feat: add client status

examples/WebApiApp/FeatBitHealthCheck.cs

@@ -0,0 +1,30 @@
+using FeatBit.Sdk.Server;
+using Microsoft.Extensions.Diagnostics.HealthChecks;
+
+namespace WebApiApp;
+
+public class FeatBitHealthCheck : IHealthCheck
+{
+    private readonly IFbClient _fbClient;
+
+    public FeatBitHealthCheck(IFbClient fbClient)
+    {
+        _fbClient = fbClient;
+    }
+
+    public Task<HealthCheckResult> CheckHealthAsync(
+        HealthCheckContext context,
+        CancellationToken cancellationToken = default)
+    {
+        var status = _fbClient.Status;
+
+        var result = status switch
+        {
+            FbClientStatus.Ready => HealthCheckResult.Healthy(),
+            FbClientStatus.Stale => HealthCheckResult.Degraded(),
+            _ => HealthCheckResult.Unhealthy()
+        };
+
+        return Task.FromResult(result);
+    }
+}

examples/WebApiApp/Program.cs

@@ -1,6 +1,9 @@
 using FeatBit.Sdk.Server;
 using FeatBit.Sdk.Server.Model;
 using FeatBit.Sdk.Server.DependencyInjection;
+using HealthChecks.UI.Client;
+using Microsoft.AspNetCore.Diagnostics.HealthChecks;
+using WebApiApp;
 
 var builder = WebApplication.CreateBuilder(args);
 
@@ -13,8 +16,16 @@
     options.StartWaitTime = TimeSpan.FromSeconds(3);
 });
 
+builder.Services.AddHealthChecks()
+    .AddCheck<FeatBitHealthCheck>("FeatBit");
+
 var app = builder.Build();
 
+app.MapHealthChecks("/healthz", new HealthCheckOptions
+{
+    ResponseWriter = UIResponseWriter.WriteHealthCheckUIResponse
+});
+
 // curl -X GET --location "http://localhost:5014/variation-detail/game-runner?fallbackValue=lol"
 app.MapGet("/variation-detail/{flagKey}", (IFbClient fbClient, string flagKey, string fallbackValue) =>
 {

examples/WebApiApp/WebApiApp.csproj

@@ -7,13 +7,12 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="FeatBit.ServerSdk" Version="1.1.4"/>
+        <!--        <PackageReference Include="FeatBit.ServerSdk" Version="1.1.4"/>-->
+        <PackageReference Include="AspNetCore.HealthChecks.UI.Client" Version="6.0.5"/>
     </ItemGroup>
 
-    <!--
-        <ItemGroup>
-            <ProjectReference Include="..\..\src\FeatBit.ServerSdk\FeatBit.ServerSdk.csproj"/>
-        </ItemGroup>
-    -->
+    <ItemGroup>
+        <ProjectReference Include="..\..\src\FeatBit.ServerSdk\FeatBit.ServerSdk.csproj"/>
+    </ItemGroup>
 
 </Project>

src/FeatBit.ServerSdk/Concurrent/AtomicBoolean.cs

@@ -10,7 +10,7 @@ namespace FeatBit.Sdk.Server.Concurrent
     /// without any explicit locking. .NET's strong memory on write guarantees might already enforce
     /// this ordering, but the addition of the MemoryBarrier guarantees it.
     /// </summary>
-    public class AtomicBoolean
+    public sealed class AtomicBoolean
     {
         private const int FalseValue = 0;
         private const int TrueValue = 1;

src/FeatBit.ServerSdk/Concurrent/StatusManager.cs

@@ -0,0 +1,56 @@
+using System;
+using System.Collections.Generic;
+
+namespace FeatBit.Sdk.Server.Concurrent;
+
+public sealed class StatusManager<TStatus> where TStatus : Enum
+{
+    private TStatus _status;
+    private readonly object _statusLock = new object();
+    private readonly Action<TStatus> _onStatusChanged;
+
+    public StatusManager(TStatus initialStatus, Action<TStatus> onStatusChanged = null)
+    {
+        _status = initialStatus;
+        _onStatusChanged = onStatusChanged;
+    }
+
+    public TStatus Status
+    {
+        get
+        {
+            lock (_statusLock)
+            {
+                return _status;
+            }
+        }
+    }
+
+    public bool CompareAndSet(TStatus expected, TStatus newStatus)
+    {
+        lock (_statusLock)
+        {
+            if (!EqualityComparer<TStatus>.Default.Equals(_status, expected))
+            {
+                return false;
+            }
+
+            SetStatus(newStatus);
+            return true;
+        }
+    }
+
+    public void SetStatus(TStatus newStatus)
+    {
+        lock (_statusLock)
+        {
+            if (EqualityComparer<TStatus>.Default.Equals(_status, newStatus))
+            {
+                return;
+            }
+
+            _status = newStatus;
+            _onStatusChanged?.Invoke(_status);
+        }
+    }
+}

src/FeatBit.ServerSdk/DataSynchronizer/DataSynchronizerStatus.cs

@@ -0,0 +1,45 @@
+namespace FeatBit.Sdk.Server.DataSynchronizer;
+
+public enum DataSynchronizerStatus
+{
+    /// <summary>
+    /// The initial state of the synchronizer when the SDK is being initialized.
+    /// </summary>
+    /// <remarks>
+    /// If it encounters an error that requires it to retry initialization, the state will remain at
+    /// <see cref="Starting"/> until it either succeeds and becomes <see cref="Stable"/>, or
+    /// permanently fails and becomes <see cref="Stopped"/>.
+    /// </remarks>
+    Starting,
+
+    /// <summary>
+    /// Indicates that the synchronizer is currently operational and has not had any problems since the
+    /// last time it received data.
+    /// </summary>
+    /// <remarks>
+    /// In streaming mode, this means that there is currently an open stream connection and that at least
+    /// one initial message has been received on the stream. In polling mode, it means that the last poll
+    /// request succeeded.
+    /// </remarks>
+    Stable,
+
+    /// <summary>
+    /// Indicates that the synchronizer encountered an error that it will attempt to recover from.
+    /// </summary>
+    /// <remarks>
+    /// In streaming mode, this means that the stream connection failed, or had to be dropped due to some
+    /// other error, and will be retried after a backoff delay. In polling mode, it means that the last poll
+    /// request failed, and a new poll request will be made after the configured polling interval.
+    /// </remarks>
+    Interrupted,
+
+    /// <summary>
+    /// Indicates that the synchronizer has been permanently shut down.
+    /// </summary>
+    /// <remarks>
+    /// This could be because it encountered an unrecoverable error (for instance, the Evaluation server
+    /// rejected the SDK key: an invalid SDK key will never become valid), or because the SDK client was
+    /// explicitly shut down.
+    /// </remarks>
+    Stopped
+}

src/FeatBit.ServerSdk/DataSynchronizer/IDataSynchronizer.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Threading.Tasks;
 
 namespace FeatBit.Sdk.Server.DataSynchronizer
@@ -9,6 +10,24 @@ public interface IDataSynchronizer
         /// </summary>
         public bool Initialized { get; }
 
+        /// <summary>
+        /// The current status of the data synchronizer.
+        /// </summary>
+        public DataSynchronizerStatus Status { get; }
+
+        /// <summary>An event for receiving notifications of status changes.</summary>
+        /// <remarks>
+        /// <para>
+        /// Any handlers attached to this event will be notified whenever any property of the status has changed.
+        /// See <see cref="T:FeatBit.Sdk.Server.DataSynchronizer.DataSynchronizerStatus" /> for an explanation of the meaning of each property and what could cause it
+        /// to change.
+        /// </para>
+        /// <para>
+        /// The listener should return as soon as possible so as not to block subsequent notifications.
+        /// </para>
+        /// </remarks>
+        event Action<DataSynchronizerStatus> StatusChanged;
+
         /// <summary>
         /// Starts the data synchronizer. This is called once from the <see cref="FbClient"/> constructor.
         /// </summary>

src/FeatBit.ServerSdk/DataSynchronizer/NullDataSynchronizer.cs

@@ -1,18 +1,36 @@
+using System;
 using System.Threading.Tasks;
+using FeatBit.Sdk.Server.Concurrent;
 
 namespace FeatBit.Sdk.Server.DataSynchronizer;
 
 internal sealed class NullDataSynchronizer : IDataSynchronizer
 {
+    private readonly StatusManager<DataSynchronizerStatus> _statusManager;
+
     public bool Initialized => true;
+    public DataSynchronizerStatus Status => _statusManager.Status;
+    public event Action<DataSynchronizerStatus> StatusChanged;
+
+    public NullDataSynchronizer()
+    {
+        _statusManager = new StatusManager<DataSynchronizerStatus>(
+            DataSynchronizerStatus.Stable,
+            OnStatusChanged
+        );
+    }
 
     public Task<bool> StartAsync()
     {
+        _statusManager.SetStatus(DataSynchronizerStatus.Stable);
         return Task.FromResult(true);
     }
 
     public Task StopAsync()
     {
+        _statusManager.SetStatus(DataSynchronizerStatus.Stopped);
         return Task.CompletedTask;
     }
+
+    private void OnStatusChanged(DataSynchronizerStatus status) => StatusChanged?.Invoke(status);
 }