src/ZB.MOM.WW.ScadaBridge.Host/Program.cs

@@ -7,6 +7,7 @@ using ZB.MOM.WW.Auth.AspNetCore;
 using ZB.MOM.WW.Health;
 using ZB.MOM.WW.Health.Akka;
 using ZB.MOM.WW.Health.EntityFrameworkCore;
+using ZB.MOM.WW.LocalDb.Replication;
 using ZB.MOM.WW.ScadaBridge.AuditLog;
 using ZB.MOM.WW.ScadaBridge.CentralUI;
 using ZB.MOM.WW.ScadaBridge.ClusterInfrastructure;
@@ -516,7 +517,8 @@ try
         builder.Services.AddGrpc();
         builder.Services.AddSingleton<ZB.MOM.WW.ScadaBridge.Communication.Grpc.SiteStreamGrpcServer>();
 
-        // Existing site service registrations
+        // Existing site service registrations (this is also where LocalDb and its
+        // replication engine are registered — see SiteServiceRegistration)
         SiteServiceRegistration.Configure(builder.Services, builder.Configuration);
 
         var app = builder.Build();
@@ -535,6 +537,14 @@ try
         // Map gRPC service — resolves the singleton SiteStreamGrpcServer from DI
         app.MapGrpcService<ZB.MOM.WW.ScadaBridge.Communication.Grpc.SiteStreamGrpcServer>();
 
+        // The passive half of LocalDb replication: the peer node dials THIS endpoint.
+        // It shares the Kestrel h2c listener the site gRPC server already uses, so no
+        // listener or port changes are needed. Mapping it is harmless with no peer
+        // configured — nothing dials it. Inbound auth is the host's job and lands in
+        // Task 8; until then this endpoint is unauthenticated, which is why
+        // replication ships default-OFF.
+        app.MapZbLocalDbSync();
+
         // Site-shutdown ordering. ApplicationStopping
         // fires BEFORE IHostedService.StopAsync runs, so the gRPC server
         // refuses new streams (Unavailable) and cancels every active stream

src/ZB.MOM.WW.ScadaBridge.Host/SiteServiceRegistration.cs

@@ -15,6 +15,7 @@ using ZB.MOM.WW.ScadaBridge.SiteEventLogging;
 using ZB.MOM.WW.ScadaBridge.SiteRuntime;
 using ZB.MOM.WW.ScadaBridge.StoreAndForward;
 using ZB.MOM.WW.LocalDb;
+using ZB.MOM.WW.LocalDb.Replication;
 using ZB.MOM.WW.Secrets.DependencyInjection;
 using ZB.MOM.WW.Telemetry;
 
@@ -56,12 +57,22 @@ public static class SiteServiceRegistration
         //
         // Storage is registered UNCONDITIONALLY and needs no flag: with no peer
         // configured, LocalDb is simply a local SQLite file and the replication
-        // initiator idles. Replication itself is opt-in via LocalDb:Replication:PeerAddress
-        // and is wired in Program.cs, which owns the gRPC host the sync endpoint needs.
+        // initiator idles.
         //
         // Design: scadaproj docs/plans/2026-07-19-scadabridge-localdb-design.md
         services.AddZbLocalDb(config, SiteLocalDbSetup.OnReady);
 
+        // The replication engine, likewise unconditional but INERT by default: with no
+        // LocalDb:Replication:PeerAddress the initiating SyncBackgroundService starts and
+        // immediately idles, and nothing dials the passive endpoint. A site pair
+        // replicates only once an operator sets a peer on both nodes.
+        //
+        // Registered HERE rather than in Program.cs (where the plan first put it) so the
+        // composition-root tests, which build this graph and not Program.cs, actually
+        // cover it. The endpoint half — MapZbLocalDbSync — has to stay in Program.cs
+        // because it needs the WebApplication.
+        services.AddZbLocalDbReplication(config);
+
         services.AddDataConnectionLayer();
         // Local SQLite store by default; a local store replicating against a shared SQL-Server hub
         // only when Secrets:Replication:Enabled is true AND a hub connection string is present.

tests/ZB.MOM.WW.ScadaBridge.Host.Tests/SiteLocalDbWiringTests.cs

@@ -2,6 +2,7 @@ using Microsoft.AspNetCore.Builder;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using ZB.MOM.WW.LocalDb;
+using ZB.MOM.WW.LocalDb.Replication;
 using ZB.MOM.WW.ScadaBridge.Host;
 using ZB.MOM.WW.ScadaBridge.Host.Actors;
 
@@ -120,6 +121,35 @@ public class SiteLocalDbWiringTests : IDisposable
         Assert.Same(first, second);
     }
 
+    [Fact]
+    public void Site_Replication_IsRegistered_AndInertWithNoPeer()
+    {
+        // Task 7's default-OFF pin. This fixture configures NO
+        // LocalDb:Replication:PeerAddress, which is the shipping default, so the
+        // engine must resolve cleanly and sit idle rather than throw, retry, or
+        // report a connection. "Default-off" here means inert, NOT unregistered —
+        // the services are always in the graph.
+        var status = _host.Services.GetService<ISyncStatus>();
+
+        Assert.NotNull(status);
+        Assert.False(status!.Connected);
+        Assert.Null(status.PeerNodeId);
+        Assert.Null(status.LastSyncUtc);
+    }
+
+    [Fact]
+    public void Site_Replication_OplogBacklog_IsZero_NotNull_OnAHealthyIdleNode()
+    {
+        // OplogBacklog is deliberately nullable: null means "unknown" (no provider
+        // wired, or the poll failed) and must never be reported as a healthy 0. On a
+        // node whose local database is present and readable, a real 0 proves the
+        // backlog provider is actually wired to the oplog — a null here would mean
+        // Task 9's health signal is about to publish "unknown" forever.
+        var status = _host.Services.GetRequiredService<ISyncStatus>();
+
+        Assert.Equal(0L, status.OplogBacklog);
+    }
+
     [Fact]
     public void Site_LocalDb_CreatesTheConfiguredFile()
     {