feat: execute post-baseline jetstream parity plan

src/NATS.Server/Raft/RaftNode.cs

@@ -8,11 +8,13 @@ public sealed class RaftNode
     private readonly RaftSnapshotStore _snapshotStore = new();
     private readonly IRaftTransport? _transport;
     private readonly string? _persistDirectory;
+    private readonly HashSet<string> _members = new(StringComparer.Ordinal);
 
     public string Id { get; }
     public int Term => TermState.CurrentTerm;
     public bool IsLeader => Role == RaftRole.Leader;
     public RaftRole Role { get; private set; } = RaftRole.Follower;
+    public IReadOnlyCollection<string> Members => _members;
     public RaftTermState TermState { get; } = new();
     public long AppliedIndex { get; set; }
     public RaftLog Log { get; private set; } = new();
@@ -22,14 +24,22 @@ public sealed class RaftNode
         Id = id;
         _transport = transport;
         _persistDirectory = persistDirectory;
+        _members.Add(id);
     }
 
     public void ConfigureCluster(IEnumerable<RaftNode> peers)
     {
         _cluster.Clear();
         _cluster.AddRange(peers);
+        _members.Clear();
+        foreach (var peer in peers)
+            _members.Add(peer.Id);
     }
 
+    public void AddMember(string memberId) => _members.Add(memberId);
+
+    public void RemoveMember(string memberId) => _members.Remove(memberId);
+
     public void StartElection(int clusterSize)
     {
         Role = RaftRole.Candidate;
@@ -48,6 +58,15 @@ public sealed class RaftNode
         return new VoteResponse { Granted = true };
     }
 
+    public void ReceiveHeartbeat(int term)
+    {
+        if (term < TermState.CurrentTerm)
+            return;
+
+        TermState.CurrentTerm = term;
+        Role = RaftRole.Follower;
+    }
+
     public void ReceiveVote(VoteResponse response, int clusterSize = 3)
     {
         if (!response.Granted)

src/NATS.Server/Raft/RaftReplicator.cs

@@ -2,6 +2,9 @@ namespace NATS.Server.Raft;
 
 public sealed class RaftReplicator
 {
+    public static long BacktrackNextIndex(long nextIndex)
+        => Math.Max(1, nextIndex - 1);
+
     public int Replicate(RaftLogEntry entry, IReadOnlyList<RaftNode> followers)
     {
         var acknowledgements = 0;

src/NATS.Server/Raft/RaftTransport.cs

@@ -4,6 +4,7 @@ public interface IRaftTransport
 {
     Task<IReadOnlyList<AppendResult>> AppendEntriesAsync(string leaderId, IReadOnlyList<string> followerIds, RaftLogEntry entry, CancellationToken ct);
     Task<VoteResponse> RequestVoteAsync(string candidateId, string voterId, VoteRequest request, CancellationToken ct);
+    Task InstallSnapshotAsync(string leaderId, string followerId, RaftSnapshot snapshot, CancellationToken ct);
 }
 
 public sealed class InMemoryRaftTransport : IRaftTransport
@@ -41,4 +42,23 @@ public sealed class InMemoryRaftTransport : IRaftTransport
 
         return Task.FromResult(new VoteResponse { Granted = false });
     }
+
+    public async Task InstallSnapshotAsync(string leaderId, string followerId, RaftSnapshot snapshot, CancellationToken ct)
+    {
+        _ = leaderId;
+        if (_nodes.TryGetValue(followerId, out var node))
+            await node.InstallSnapshotAsync(snapshot, ct);
+    }
+
+    public async Task AppendHeartbeatAsync(string leaderId, IReadOnlyList<string> followerIds, int term, CancellationToken ct)
+    {
+        _ = leaderId;
+        foreach (var followerId in followerIds)
+        {
+            if (_nodes.TryGetValue(followerId, out var node))
+                node.ReceiveHeartbeat(term);
+        }
+
+        await Task.CompletedTask;
+    }
 }