feat: implement strict raft consensus and convergence parity

src/NATS.Server/Raft/RaftNode.cs

@@ -49,12 +49,24 @@ public sealed class RaftNode
         TryBecomeLeader(clusterSize);
     }
 
-    public VoteResponse GrantVote(int term)
+    public VoteResponse GrantVote(int term, string candidateId = "")
     {
         if (term < TermState.CurrentTerm)
             return new VoteResponse { Granted = false };
 
-        TermState.CurrentTerm = term;
+        if (term > TermState.CurrentTerm)
+        {
+            TermState.CurrentTerm = term;
+            TermState.VotedFor = null;
+        }
+
+        if (!string.IsNullOrEmpty(TermState.VotedFor)
+            && !string.Equals(TermState.VotedFor, candidateId, StringComparison.Ordinal))
+        {
+            return new VoteResponse { Granted = false };
+        }
+
+        TermState.VotedFor = candidateId;
         return new VoteResponse { Granted = true };
     }
 

src/NATS.Server/Raft/RaftSnapshotStore.cs

@@ -1,17 +1,41 @@
+using System.Text.Json;
+
 namespace NATS.Server.Raft;
 
 public sealed class RaftSnapshotStore
 {
     private RaftSnapshot? _snapshot;
+    private readonly string? _snapshotPath;
+
+    public RaftSnapshotStore(string? snapshotPath = null)
+    {
+        _snapshotPath = snapshotPath;
+    }
 
     public Task SaveAsync(RaftSnapshot snapshot, CancellationToken ct)
     {
         _snapshot = snapshot;
+        if (!string.IsNullOrWhiteSpace(_snapshotPath))
+        {
+            var dir = Path.GetDirectoryName(_snapshotPath);
+            if (!string.IsNullOrWhiteSpace(dir))
+                Directory.CreateDirectory(dir);
+
+            File.WriteAllText(_snapshotPath, JsonSerializer.Serialize(snapshot));
+        }
+
         return Task.CompletedTask;
     }
 
     public Task<RaftSnapshot?> LoadAsync(CancellationToken ct)
     {
+        if (_snapshot == null
+            && !string.IsNullOrWhiteSpace(_snapshotPath)
+            && File.Exists(_snapshotPath))
+        {
+            _snapshot = JsonSerializer.Deserialize<RaftSnapshot>(File.ReadAllText(_snapshotPath));
+        }
+
         return Task.FromResult(_snapshot);
     }
 }

src/NATS.Server/Raft/RaftTransport.cs

@@ -38,7 +38,7 @@ public sealed class InMemoryRaftTransport : IRaftTransport
     public Task<VoteResponse> RequestVoteAsync(string candidateId, string voterId, VoteRequest request, CancellationToken ct)
     {
         if (_nodes.TryGetValue(voterId, out var node))
-            return Task.FromResult(node.GrantVote(request.Term));
+            return Task.FromResult(node.GrantVote(request.Term, string.IsNullOrWhiteSpace(request.CandidateId) ? candidateId : request.CandidateId));
 
         return Task.FromResult(new VoteResponse { Granted = false });
     }