From 70ffd2849df030773fbefb89ca0f6f1532a8590a Mon Sep 17 00:00:00 2001
From: Joseph Doherty <dohejw01@gmail.com>
Date: Tue, 26 May 2026 11:21:11 -0400
Subject: [PATCH] feat(opcua): OpcUaApplicationHost publishes peer URIs in
 Server.ServerArray

---
 .../OpcUaApplicationHost.cs                   | 30 ++++++++++
 .../OpcUaApplicationHostServerArrayTests.cs   | 59 +++++++++++++++++++
 2 files changed, 89 insertions(+)
 create mode 100644 tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/OpcUaApplicationHostServerArrayTests.cs

diff --git a/src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OpcUaApplicationHost.cs b/src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OpcUaApplicationHost.cs
index 20978b9..0509e60 100644
--- a/src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OpcUaApplicationHost.cs
+++ b/src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OpcUaApplicationHost.cs
@@ -63,6 +63,15 @@ public sealed class OpcUaApplicationHostOptions
     /// the Admin UI). Has no effect on <c>None</c> endpoints, which don't exchange certs.
     /// </summary>
     public bool AutoAcceptUntrustedClientCertificates { get; set; }
+
+    /// <summary>
+    /// Peer server URIs published in <c>Server.ServerArray</c> after start, in addition to
+    /// the local <see cref="ApplicationUri"/>. Empty by default — set this on warm-redundancy
+    /// deployments so OPC UA clients can discover the partner endpoint via the standard
+    /// Server.ServerArray property (NodeId i=2254). Order does not matter; the local URI
+    /// is always element 0.
+    /// </summary>
+    public IList<string> PeerApplicationUris { get; set; } = new List<string>();
 }
 
 /// <summary>
@@ -112,6 +121,7 @@ public sealed class OpcUaApplicationHost : IAsyncDisposable
         await _application.Start(server).ConfigureAwait(false);
 
         AttachUserAuthenticator();
+        PopulateServerArray();
 
         _logger.LogInformation("OPC UA server started on opc.tcp://{Host}:{Port}",
             _options.PublicHostname, _options.OpcUaPort);
@@ -143,6 +153,26 @@ public sealed class OpcUaApplicationHost : IAsyncDisposable
         sessionManager.ImpersonateUser += _impersonateHandler;
     }
 
+    /// <summary>
+    /// Writes the union of <see cref="OpcUaApplicationHostOptions.ApplicationUri"/> and
+    /// <see cref="OpcUaApplicationHostOptions.PeerApplicationUris"/> to the OPC UA standard
+    /// <c>Server.ServerArray</c> property (NodeId i=2254). Clients in a warm-redundancy
+    /// deployment discover the partner endpoint by reading this property.
+    /// </summary>
+    private void PopulateServerArray()
+    {
+        var serverObject = _server?.CurrentInstance?.ServerObject;
+        if (serverObject is null) return;
+
+        var uris = new List<string> { _options.ApplicationUri };
+        foreach (var peer in _options.PeerApplicationUris)
+        {
+            if (!string.IsNullOrWhiteSpace(peer) && !uris.Contains(peer))
+                uris.Add(peer);
+        }
+        serverObject.ServerArray.Value = uris.ToArray();
+    }
+
     private void OnImpersonateUser(Session session, ImpersonateEventArgs args) =>
         HandleImpersonation(_userAuthenticator, args, _logger);
 
diff --git a/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/OpcUaApplicationHostServerArrayTests.cs b/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/OpcUaApplicationHostServerArrayTests.cs
new file mode 100644
index 0000000..c95d811
--- /dev/null
+++ b/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/OpcUaApplicationHostServerArrayTests.cs
@@ -0,0 +1,59 @@
+using System.IO;
+using System.Net.Sockets;
+using System.Net;
+using Microsoft.Extensions.Logging.Abstractions;
+using Opc.Ua;
+using Opc.Ua.Server;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.OpcUaServer;
+
+namespace ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests;
+
+/// <summary>
+/// Audit gap closeout — verifies <see cref="OpcUaApplicationHostOptions.PeerApplicationUris"/>
+/// is reflected in <c>Server.ServerArray</c> after start. Single-server in-process check; the
+/// cross-server visibility check lives in <c>OtOpcUa.OpcUaServer.IntegrationTests</c>.
+/// </summary>
+public sealed class OpcUaApplicationHostServerArrayTests
+{
+    [Fact]
+    public async Task ServerArray_contains_local_uri_and_configured_peers_after_start()
+    {
+        var pkiRoot = Path.Combine(Path.GetTempPath(), $"otopcua-pki-{Guid.NewGuid():N}");
+        try
+        {
+            var options = new OpcUaApplicationHostOptions
+            {
+                ApplicationName = "OtOpcUa.UnitTest",
+                ApplicationUri = "urn:OtOpcUa.UnitTest.NodeA",
+                OpcUaPort = AllocateFreePort(),
+                PublicHostname = "127.0.0.1",
+                PkiStoreRoot = pkiRoot,
+                PeerApplicationUris = new[] { "urn:OtOpcUa.UnitTest.NodeB" },
+            };
+
+            var server = new StandardServer();
+            await using var host = new OpcUaApplicationHost(options, NullLogger<OpcUaApplicationHost>.Instance);
+            await host.StartAsync(server, CancellationToken.None);
+
+            var serverArray = server.CurrentInstance.ServerObject.ServerArray.Value;
+            serverArray.ShouldNotBeNull();
+            serverArray.ShouldContain("urn:OtOpcUa.UnitTest.NodeA");
+            serverArray.ShouldContain("urn:OtOpcUa.UnitTest.NodeB");
+        }
+        finally
+        {
+            if (Directory.Exists(pkiRoot)) Directory.Delete(pkiRoot, recursive: true);
+        }
+    }
+
+    private static int AllocateFreePort()
+    {
+        var listener = new TcpListener(IPAddress.Loopback, 0);
+        listener.Start();
+        var port = ((IPEndPoint)listener.LocalEndpoint).Port;
+        listener.Stop();
+        return port;
+    }
+}