            return new VoteResponse { Granted = false };
+
+        TermState.CurrentTerm = term;
+        return new VoteResponse { Granted = true };
+    }
+
+    public void ReceiveVote(VoteResponse response, int clusterSize = 3)
+    {
+        if (!response.Granted)
+            return;
+
+        _votesReceived++;
+        TryBecomeLeader(clusterSize);
+    }
+
+    private void TryBecomeLeader(int clusterSize)
+    {
+        var quorum = (clusterSize / 2) + 1;
+        if (_votesReceived >= quorum)
+            Role = RaftRole.Leader;
+    }
+
+    public void RequestStepDown()
+    {
+        Role = RaftRole.Follower;
+    }
+}

src/NATS.Server/Raft/RaftRpcContracts.cs

@@ -0,0 +1,12 @@
+namespace NATS.Server.Raft;
+
+public sealed class VoteRequest
+{
+    public int Term { get; init; }
+    public string CandidateId { get; init; } = string.Empty;
+}
+
+public sealed class VoteResponse
+{
+    public bool Granted { get; init; }
+}

src/NATS.Server/Raft/RaftTermState.cs

@@ -0,0 +1,14 @@
+namespace NATS.Server.Raft;
+
+public sealed class RaftTermState
+{
+    public int CurrentTerm { get; set; }
+    public string? VotedFor { get; set; }
+}
+
+public enum RaftRole
+{
+    Follower,
+    Candidate,
+    Leader,
+}

tests/NATS.Server.Tests/RaftElectionTests.cs

@@ -0,0 +1,43 @@
+using NATS.Server.Raft;
+
+namespace NATS.Server.Tests;
+
+public class RaftElectionTests
+{
+    [Fact]
+    public async Task Candidate_becomes_leader_after_majority_votes()
+    {
+        var cluster = RaftTestCluster.Create(3);
+        var leader = await cluster.ElectLeaderAsync();
+
+        leader.Role.ShouldBe(RaftRole.Leader);
+        leader.Term.ShouldBe(1);
+    }
+}
+
+internal sealed class RaftTestCluster
+{
+    public List<RaftNode> Nodes { get; }
+
+    private RaftTestCluster(List<RaftNode> nodes)
+    {
+        Nodes = nodes;
+    }
+
+    public static RaftTestCluster Create(int nodes)
+    {
+        var created = Enumerable.Range(1, nodes).Select(i => new RaftNode($"n{i}")).ToList();
+        return new RaftTestCluster(created);
+    }
+
+    public Task<RaftNode> ElectLeaderAsync()
+    {
+        var candidate = Nodes[0];
+        candidate.StartElection(Nodes.Count);
+
+        foreach (var voter in Nodes.Skip(1))
+            candidate.ReceiveVote(voter.GrantVote(candidate.Term));
+
+        return Task.FromResult(candidate);
+    }
+}