Phase 6.3 Stream A — RedundancyTopology + ClusterTopologyLoader + RedundancyCoordinator

Lands the data path that feeds the Phase 6.3 ServiceLevelCalculator shipped in
PR #89. OPC UA node wiring (ServiceLevel variable + ServerUriArray +
RedundancySupport) still deferred to task #147; peer-probe loops (Stream B.1/B.2
runtime layer beyond the calculator logic) deferred.

Server.Redundancy additions:
- RedundancyTopology record — immutable snapshot (ClusterId, SelfNodeId,
  SelfRole, Mode, Peers[], SelfApplicationUri). ServerUriArray() emits the
  OPC UA Part 4 §6.6.2.2 shape (self first, peers lexicographically by
  NodeId). RedundancyPeer record with per-peer Host/OpcUaPort/DashboardPort/
  ApplicationUri so the follow-up peer-probe loops know where to probe.
- ClusterTopologyLoader — pure fn from ServerCluster + ClusterNode[] to
  RedundancyTopology. Enforces Phase 6.3 Stream A.1 invariants:
    * At least one node per cluster.
    * At most 2 nodes (decision #83, v2.0 cap).
    * Every node belongs to the target cluster.
    * Unique ApplicationUri across the cluster (OPC UA Part 4 trust pin,
      decision #86).
    * At most 1 Primary per cluster in Warm/Hot modes (decision #84).
    * Self NodeId must be a member of the cluster.
  Violations throw InvalidTopologyException with a decision-ID-tagged message
  so operators know which invariant + what to fix.
- RedundancyCoordinator singleton — holds the current topology + IsTopologyValid
  flag. InitializeAsync throws on invariant violation (startup fails fast).
  RefreshAsync logs + flips IsTopologyValid=false (runtime won't tear down a
  running server; ServiceLevelCalculator falls to InvalidTopology band = 2
  which surfaces the problem to clients without crashing). CAS-style swap
  via Volatile.Write so readers always see a coherent snapshot.

Tests (10 new ClusterTopologyLoaderTests):
- Single-node standalone loads + empty peer list.
- Two-node cluster loads self + peer.
- ServerUriArray puts self first + peers sort lexicographically.
- Empty-nodes throws.
- Self-not-in-cluster throws.
- Three-node cluster rejected with decision #83 message.
- Duplicate ApplicationUri rejected with decision #86 shape reference.
- Two Primaries in Warm mode rejected (decision #84 + runtime-band reference).
- Cross-cluster node rejected.
- None-mode allows any role mix (standalone clusters don't enforce Primary count).

Full solution dotnet test: 1178 passing (was 1168, +10). Pre-existing
Client.CLI Subscribe flake unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/v2/v2-release-readiness.md

@@ -1,7 +1,7 @@
 # v2 Release Readiness
 
-> **Last updated**: 2026-04-19 (release blocker #1 closed — Phase 6.2 dispatch wiring shipped)
-> **Status**: **NOT YET RELEASE-READY** — two of three release blockers remain (Phase 6.1 Stream D config-cache wiring + Phase 6.3 Streams A/C/F redundancy runtime).
+> **Last updated**: 2026-04-19 (release blockers #1 + #2 closed; Phase 6.3 redundancy runtime is the last)
+> **Status**: **NOT YET RELEASE-READY** — one of three release blockers remains (Phase 6.3 Streams A/C/F redundancy-coordinator + OPC UA node wiring + client interop).
 
 This doc is the single view of where v2 stands against its release criteria. Update it whenever a deferred follow-up closes or a new release blocker is discovered.
 
@@ -41,15 +41,16 @@ Additional Stream C surfaces (not release-blocking, hardening only):
 
 These are additional hardening — the three highest-value surfaces (Read / Write / HistoryRead) are now gated, which covers the base-security gap for v2 GA.
 
-### Config fallback — Phase 6.1 Stream D wiring (task #136)
+### ~~Config fallback — Phase 6.1 Stream D wiring~~ (task #136 — **CLOSED** 2026-04-19, PR #96)
 
-`ResilientConfigReader` + `GenerationSealedCache` + `StaleConfigFlag` all exist but nothing consumes them. The `NodeBootstrap` path still uses the original single-file `LiteDbConfigCache` via `ILocalConfigCache`; `sp_PublishGeneration` doesn't call `GenerationSealedCache.SealAsync` after commit; the Configuration read services don't wrap queries in `ResilientConfigReader.ReadAsync`.
+**Closed**. `SealedBootstrap` consumes `ResilientConfigReader` + `GenerationSealedCache` + `StaleConfigFlag` end-to-end: bootstrap calls go through the timeout → retry → fallback-to-sealed pipeline; every central-DB success writes a fresh sealed snapshot so the next cache-miss has a known-good fallback; `StaleConfigFlag.IsStale` is now consumed by `HealthEndpointsHost.usingStaleConfig` so `/healthz` body reports reality.
 
-Closing this requires:
+Production activation: Program.cs switches `NodeBootstrap → SealedBootstrap` + constructs `OpcUaApplicationHost` with the `StaleConfigFlag` as an optional ctor parameter.
 
-- `sp_PublishGeneration` (or its EF-side wrapper) calls `SealAsync` after successful commit.
-- DriverInstance enumeration, LdapGroupRoleMapping fetches, cluster + namespace metadata reads route through `ResilientConfigReader.ReadAsync`.
-- Integration test: SQL container kill mid-operation → serves sealed snapshot, `UsingStaleConfig` = true, driver stays Healthy, `/healthz` body reflects the flag.
+Remaining follow-ups (hardening, not release-blocking):
+
+- A `HostedService` that polls `sp_GetCurrentGenerationForCluster` periodically so peer-published generations land in this node's cache without a restart.
+- Richer snapshot payload via `sp_GetGenerationContent` so fallback can serve the full generation content (DriverInstance enumeration, ACL rows, etc.) from the sealed cache alone.
 
 ### Redundancy — Phase 6.3 Streams A/C/F (tasks #145, #147, #150)
 
@@ -97,6 +98,7 @@ v2 GA requires all of the following:
 
 ## Change log
 
+- **2026-04-19** — Release blocker #2 **closed** (PR #96). `SealedBootstrap` consumes `ResilientConfigReader` + `GenerationSealedCache` + `StaleConfigFlag`; `/healthz` now surfaces the stale flag. Remaining follow-ups (periodic poller + richer snapshot payload) downgraded to hardening.
 - **2026-04-19** — Release blocker #1 **closed** (PR #94). `AuthorizationGate` wired into `DriverNodeManager` Read / Write / HistoryRead dispatch. Remaining Stream C surfaces (Browse / Subscribe / Alarm / Call + finer-grained scope resolution) downgraded to hardening follow-ups — no longer release-blocking.
 - **2026-04-19** — Phase 6.4 data layer merged (PRs #91–92). Phase 6 core complete. Capstone doc created.
 - **2026-04-19** — Phase 6.3 core merged (PRs #89–90). `ServiceLevelCalculator` + `RecoveryStateManager` + `ApplyLeaseRegistry` land as pure logic; coordinator / UA-node wiring / Admin UI / interop deferred.

src/ZB.MOM.WW.OtOpcUa.Server/OpcUa/OpcUaApplicationHost.cs

@@ -1,6 +1,7 @@
 using Microsoft.Extensions.Logging;
 using Opc.Ua;
 using Opc.Ua.Configuration;
+using ZB.MOM.WW.OtOpcUa.Configuration.LocalCache;
 using ZB.MOM.WW.OtOpcUa.Core.Hosting;
 using ZB.MOM.WW.OtOpcUa.Core.OpcUa;
 using ZB.MOM.WW.OtOpcUa.Core.Resilience;
@@ -25,6 +26,7 @@ public sealed class OpcUaApplicationHost : IAsyncDisposable
     private readonly DriverResiliencePipelineBuilder _pipelineBuilder;
     private readonly AuthorizationGate? _authzGate;
     private readonly NodeScopeResolver? _scopeResolver;
+    private readonly StaleConfigFlag? _staleConfigFlag;
     private readonly ILoggerFactory _loggerFactory;
     private readonly ILogger<OpcUaApplicationHost> _logger;
     private ApplicationInstance? _application;
@@ -36,7 +38,8 @@ public sealed class OpcUaApplicationHost : IAsyncDisposable
         IUserAuthenticator authenticator, ILoggerFactory loggerFactory, ILogger<OpcUaApplicationHost> logger,
         DriverResiliencePipelineBuilder? pipelineBuilder = null,
         AuthorizationGate? authzGate = null,
-        NodeScopeResolver? scopeResolver = null)
+        NodeScopeResolver? scopeResolver = null,
+        StaleConfigFlag? staleConfigFlag = null)
     {
         _options = options;
         _driverHost = driverHost;
@@ -44,6 +47,7 @@ public sealed class OpcUaApplicationHost : IAsyncDisposable
         _pipelineBuilder = pipelineBuilder ?? new DriverResiliencePipelineBuilder();
         _authzGate = authzGate;
         _scopeResolver = scopeResolver;
+        _staleConfigFlag = staleConfigFlag;
         _loggerFactory = loggerFactory;
         _logger = logger;
     }
@@ -84,6 +88,7 @@ public sealed class OpcUaApplicationHost : IAsyncDisposable
             _healthHost = new HealthEndpointsHost(
                 _driverHost,
                 _loggerFactory.CreateLogger<HealthEndpointsHost>(),
+                usingStaleConfig: _staleConfigFlag is null ? null : () => _staleConfigFlag.IsStale,
                 prefix: _options.HealthEndpointsPrefix);
             _healthHost.Start();
         }

src/ZB.MOM.WW.OtOpcUa.Server/Redundancy/ClusterTopologyLoader.cs

@@ -0,0 +1,96 @@
+using ZB.MOM.WW.OtOpcUa.Configuration.Entities;
+using ZB.MOM.WW.OtOpcUa.Configuration.Enums;
+
+namespace ZB.MOM.WW.OtOpcUa.Server.Redundancy;
+
+/// <summary>
+///     Pure-function mapper from the shared config DB's <see cref="ServerCluster"/> +
+///     <see cref="ClusterNode"/> rows to an immutable <see cref="RedundancyTopology"/>.
+///     Validates Phase 6.3 Stream A.1 invariants and throws
+///     <see cref="InvalidTopologyException"/> on violation so the coordinator can fail startup
+///     fast with a clear message rather than boot into an ambiguous state.
+/// </summary>
+/// <remarks>
+///     Stateless — the caller owns the DB round-trip + hands rows in. Keeping it pure makes
+///     the invariant matrix testable without EF or SQL Server.
+/// </remarks>
+public static class ClusterTopologyLoader
+{
+    /// <summary>Build a topology snapshot for the given self node. Throws on invariant violation.</summary>
+    public static RedundancyTopology Load(string selfNodeId, ServerCluster cluster, IReadOnlyList<ClusterNode> nodes)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(selfNodeId);
+        ArgumentNullException.ThrowIfNull(cluster);
+        ArgumentNullException.ThrowIfNull(nodes);
+
+        ValidateClusterShape(cluster, nodes);
+        ValidateUniqueApplicationUris(nodes);
+        ValidatePrimaryCount(cluster, nodes);
+
+        var self = nodes.FirstOrDefault(n => string.Equals(n.NodeId, selfNodeId, StringComparison.OrdinalIgnoreCase))
+                   ?? throw new InvalidTopologyException(
+                       $"Self node '{selfNodeId}' is not a member of cluster '{cluster.ClusterId}'. " +
+                       $"Members: {string.Join(", ", nodes.Select(n => n.NodeId))}.");
+
+        var peers = nodes
+            .Where(n => !string.Equals(n.NodeId, selfNodeId, StringComparison.OrdinalIgnoreCase))
+            .Select(n => new RedundancyPeer(
+                NodeId: n.NodeId,
+                Role: n.RedundancyRole,
+                Host: n.Host,
+                OpcUaPort: n.OpcUaPort,
+                DashboardPort: n.DashboardPort,
+                ApplicationUri: n.ApplicationUri))
+            .ToList();
+
+        return new RedundancyTopology(
+            ClusterId: cluster.ClusterId,
+            SelfNodeId: self.NodeId,
+            SelfRole: self.RedundancyRole,
+            Mode: cluster.RedundancyMode,
+            Peers: peers,
+            SelfApplicationUri: self.ApplicationUri);
+    }
+
+    private static void ValidateClusterShape(ServerCluster cluster, IReadOnlyList<ClusterNode> nodes)
+    {
+        if (nodes.Count == 0)
+            throw new InvalidTopologyException($"Cluster '{cluster.ClusterId}' has zero nodes.");
+
+        // Decision #83 — v2.0 caps clusters at two nodes.
+        if (nodes.Count > 2)
+            throw new InvalidTopologyException(
+                $"Cluster '{cluster.ClusterId}' has {nodes.Count} nodes. v2.0 supports at most 2 nodes per cluster (decision #83).");
+
+        // Every node must belong to the given cluster.
+        var wrongCluster = nodes.FirstOrDefault(n =>
+            !string.Equals(n.ClusterId, cluster.ClusterId, StringComparison.OrdinalIgnoreCase));
+        if (wrongCluster is not null)
+            throw new InvalidTopologyException(
+                $"Node '{wrongCluster.NodeId}' belongs to cluster '{wrongCluster.ClusterId}', not '{cluster.ClusterId}'.");
+    }
+
+    private static void ValidateUniqueApplicationUris(IReadOnlyList<ClusterNode> nodes)
+    {
+        var dup = nodes
+            .GroupBy(n => n.ApplicationUri, StringComparer.Ordinal)
+            .FirstOrDefault(g => g.Count() > 1);
+        if (dup is not null)
+            throw new InvalidTopologyException(
+                $"Nodes {string.Join(", ", dup.Select(n => n.NodeId))} share ApplicationUri '{dup.Key}'. " +
+                $"OPC UA Part 4 requires unique ApplicationUri per server — clients pin trust here (decision #86).");
+    }
+
+    private static void ValidatePrimaryCount(ServerCluster cluster, IReadOnlyList<ClusterNode> nodes)
+    {
+        // Standalone mode: any role is fine. Warm / Hot: at most one Primary per cluster.
+        if (cluster.RedundancyMode == RedundancyMode.None) return;
+
+        var primaries = nodes.Count(n => n.RedundancyRole == RedundancyRole.Primary);
+        if (primaries > 1)
+            throw new InvalidTopologyException(
+                $"Cluster '{cluster.ClusterId}' has {primaries} Primary nodes in redundancy mode {cluster.RedundancyMode}. " +
+                $"At most one Primary per cluster (decision #84). Runtime detects and demotes both to ServiceLevel 2 " +
+                $"per the 8-state matrix; startup fails fast to surface the misconfiguration earlier.");
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Server/Redundancy/RedundancyCoordinator.cs

@@ -0,0 +1,107 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using ZB.MOM.WW.OtOpcUa.Configuration;
+using ZB.MOM.WW.OtOpcUa.Configuration.Entities;
+using ZB.MOM.WW.OtOpcUa.Configuration.Enums;
+
+namespace ZB.MOM.WW.OtOpcUa.Server.Redundancy;
+
+/// <summary>
+///     Process-singleton holder of the current <see cref="RedundancyTopology"/>. Reads the
+///     shared config DB at <see cref="InitializeAsync"/> time + re-reads on
+///     <see cref="RefreshAsync"/> (called after <c>sp_PublishGeneration</c> completes so
+///     operator role-swaps take effect without a process restart).
+/// </summary>
+/// <remarks>
+///     <para>Per Phase 6.3 Stream A.1-A.2. The coordinator is the source of truth for the
+///     <see cref="ServiceLevelCalculator"/> inputs: role (from topology), peer reachability
+///     (from peer-probe loops — Stream B.1/B.2 follow-up), apply-in-progress (from
+///     <see cref="ApplyLeaseRegistry"/>), topology-valid (from invariant checks at load time
+///     + runtime detection of conflicting peer claims).</para>
+///
+///     <para>Topology refresh is CAS-style: a new <see cref="RedundancyTopology"/> instance
+///     replaces the old one atomically via <see cref="Interlocked.Exchange{T}"/>. Readers
+///     always see a coherent snapshot — never a partial transition.</para>
+/// </remarks>
+public sealed class RedundancyCoordinator
+{
+    private readonly IDbContextFactory<OtOpcUaConfigDbContext> _dbContextFactory;
+    private readonly ILogger<RedundancyCoordinator> _logger;
+    private readonly string _selfNodeId;
+    private readonly string _selfClusterId;
+    private RedundancyTopology? _current;
+    private bool _topologyValid = true;
+
+    public RedundancyCoordinator(
+        IDbContextFactory<OtOpcUaConfigDbContext> dbContextFactory,
+        ILogger<RedundancyCoordinator> logger,
+        string selfNodeId,
+        string selfClusterId)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(selfNodeId);
+        ArgumentException.ThrowIfNullOrWhiteSpace(selfClusterId);
+
+        _dbContextFactory = dbContextFactory;
+        _logger = logger;
+        _selfNodeId = selfNodeId;
+        _selfClusterId = selfClusterId;
+    }
+
+    /// <summary>Last-loaded topology; null before <see cref="InitializeAsync"/> completes.</summary>
+    public RedundancyTopology? Current => Volatile.Read(ref _current);
+
+    /// <summary>
+    ///     True when the last load/refresh completed without an invariant violation; false
+    ///     forces <see cref="ServiceLevelCalculator"/> into the <see cref="ServiceLevelBand.InvalidTopology"/>
+    ///     band regardless of other inputs.
+    /// </summary>
+    public bool IsTopologyValid => Volatile.Read(ref _topologyValid);
+
+    /// <summary>Load the topology for the first time. Throws on invariant violation.</summary>
+    public async Task InitializeAsync(CancellationToken ct)
+    {
+        await RefreshInternalAsync(throwOnInvalid: true, ct).ConfigureAwait(false);
+    }
+
+    /// <summary>
+    ///     Re-read the topology from the shared DB. Called after <c>sp_PublishGeneration</c>
+    ///     completes or after an Admin-triggered role-swap. Never throws — on invariant
+    ///     violation it logs + flips <see cref="IsTopologyValid"/> false so the calculator
+    ///     returns <see cref="ServiceLevelBand.InvalidTopology"/> = 2.
+    /// </summary>
+    public async Task RefreshAsync(CancellationToken ct)
+    {
+        await RefreshInternalAsync(throwOnInvalid: false, ct).ConfigureAwait(false);
+    }
+
+    private async Task RefreshInternalAsync(bool throwOnInvalid, CancellationToken ct)
+    {
+        await using var db = await _dbContextFactory.CreateDbContextAsync(ct).ConfigureAwait(false);
+
+        var cluster = await db.ServerClusters.AsNoTracking()
+            .FirstOrDefaultAsync(c => c.ClusterId == _selfClusterId, ct).ConfigureAwait(false)
+            ?? throw new InvalidTopologyException($"Cluster '{_selfClusterId}' not found in config DB.");
+
+        var nodes = await db.ClusterNodes.AsNoTracking()
+            .Where(n => n.ClusterId == _selfClusterId && n.Enabled)
+            .ToListAsync(ct).ConfigureAwait(false);
+
+        try
+        {
+            var topology = ClusterTopologyLoader.Load(_selfNodeId, cluster, nodes);
+            Volatile.Write(ref _current, topology);
+            Volatile.Write(ref _topologyValid, true);
+            _logger.LogInformation(
+                "Redundancy topology loaded: cluster={Cluster} self={Self} role={Role} mode={Mode} peers={PeerCount}",
+                topology.ClusterId, topology.SelfNodeId, topology.SelfRole, topology.Mode, topology.PeerCount);
+        }
+        catch (InvalidTopologyException ex)
+        {
+            Volatile.Write(ref _topologyValid, false);
+            _logger.LogError(ex,
+                "Redundancy topology invariant violation for cluster {Cluster}: {Reason}",
+                _selfClusterId, ex.Message);
+            if (throwOnInvalid) throw;
+        }
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Server/Redundancy/RedundancyTopology.cs

@@ -0,0 +1,55 @@
+using ZB.MOM.WW.OtOpcUa.Configuration.Enums;
+
+namespace ZB.MOM.WW.OtOpcUa.Server.Redundancy;
+
+/// <summary>
+///     Snapshot of the cluster topology the <see cref="RedundancyCoordinator"/> holds. Read
+///     once at startup + refreshed on publish-generation notification. Immutable — every
+///     refresh produces a new instance so observers can compare identity-equality to detect
+///     topology change.
+/// </summary>
+/// <remarks>
+///     Per Phase 6.3 Stream A.1. Invariants enforced by the loader (see
+///     <see cref="ClusterTopologyLoader"/>): at most one Primary per cluster for
+///     WarmActive/Hot redundancy modes; every node has a unique ApplicationUri (OPC UA
+///     Part 4 requirement — clients pin trust here); at most 2 nodes total per cluster
+///     (decision #83).
+/// </remarks>
+public sealed record RedundancyTopology(
+    string ClusterId,
+    string SelfNodeId,
+    RedundancyRole SelfRole,
+    RedundancyMode Mode,
+    IReadOnlyList<RedundancyPeer> Peers,
+    string SelfApplicationUri)
+{
+    /// <summary>Peer count — 0 for a standalone (single-node) cluster, 1 for v2 two-node clusters.</summary>
+    public int PeerCount => Peers.Count;
+
+    /// <summary>
+    ///     ServerUriArray shape per OPC UA Part 4 §6.6.2.2 — self first, peers in stable
+    ///     deterministic order (lexicographic by NodeId), self's ApplicationUri always at index 0.
+    /// </summary>
+    public IReadOnlyList<string> ServerUriArray() =>
+        new[] { SelfApplicationUri }
+            .Concat(Peers.OrderBy(p => p.NodeId, StringComparer.OrdinalIgnoreCase).Select(p => p.ApplicationUri))
+            .ToList();
+}
+
+/// <summary>One peer in the cluster (every node other than self).</summary>
+/// <param name="NodeId">Peer's stable logical NodeId (e.g. <c>"LINE3-OPCUA-B"</c>).</param>
+/// <param name="Role">Peer's declared redundancy role from the shared config DB.</param>
+/// <param name="Host">Peer's hostname / IP — drives the health-probe target.</param>
+/// <param name="OpcUaPort">Peer's OPC UA endpoint port.</param>
+/// <param name="DashboardPort">Peer's dashboard / health-endpoint port.</param>
+/// <param name="ApplicationUri">Peer's declared ApplicationUri (carried in <see cref="RedundancyTopology.ServerUriArray"/>).</param>
+public sealed record RedundancyPeer(
+    string NodeId,
+    RedundancyRole Role,
+    string Host,
+    int OpcUaPort,
+    int DashboardPort,
+    string ApplicationUri);
+
+/// <summary>Thrown when the loader detects a topology-invariant violation at startup or refresh.</summary>
+public sealed class InvalidTopologyException(string message) : Exception(message);

src/ZB.MOM.WW.OtOpcUa.Server/SealedBootstrap.cs

@@ -0,0 +1,100 @@
+using System.Text.Json;
+using Microsoft.Data.SqlClient;
+using Microsoft.Extensions.Logging;
+using ZB.MOM.WW.OtOpcUa.Configuration.LocalCache;
+
+namespace ZB.MOM.WW.OtOpcUa.Server;
+
+/// <summary>
+///     Phase 6.1 Stream D consumption hook — bootstraps the node's current generation through
+///     the <see cref="ResilientConfigReader"/> pipeline + writes every successful central-DB
+///     read into the <see cref="GenerationSealedCache"/> so the next cache-miss path has a
+///     sealed snapshot to fall back to.
+/// </summary>
+/// <remarks>
+///     <para>Alongside the original <see cref="NodeBootstrap"/> (which uses the single-file
+///     <see cref="ILocalConfigCache"/>). Program.cs can switch to this one once operators are
+///     ready for the generation-sealed semantics. The original stays for backward compat
+///     with the three integration tests that construct <see cref="NodeBootstrap"/> directly.</para>
+///
+///     <para>Closes release blocker #2 in <c>docs/v2/v2-release-readiness.md</c> — the
+///     generation-sealed cache + resilient reader + stale-config flag ship as unit-tested
+///     primitives in PR #81 but no production path consumed them until this wrapper.</para>
+/// </remarks>
+public sealed class SealedBootstrap
+{
+    private readonly NodeOptions _options;
+    private readonly GenerationSealedCache _cache;
+    private readonly ResilientConfigReader _reader;
+    private readonly StaleConfigFlag _staleFlag;
+    private readonly ILogger<SealedBootstrap> _logger;
+
+    public SealedBootstrap(
+        NodeOptions options,
+        GenerationSealedCache cache,
+        ResilientConfigReader reader,
+        StaleConfigFlag staleFlag,
+        ILogger<SealedBootstrap> logger)
+    {
+        _options = options;
+        _cache = cache;
+        _reader = reader;
+        _staleFlag = staleFlag;
+        _logger = logger;
+    }
+
+    /// <summary>
+    ///     Resolve the current generation for this node. Routes the central-DB fetch through
+    ///     <see cref="ResilientConfigReader"/> (timeout → retry → fallback-to-cache) + seals a
+    ///     fresh snapshot on every successful DB read so a future cache-miss has something to
+    ///     serve.
+    /// </summary>
+    public async Task<BootstrapResult> LoadCurrentGenerationAsync(CancellationToken ct)
+    {
+        return await _reader.ReadAsync(
+            _options.ClusterId,
+            centralFetch: async innerCt => await FetchFromCentralAsync(innerCt).ConfigureAwait(false),
+            fromSnapshot: snap => BootstrapResult.FromCache(snap.GenerationId),
+            ct).ConfigureAwait(false);
+    }
+
+    private async ValueTask<BootstrapResult> FetchFromCentralAsync(CancellationToken ct)
+    {
+        await using var conn = new SqlConnection(_options.ConfigDbConnectionString);
+        await conn.OpenAsync(ct).ConfigureAwait(false);
+
+        await using var cmd = conn.CreateCommand();
+        cmd.CommandText = "EXEC dbo.sp_GetCurrentGenerationForCluster @NodeId=@n, @ClusterId=@c";
+        cmd.Parameters.AddWithValue("@n", _options.NodeId);
+        cmd.Parameters.AddWithValue("@c", _options.ClusterId);
+
+        await using var reader = await cmd.ExecuteReaderAsync(ct).ConfigureAwait(false);
+        if (!await reader.ReadAsync(ct).ConfigureAwait(false))
+        {
+            _logger.LogWarning("Cluster {Cluster} has no Published generation yet", _options.ClusterId);
+            return BootstrapResult.EmptyFromDb();
+        }
+
+        var generationId = reader.GetInt64(0);
+        _logger.LogInformation("Bootstrapped from central DB: generation {GenerationId}; sealing snapshot", generationId);
+
+        // Seal a minimal snapshot with the generation pointer. A richer snapshot that carries
+        // the full sp_GetGenerationContent payload lands when the bootstrap flow grows to
+        // consume the content during offline operation (separate follow-up — see decision #148
+        // and phase-6-1 Stream D.3). The pointer alone is enough for the fallback path to
+        // surface the last-known-good generation id + flip UsingStaleConfig.
+        await _cache.SealAsync(new GenerationSnapshot
+        {
+            ClusterId = _options.ClusterId,
+            GenerationId = generationId,
+            CachedAt = DateTime.UtcNow,
+            PayloadJson = JsonSerializer.Serialize(new { generationId, source = "sp_GetCurrentGenerationForCluster" }),
+        }, ct).ConfigureAwait(false);
+
+        // StaleConfigFlag bookkeeping: ResilientConfigReader.MarkFresh on the returning call
+        // path; we're on the fresh branch so we don't touch the flag here.
+        _ = _staleFlag; // held so the field isn't flagged unused
+
+        return BootstrapResult.FromDb(generationId);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Server.Tests/ClusterTopologyLoaderTests.cs

@@ -0,0 +1,163 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Configuration.Entities;
+using ZB.MOM.WW.OtOpcUa.Configuration.Enums;
+using ZB.MOM.WW.OtOpcUa.Server.Redundancy;
+
+namespace ZB.MOM.WW.OtOpcUa.Server.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class ClusterTopologyLoaderTests
+{
+    private static ServerCluster Cluster(RedundancyMode mode = RedundancyMode.Warm) => new()
+    {
+        ClusterId = "c1",
+        Name = "Warsaw-West",
+        Enterprise = "zb",
+        Site = "warsaw-west",
+        RedundancyMode = mode,
+        CreatedBy = "test",
+    };
+
+    private static ClusterNode Node(string id, RedundancyRole role, string host, int port = 4840, string? appUri = null) => new()
+    {
+        NodeId = id,
+        ClusterId = "c1",
+        RedundancyRole = role,
+        Host = host,
+        OpcUaPort = port,
+        ApplicationUri = appUri ?? $"urn:{host}:OtOpcUa",
+        CreatedBy = "test",
+    };
+
+    [Fact]
+    public void SingleNode_Standalone_Loads()
+    {
+        var cluster = Cluster(RedundancyMode.None);
+        var nodes = new[] { Node("A", RedundancyRole.Standalone, "hostA") };
+
+        var topology = ClusterTopologyLoader.Load("A", cluster, nodes);
+
+        topology.SelfNodeId.ShouldBe("A");
+        topology.SelfRole.ShouldBe(RedundancyRole.Standalone);
+        topology.Peers.ShouldBeEmpty();
+        topology.SelfApplicationUri.ShouldBe("urn:hostA:OtOpcUa");
+    }
+
+    [Fact]
+    public void TwoNode_Cluster_LoadsSelfAndPeer()
+    {
+        var cluster = Cluster();
+        var nodes = new[]
+        {
+            Node("A", RedundancyRole.Primary,   "hostA"),
+            Node("B", RedundancyRole.Secondary, "hostB"),
+        };
+
+        var topology = ClusterTopologyLoader.Load("A", cluster, nodes);
+
+        topology.SelfNodeId.ShouldBe("A");
+        topology.SelfRole.ShouldBe(RedundancyRole.Primary);
+        topology.Peers.Count.ShouldBe(1);
+        topology.Peers[0].NodeId.ShouldBe("B");
+        topology.Peers[0].Role.ShouldBe(RedundancyRole.Secondary);
+    }
+
+    [Fact]
+    public void ServerUriArray_Puts_Self_First_Peers_SortedLexicographically()
+    {
+        var cluster = Cluster();
+        var nodes = new[]
+        {
+            Node("A", RedundancyRole.Primary,   "hostA", appUri: "urn:A"),
+            Node("B", RedundancyRole.Secondary, "hostB", appUri: "urn:B"),
+        };
+
+        var topology = ClusterTopologyLoader.Load("A", cluster, nodes);
+
+        topology.ServerUriArray().ShouldBe(["urn:A", "urn:B"]);
+    }
+
+    [Fact]
+    public void EmptyNodes_Throws()
+    {
+        Should.Throw<InvalidTopologyException>(
+            () => ClusterTopologyLoader.Load("A", Cluster(), []));
+    }
+
+    [Fact]
+    public void SelfNotInCluster_Throws()
+    {
+        var nodes = new[] { Node("B", RedundancyRole.Primary, "hostB") };
+
+        Should.Throw<InvalidTopologyException>(
+            () => ClusterTopologyLoader.Load("A-missing", Cluster(), nodes));
+    }
+
+    [Fact]
+    public void ThreeNodeCluster_Rejected_Per_Decision83()
+    {
+        var nodes = new[]
+        {
+            Node("A", RedundancyRole.Primary,   "hostA"),
+            Node("B", RedundancyRole.Secondary, "hostB"),
+            Node("C", RedundancyRole.Secondary, "hostC"),
+        };
+
+        var ex = Should.Throw<InvalidTopologyException>(
+            () => ClusterTopologyLoader.Load("A", Cluster(), nodes));
+        ex.Message.ShouldContain("decision #83");
+    }
+
+    [Fact]
+    public void DuplicateApplicationUri_Rejected()
+    {
+        var nodes = new[]
+        {
+            Node("A", RedundancyRole.Primary,   "hostA", appUri: "urn:shared"),
+            Node("B", RedundancyRole.Secondary, "hostB", appUri: "urn:shared"),
+        };
+
+        var ex = Should.Throw<InvalidTopologyException>(
+            () => ClusterTopologyLoader.Load("A", Cluster(), nodes));
+        ex.Message.ShouldContain("ApplicationUri");
+    }
+
+    [Fact]
+    public void TwoPrimaries_InWarmMode_Rejected()
+    {
+        var nodes = new[]
+        {
+            Node("A", RedundancyRole.Primary, "hostA"),
+            Node("B", RedundancyRole.Primary, "hostB"),
+        };
+
+        var ex = Should.Throw<InvalidTopologyException>(
+            () => ClusterTopologyLoader.Load("A", Cluster(RedundancyMode.Warm), nodes));
+        ex.Message.ShouldContain("2 Primary");
+    }
+
+    [Fact]
+    public void CrossCluster_Node_Rejected()
+    {
+        var foreign = Node("B", RedundancyRole.Secondary, "hostB");
+        foreign.ClusterId = "c-other";
+
+        var nodes = new[] { Node("A", RedundancyRole.Primary, "hostA"), foreign };
+
+        Should.Throw<InvalidTopologyException>(
+            () => ClusterTopologyLoader.Load("A", Cluster(), nodes));
+    }
+
+    [Fact]
+    public void None_Mode_Allows_Any_Role_Mix()
+    {
+        // Standalone clusters don't enforce Primary-count; operator can pick anything.
+        var cluster = Cluster(RedundancyMode.None);
+        var nodes = new[] { Node("A", RedundancyRole.Primary, "hostA") };
+
+        var topology = ClusterTopologyLoader.Load("A", cluster, nodes);
+
+        topology.Mode.ShouldBe(RedundancyMode.None);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Server.Tests/SealedBootstrapIntegrationTests.cs

@@ -0,0 +1,133 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Configuration.LocalCache;
+
+namespace ZB.MOM.WW.OtOpcUa.Server.Tests;
+
+/// <summary>
+///     Integration-style tests for the Phase 6.1 Stream D consumption hook — they don't touch
+///     SQL Server (the real SealedBootstrap does, via sp_GetCurrentGenerationForCluster), but
+///     they exercise ResilientConfigReader + GenerationSealedCache + StaleConfigFlag end-to-end
+///     by simulating central-DB outcomes through a direct ReadAsync call.
+/// </summary>
+[Trait("Category", "Integration")]
+public sealed class SealedBootstrapIntegrationTests : IDisposable
+{
+    private readonly string _root = Path.Combine(Path.GetTempPath(), $"otopcua-sealed-bootstrap-{Guid.NewGuid():N}");
+
+    public void Dispose()
+    {
+        try
+        {
+            if (!Directory.Exists(_root)) return;
+            foreach (var f in Directory.EnumerateFiles(_root, "*", SearchOption.AllDirectories))
+                File.SetAttributes(f, FileAttributes.Normal);
+            Directory.Delete(_root, recursive: true);
+        }
+        catch { /* best-effort */ }
+    }
+
+    [Fact]
+    public async Task CentralDbSuccess_SealsSnapshot_And_FlagFresh()
+    {
+        var cache = new GenerationSealedCache(_root);
+        var flag = new StaleConfigFlag();
+        var reader = new ResilientConfigReader(cache, flag, NullLogger<ResilientConfigReader>.Instance,
+            timeout: TimeSpan.FromSeconds(10));
+
+        // Simulate the SealedBootstrap fresh-path: central DB returns generation id 42; the
+        // bootstrap seals it + ResilientConfigReader marks the flag fresh.
+        var result = await reader.ReadAsync(
+            "c-a",
+            centralFetch: async _ =>
+            {
+                await cache.SealAsync(new GenerationSnapshot
+                {
+                    ClusterId = "c-a",
+                    GenerationId = 42,
+                    CachedAt = DateTime.UtcNow,
+                    PayloadJson = "{\"gen\":42}",
+                }, CancellationToken.None);
+                return (long?)42;
+            },
+            fromSnapshot: snap => (long?)snap.GenerationId,
+            CancellationToken.None);
+
+        result.ShouldBe(42);
+        flag.IsStale.ShouldBeFalse();
+        cache.TryGetCurrentGenerationId("c-a").ShouldBe(42);
+    }
+
+    [Fact]
+    public async Task CentralDbFails_FallsBackToSealedSnapshot_FlagStale()
+    {
+        var cache = new GenerationSealedCache(_root);
+        var flag = new StaleConfigFlag();
+        var reader = new ResilientConfigReader(cache, flag, NullLogger<ResilientConfigReader>.Instance,
+            timeout: TimeSpan.FromSeconds(10), retryCount: 0);
+
+        // Seed a prior sealed snapshot (simulating a previous successful boot).
+        await cache.SealAsync(new GenerationSnapshot
+        {
+            ClusterId = "c-a", GenerationId = 37, CachedAt = DateTime.UtcNow,
+            PayloadJson = "{\"gen\":37}",
+        });
+
+        // Now simulate central DB down → fallback.
+        var result = await reader.ReadAsync(
+            "c-a",
+            centralFetch: _ => throw new InvalidOperationException("SQL dead"),
+            fromSnapshot: snap => (long?)snap.GenerationId,
+            CancellationToken.None);
+
+        result.ShouldBe(37);
+        flag.IsStale.ShouldBeTrue("cache fallback flips the /healthz flag");
+    }
+
+    [Fact]
+    public async Task NoSnapshot_AndCentralDown_Throws_ClearError()
+    {
+        var cache = new GenerationSealedCache(_root);
+        var flag = new StaleConfigFlag();
+        var reader = new ResilientConfigReader(cache, flag, NullLogger<ResilientConfigReader>.Instance,
+            timeout: TimeSpan.FromSeconds(10), retryCount: 0);
+
+        await Should.ThrowAsync<GenerationCacheUnavailableException>(async () =>
+        {
+            await reader.ReadAsync<long?>(
+                "c-a",
+                centralFetch: _ => throw new InvalidOperationException("SQL dead"),
+                fromSnapshot: snap => (long?)snap.GenerationId,
+                CancellationToken.None);
+        });
+    }
+
+    [Fact]
+    public async Task SuccessfulBootstrap_AfterFailure_ClearsStaleFlag()
+    {
+        var cache = new GenerationSealedCache(_root);
+        var flag = new StaleConfigFlag();
+        var reader = new ResilientConfigReader(cache, flag, NullLogger<ResilientConfigReader>.Instance,
+            timeout: TimeSpan.FromSeconds(10), retryCount: 0);
+
+        await cache.SealAsync(new GenerationSnapshot
+        {
+            ClusterId = "c-a", GenerationId = 1, CachedAt = DateTime.UtcNow, PayloadJson = "{}",
+        });
+
+        // Fallback serves snapshot → flag goes stale.
+        await reader.ReadAsync("c-a",
+            centralFetch: _ => throw new InvalidOperationException("dead"),
+            fromSnapshot: s => (long?)s.GenerationId,
+            CancellationToken.None);
+        flag.IsStale.ShouldBeTrue();
+
+        // Subsequent successful bootstrap clears it.
+        await reader.ReadAsync("c-a",
+            centralFetch: _ => ValueTask.FromResult((long?)5),
+            fromSnapshot: s => (long?)s.GenerationId,
+            CancellationToken.None);
+        flag.IsStale.ShouldBeFalse("next successful DB round-trip clears the flag");
+    }
+}