feat: implement raft log replication and apply

src/NATS.Server/Raft/RaftLog.cs

@@ -0,0 +1,25 @@
+namespace NATS.Server.Raft;
+
+public sealed class RaftLog
+{
+    private readonly List<RaftLogEntry> _entries = [];
+
+    public IReadOnlyList<RaftLogEntry> Entries => _entries;
+
+    public RaftLogEntry Append(int term, string command)
+    {
+        var entry = new RaftLogEntry(_entries.Count + 1, term, command);
+        _entries.Add(entry);
+        return entry;
+    }
+
+    public void AppendReplicated(RaftLogEntry entry)
+    {
+        if (_entries.Any(e => e.Index == entry.Index))
+            return;
+
+        _entries.Add(entry);
+    }
+}
+
+public sealed record RaftLogEntry(long Index, int Term, string Command);

src/NATS.Server/Raft/RaftNode.cs

@@ -3,18 +3,27 @@ namespace NATS.Server.Raft;
 public sealed class RaftNode
 {
     private int _votesReceived;
+    private readonly List<RaftNode> _cluster = [];
+    private readonly RaftReplicator _replicator = new();
 
     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 RaftLog Log { get; } = new();
 
     public RaftNode(string id)
     {
         Id = id;
     }
 
+    public void ConfigureCluster(IEnumerable<RaftNode> peers)
+    {
+        _cluster.Clear();
+        _cluster.AddRange(peers);
+    }
+
     public void StartElection(int clusterSize)
     {
         Role = RaftRole.Candidate;
@@ -42,15 +51,41 @@ public sealed class RaftNode
         TryBecomeLeader(clusterSize);
     }
 
-    private void TryBecomeLeader(int clusterSize)
+    public async ValueTask<long> ProposeAsync(string command, CancellationToken ct)
     {
-        var quorum = (clusterSize / 2) + 1;
-        if (_votesReceived >= quorum)
-            Role = RaftRole.Leader;
+        if (Role != RaftRole.Leader)
+            throw new InvalidOperationException("Only leader can propose entries.");
+
+        var entry = Log.Append(TermState.CurrentTerm, command);
+        var followers = _cluster.Where(n => n.Id != Id).ToList();
+        var acknowledgements = _replicator.Replicate(entry, followers);
+
+        var quorum = (_cluster.Count / 2) + 1;
+        if (acknowledgements + 1 >= quorum)
+        {
+            AppliedIndex = entry.Index;
+            foreach (var node in _cluster)
+                node.AppliedIndex = Math.Max(node.AppliedIndex, entry.Index);
+        }
+
+        await Task.CompletedTask;
+        return entry.Index;
+    }
+
+    public void ReceiveReplicatedEntry(RaftLogEntry entry)
+    {
+        Log.AppendReplicated(entry);
     }
 
     public void RequestStepDown()
     {
         Role = RaftRole.Follower;
     }
+
+    private void TryBecomeLeader(int clusterSize)
+    {
+        var quorum = (clusterSize / 2) + 1;
+        if (_votesReceived >= quorum)
+            Role = RaftRole.Leader;
+    }
 }

src/NATS.Server/Raft/RaftReplicator.cs

@@ -0,0 +1,16 @@
+namespace NATS.Server.Raft;
+
+public sealed class RaftReplicator
+{
+    public int Replicate(RaftLogEntry entry, IReadOnlyList<RaftNode> followers)
+    {
+        var acknowledgements = 0;
+        foreach (var follower in followers)
+        {
+            follower.ReceiveReplicatedEntry(entry);
+            acknowledgements++;
+        }
+
+        return acknowledgements;
+    }
+}

tests/NATS.Server.Tests/RaftElectionTests.cs

@@ -27,6 +27,8 @@ internal sealed class RaftTestCluster
     public static RaftTestCluster Create(int nodes)
     {
         var created = Enumerable.Range(1, nodes).Select(i => new RaftNode($"n{i}")).ToList();
+        foreach (var node in created)
+            node.ConfigureCluster(created);
         return new RaftTestCluster(created);
     }
 
@@ -40,4 +42,16 @@ internal sealed class RaftTestCluster
 
         return Task.FromResult(candidate);
     }
+
+    public async Task WaitForAppliedAsync(long index)
+    {
+        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(2));
+        while (!timeout.IsCancellationRequested)
+        {
+            if (Nodes.All(n => n.AppliedIndex >= index))
+                return;
+
+            await Task.Delay(20, timeout.Token).ContinueWith(_ => { }, TaskScheduler.Default);
+        }
+    }
 }

tests/NATS.Server.Tests/RaftReplicationTests.cs

@@ -0,0 +1,19 @@
+using NATS.Server.Raft;
+
+namespace NATS.Server.Tests;
+
+public class RaftReplicationTests
+{
+    [Fact]
+    public async Task Leader_replicates_entry_to_quorum_and_applies()
+    {
+        var cluster = RaftTestCluster.Create(3);
+        var leader = await cluster.ElectLeaderAsync();
+
+        var idx = await leader.ProposeAsync("create-stream", default);
+        idx.ShouldBeGreaterThan(0);
+
+        await cluster.WaitForAppliedAsync(idx);
+        cluster.Nodes.All(n => n.AppliedIndex >= idx).ShouldBeTrue();
+    }
+}