feat: implement jsz and live jetstream monitoring

src/NATS.Server/Monitoring/JszHandler.cs

@@ -0,0 +1,62 @@
+using System.Text.Json.Serialization;
+
+namespace NATS.Server.Monitoring;
+
+public sealed class JszHandler
+{
+    private readonly NatsServer _server;
+    private readonly NatsOptions _options;
+
+    public JszHandler(NatsServer server, NatsOptions options)
+    {
+        _server = server;
+        _options = options;
+    }
+
+    public JszResponse Build()
+    {
+        return new JszResponse
+        {
+            ServerId = _server.ServerId,
+            Now = DateTime.UtcNow,
+            Enabled = _server.Stats.JetStreamEnabled,
+            Memory = 0,
+            Storage = 0,
+            Streams = _server.JetStreamStreams,
+            Consumers = _server.JetStreamConsumers,
+            Config = new JetStreamConfig
+            {
+                MaxMemory = _options.JetStream?.MaxMemoryStore ?? 0,
+                MaxStorage = _options.JetStream?.MaxFileStore ?? 0,
+                StoreDir = _options.JetStream?.StoreDir ?? string.Empty,
+            },
+        };
+    }
+}
+
+public sealed class JszResponse
+{
+    [JsonPropertyName("server_id")]
+    public string ServerId { get; set; } = string.Empty;
+
+    [JsonPropertyName("now")]
+    public DateTime Now { get; set; }
+
+    [JsonPropertyName("enabled")]
+    public bool Enabled { get; set; }
+
+    [JsonPropertyName("memory")]
+    public ulong Memory { get; set; }
+
+    [JsonPropertyName("storage")]
+    public ulong Storage { get; set; }
+
+    [JsonPropertyName("streams")]
+    public int Streams { get; set; }
+
+    [JsonPropertyName("consumers")]
+    public int Consumers { get; set; }
+
+    [JsonPropertyName("config")]
+    public JetStreamConfig Config { get; set; } = new();
+}

src/NATS.Server/Monitoring/MonitorServer.cs

@@ -16,6 +16,7 @@ public sealed class MonitorServer : IAsyncDisposable
     private readonly VarzHandler _varzHandler;
     private readonly ConnzHandler _connzHandler;
     private readonly SubszHandler _subszHandler;
+    private readonly JszHandler _jszHandler;
 
     public MonitorServer(NatsServer server, NatsOptions options, ServerStats stats, ILoggerFactory loggerFactory)
     {
@@ -31,6 +32,7 @@ public sealed class MonitorServer : IAsyncDisposable
         _varzHandler = new VarzHandler(server, options);
         _connzHandler = new ConnzHandler(server);
         _subszHandler = new SubszHandler(server);
+        _jszHandler = new JszHandler(server, options);
 
         _app.MapGet(basePath + "/", () =>
         {
@@ -100,7 +102,7 @@ public sealed class MonitorServer : IAsyncDisposable
         _app.MapGet(basePath + "/jsz", () =>
         {
             stats.HttpReqStats.AddOrUpdate("/jsz", 1, (_, v) => v + 1);
-            return Results.Ok(new { });
+            return Results.Ok(_jszHandler.Build());
         });
     }
 

src/NATS.Server/Monitoring/Varz.cs

@@ -422,6 +422,12 @@ public sealed class JetStreamStats
     [JsonPropertyName("ha_assets")]
     public int HaAssets { get; set; }
 
+    [JsonPropertyName("streams")]
+    public int Streams { get; set; }
+
+    [JsonPropertyName("consumers")]
+    public int Consumers { get; set; }
+
     [JsonPropertyName("api")]
     public JetStreamApiStats Api { get; set; } = new();
 }

src/NATS.Server/Monitoring/VarzHandler.cs

@@ -121,6 +121,22 @@ public sealed class VarzHandler : IDisposable
                 Subscriptions = _server.SubList.Count,
                 ConfigLoadTime = _server.StartTime,
                 HttpReqStats = stats.HttpReqStats.ToDictionary(kv => kv.Key, kv => (ulong)kv.Value),
+                JetStream = new JetStreamVarz
+                {
+                    Config = new JetStreamConfig
+                    {
+                        MaxMemory = _options.JetStream?.MaxMemoryStore ?? 0,
+                        MaxStorage = _options.JetStream?.MaxFileStore ?? 0,
+                        StoreDir = _options.JetStream?.StoreDir ?? string.Empty,
+                    },
+                    Stats = new JetStreamStats
+                    {
+                        Accounts = _options.JetStream is null ? 0 : 1,
+                        HaAssets = _server.JetStreamStreams,
+                        Streams = _server.JetStreamStreams,
+                        Consumers = _server.JetStreamConsumers,
+                    },
+                },
             };
         }
         finally

src/NATS.Server/NatsServer.cs

@@ -91,6 +91,8 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     public bool IsLameDuckMode => Volatile.Read(ref _lameDuck) != 0;
     public string? ClusterListen => _routeManager?.ListenEndpoint;
     public JetStreamApiRouter? JetStreamApiRouter => _jetStreamApiRouter;
+    public int JetStreamStreams => _jetStreamStreamManager?.StreamNames.Count ?? 0;
+    public int JetStreamConsumers => _jetStreamConsumerManager?.ConsumerCount ?? 0;
     public Action? ReOpenLogFile { get; set; }
     public IEnumerable<NatsClient> GetClients() => _clients.Values;
 

tests/NATS.Server.Tests/JszMonitorTests.cs

@@ -0,0 +1,112 @@
+using System.Net;
+using System.Net.Http.Json;
+using System.Net.Sockets;
+using System.Text;
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Server.Configuration;
+using NATS.Server.Monitoring;
+
+namespace NATS.Server.Tests;
+
+public class JszMonitorTests
+{
+    [Fact]
+    public async Task Jsz_reports_live_stream_and_consumer_counts()
+    {
+        await using var fixture = await JetStreamMonitoringFixture.StartWithStreamAndConsumerAsync();
+
+        var jsz = await fixture.GetJszAsync();
+        jsz.Streams.ShouldBeGreaterThan(0);
+        jsz.Consumers.ShouldBeGreaterThan(0);
+    }
+}
+
+internal sealed class JetStreamMonitoringFixture : IAsyncDisposable
+{
+    private readonly NatsServer _server;
+    private readonly int _monitorPort;
+    private readonly CancellationTokenSource _cts = new();
+    private readonly HttpClient _http = new();
+
+    private JetStreamMonitoringFixture(NatsServer server, int monitorPort)
+    {
+        _server = server;
+        _monitorPort = monitorPort;
+    }
+
+    public static async Task<JetStreamMonitoringFixture> StartWithStreamAndConsumerAsync()
+    {
+        var natsPort = GetFreePort();
+        var monitorPort = GetFreePort();
+        var options = new NatsOptions
+        {
+            Host = "127.0.0.1",
+            Port = natsPort,
+            MonitorHost = "127.0.0.1",
+            MonitorPort = monitorPort,
+            JetStream = new JetStreamOptions
+            {
+                StoreDir = Path.Combine(Path.GetTempPath(), "natsdotnet-jsz"),
+                MaxMemoryStore = 1_024 * 1_024,
+                MaxFileStore = 10 * 1_024 * 1_024,
+            },
+        };
+
+        var server = new NatsServer(options, NullLoggerFactory.Instance);
+        var fixture = new JetStreamMonitoringFixture(server, monitorPort);
+
+        _ = server.StartAsync(fixture._cts.Token);
+        await server.WaitForReadyAsync();
+        await fixture.WaitForHealthAsync();
+
+        var router = server.JetStreamApiRouter ?? throw new InvalidOperationException("JetStream API router unavailable.");
+        _ = router.Route("$JS.API.STREAM.CREATE.ORDERS", Encoding.UTF8.GetBytes("{\"name\":\"ORDERS\",\"subjects\":[\"orders.*\"]}"));
+        _ = router.Route("$JS.API.CONSUMER.CREATE.ORDERS.DUR", Encoding.UTF8.GetBytes("{\"durable_name\":\"DUR\",\"filter_subject\":\"orders.*\"}"));
+
+        return fixture;
+    }
+
+    public async Task<JszResponse> GetJszAsync()
+    {
+        var response = await _http.GetAsync($"http://127.0.0.1:{_monitorPort}/jsz");
+        response.StatusCode.ShouldBe(HttpStatusCode.OK);
+
+        var jsz = await response.Content.ReadFromJsonAsync<JszResponse>();
+        return jsz ?? throw new InvalidOperationException("Failed to deserialize /jsz.");
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        _http.Dispose();
+        await _cts.CancelAsync();
+        _server.Dispose();
+    }
+
+    private async Task WaitForHealthAsync()
+    {
+        for (int i = 0; i < 50; i++)
+        {
+            try
+            {
+                var response = await _http.GetAsync($"http://127.0.0.1:{_monitorPort}/healthz");
+                if (response.IsSuccessStatusCode)
+                    return;
+            }
+            catch (HttpRequestException)
+            {
+                // server not ready
+            }
+
+            await Task.Delay(50);
+        }
+
+        throw new TimeoutException("Monitoring endpoint did not become healthy.");
+    }
+
+    private static int GetFreePort()
+    {
+        using var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        sock.Bind(new System.Net.IPEndPoint(System.Net.IPAddress.Loopback, 0));
+        return ((System.Net.IPEndPoint)sock.LocalEndPoint!).Port;
+    }
+}