fix(alarms): refuse a keyed native ack with no Galaxy identity synchronously, from a cached key set (lmxopcua#765)

Item 3 of #765. A keyed (API-key) session whose key had no Galaxy ack identity
got Good and then a revert, because whether the key has an identity lives in
the Secrets store, which must not be read under the SDK lock.

GalaxyAckIdentityDirectory (Driver.Galaxy) keeps a node-wide set of the key ids
that have a galaxy-ack-user secret. It lists this node's Secrets store
(metadata only, nothing decrypted) and is read lock-free.
GalaxyDriver.GetAcknowledgeRefusal (IAlarmAcknowledgePreflight, published
through NativeAlarmAckPreflightView as in #760) now refuses a keyed session
whose key the directory reports Absent, when the driver has no fallback ack
account. A refusal also asks for a refresh.

Fails open: before the first refresh, once the snapshot is older than
StaleAfter, and from a signalled identity-secret change until the next
refresh, the lookup answers Unknown and the ack takes the old Good-then-revert
path.

Refresh: GalaxyAckIdentityRefreshService runs on driver nodes, every
Drivers:Galaxy:AckIdentityCache:RefreshIntervalSeconds (default 30;
StaleAfterSeconds 0 = three intervals; validated at start). It also refreshes
early on secret changes. The Secrets library has no change event, so
SecretsRegistration decorates ISecretCacheInvalidator, which is called by
AdminUI /api-keys, /admin/secrets and the replication appliers. API-key
changes need no event: a revoked key is already denied before the pre-check.

Tests: Galaxy.Tests GalaxyAckIdentityDirectoryTests (16) and Host.Tests
GalaxyAckIdentityCacheWiringTests (9). Seven separate mutations each turn at
least one test red: pre-check ignores the directory, factory drops it, no
invalidator decoration, no staleness check, no change-pending check, no
refresh on refusal, loop never wakes early.
Docs: Galaxy.md, AlarmTracking.md, Configuration.md, .claude/rules/alarms.md.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

.claude/rules/alarms.md

@@ -32,10 +32,14 @@ SDK's ack unconfirms any confirm-capable condition and ScadaBridge maps that fie
 `ConditionMethodEventTests` (exactly-once after a sentinel, folder + `i=2253`).
 
 **A native ack the driver refuses or fails is not left Acknowledged (#760).** Two layers: (1) a refusal the
-driver can decide without I/O (`IAlarmAcknowledgePreflight` — Galaxy: anonymous session and no fallback ack
-account) fails the Part 9 call `BadUserAccessDenied` via `NativeAlarmAckPreflight`, read lock-free from the
+driver can decide without I/O (`IAlarmAcknowledgePreflight` — Galaxy, no fallback ack account: an anonymous session, or a keyed
+session whose key `GalaxyAckIdentityDirectory` reports Absent, #765) fails the Part 9 call `BadUserAccessDenied` via `NativeAlarmAckPreflight`, read lock-free from the
 `NativeAlarmAckPreflightView` snapshot `DriverHostActor` publishes; never do I/O (e.g. a Secrets lookup) in a
-pre-check — it runs under the SDK `Lock`. (2) anything else (keyed session without an identity, MXAccess
+pre-check — it runs under the SDK `Lock`; that is why the keyed case reads a cached, periodically refreshed key set
+(`Drivers:Galaxy:AckIdentityCache`), which must answer Unknown (= let it through) whenever it is not certain — no
+refresh yet, stale, or an identity-secret change signalled through the decorated `ISecretCacheInvalidator` and not
+yet re-listed. Only an identity written straight into the store by the `secret` CLI (no in-process signal) can be
+refused until the next refresh, which the refusal itself requests. (2) anything else (a keyed session the cache cannot vouch for, MXAccess
 non-zero `hresult`, driver still connecting/reconnecting) comes back as `DriverInstanceActor.AlarmAckFailed` — or, for a
 disabled/unspawned/stubbed driver, is decided in the host itself (#765) — and `DriverHostActor` re-projects the condition unacknowledged — only while the ack is still pending in
 `NativeAlarmProjector` (any driver transition since then wins). A host that came up **Stale** cannot route any

docs/AlarmTracking.md

@@ -468,8 +468,10 @@ from the scripted `AlarmCommandRouter`:
    not yet spawned) or is a stub on this node is reverted the same way by
    the host itself (#765). Only an ack still pending in `NativeAlarmProjector` is
    reverted: a driver transition projected since then is authoritative. A
-   refusal the driver can decide without I/O (Galaxy: an anonymous session
-   and no fallback ack account) is instead returned synchronously as
+   refusal the driver can decide without I/O (Galaxy, with no fallback ack
+   account: an anonymous session, or a keyed session whose key the cached
+   `GalaxyAckIdentityDirectory` knows has no ack identity — #765) is instead
+   returned synchronously as
    `BadUserAccessDenied` by the node manager's `NativeAlarmAckPreflight`
    before anything is applied — as is every native ack while the driver host
    is Stale (configuration database unreachable at boot), when no ack can be

docs/Configuration.md

@@ -536,6 +536,21 @@ deploy that recreates the driver), not on `ReinitializeAsync`. The init log line
 `transport=Libplctag (node default)` when the pin is what selected the stack. There is **no AbLegacy
 counterpart**: an AbLegacy driver with no key always runs libplctag (Native is opt-in per driver).
 
+### `Drivers:Galaxy:AckIdentityCache` (cached API-key → Galaxy ack identity set, #765)
+
+- **Purpose:** the node-wide set of API key ids that have a Galaxy ack identity (a `galaxy-ack-user` secret),
+  which the Galaxy driver's synchronous ack pre-check reads to refuse a keyed session without one
+  (`BadUserAccessDenied`) instead of returning Good and reverting. See
+  [`drivers/Galaxy.md` § Alarm acknowledge credential](drivers/Galaxy.md#alarm-acknowledge-credential).
+- **Options class:** `GalaxyAckIdentityCacheOptions` (`SectionName = "Drivers:Galaxy:AckIdentityCache"`) —
+  `src/Server/ZB.MOM.WW.OtOpcUa.Host/Configuration/GalaxyAckIdentityCacheOptions.cs`; bound and validated
+  in `Program.cs`. The directory and its refresh loop exist on driver-role nodes only.
+
+| Key | Type | Default | Meaning |
+|---|---|---|---|
+| `RefreshIntervalSeconds` | int | `30` | How often the Secrets store is re-listed (1..3600). An in-process identity change or a refusal also triggers an early refresh. |
+| `StaleAfterSeconds` | int | `0` | Snapshot age after which keyed acks fall back to Good-then-revert (fail open). `0` = three refresh intervals; otherwise must exceed `RefreshIntervalSeconds`. A bad value fails host start. |
+
 ---
 
 ## Environment variables

docs/drivers/Galaxy.md

@@ -108,12 +108,23 @@ Galaxy password, so `GalaxyAckCredentialResolver` picks the account per acknowle
 3. **Neither** → the driver throws `DriverOperationRefusedException`; **nothing is sent to the
    gateway**. The refusal is excluded from the resilience breaker (it says nothing about gateway
    health). How the client learns of it (#760):
-   - **Synchronously** when the driver can tell without I/O — an *anonymous* session on a driver with
-     no fallback account: the Part 9 `Acknowledge` call itself fails `BadUserAccessDenied` with the
-     reason, and nothing is applied (`IAlarmAcknowledgePreflight`, published per condition by
-     `DriverHostActor` into `NativeAlarmAckPreflightView`).
-   - **By revert** otherwise — a keyed session whose key has no identity (that lives in the Secrets
-     store, which is not read under the SDK lock), or MXAccess rejecting the account (a non-zero
+   - **Synchronously** when the driver can tell without I/O, on a driver with no fallback account —
+     an *anonymous* session (#760), or a *keyed* session whose key has no identity (#765): the Part 9
+     `Acknowledge` call itself fails `BadUserAccessDenied` with the reason, and nothing is applied
+     (`IAlarmAcknowledgePreflight`, published per condition by `DriverHostActor` into
+     `NativeAlarmAckPreflightView`). The keyed case reads `GalaxyAckIdentityDirectory`, a node-wide
+     cached set of the key ids that have a `galaxy-ack-user` secret, listed from this node's Secrets
+     store (metadata only — nothing decrypted) every `Drivers:Galaxy:AckIdentityCache:RefreshIntervalSeconds`
+     (default 30) by `GalaxyAckIdentityRefreshService`, and early whenever an identity secret changes
+     in-process (the Secrets cache invalidator is decorated, so AdminUI `/api-keys`, `/admin/secrets` and
+     replication all signal it) or a refusal asks for one. ⚠️ It **fails open**: before the first refresh,
+     once the snapshot is older than `StaleAfterSeconds` (default three intervals — e.g. the store cannot
+     be listed), and from a signalled identity change until the refresh that follows it, every keyed ack
+     takes the revert path below instead of being refused. An identity written straight into the store
+     by the `secret` CLI is seen at the next refresh; the refusal itself requests one, so a retry a moment
+     later goes through.
+   - **By revert** otherwise — a keyed session the directory cannot vouch for (see above), a key whose
+     identity is broken (a user with no password secret), or MXAccess rejecting the account (a non-zero
      native `hresult`, e.g. a wrong password, now thrown as a refusal): the call returned `Good`, then
      `DriverHostActor` re-projects the condition **unacknowledged** (a fresh condition event,
      `Retain=true`) and logs the reason. A driver transition projected after the ack wins over a late

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriver.cs

@@ -119,6 +119,9 @@ public sealed class GalaxyDriver
     // lmxopcua#749: resolves the Galaxy account each acknowledge authenticates as (API key identity →
     // fallback account → refused). Built in the ctor — it needs only options + the secret resolver.
     private readonly IGalaxyAckCredentialResolver _ackCredentialResolver;
+    // lmxopcua#765: the cached key → has-Galaxy-identity set the synchronous ack pre-check reads (no I/O).
+    // Null = no directory wired, so a keyed session is never pre-refused (the pre-#765 behaviour).
+    private readonly IGalaxyAckIdentityDirectory? _ackIdentityDirectory;
     private IGalaxyAlarmFeed? _alarmFeed;
     private readonly Lock _alarmHandlersLock = new();
     private readonly Lock _alarmFeedLock = new();
@@ -262,15 +265,21 @@ public sealed class GalaxyDriver
     /// <param name="options">The Galaxy driver configuration options.</param>
     /// <param name="secretResolver">Resolves the <c>secret:</c> arm of <c>Gateway.ApiKeySecretRef</c>.</param>
     /// <param name="logger">Optional logger instance for diagnostics.</param>
+    /// <param name="ackIdentityDirectory">
+    ///     Optional cached set of API keys with a Galaxy ack identity (lmxopcua#765), consulted by the synchronous
+    ///     ack pre-check. Null = keyed sessions are never pre-refused.
+    /// </param>
     public GalaxyDriver(
         string driverInstanceId,
         GalaxyDriverOptions options,
         ISecretResolver secretResolver,
-        ILogger<GalaxyDriver>? logger = null)
+        ILogger<GalaxyDriver>? logger = null,
+        IGalaxyAckIdentityDirectory? ackIdentityDirectory = null)
         : this(driverInstanceId, options,
               hierarchySource: null, dataReader: null, dataWriter: null, subscriber: null,
               alarmAcknowledger: null, alarmFeed: null, logger,
-              secretResolver: secretResolver ?? throw new ArgumentNullException(nameof(secretResolver)))
+              secretResolver: secretResolver ?? throw new ArgumentNullException(nameof(secretResolver)),
+              ackIdentityDirectory: ackIdentityDirectory)
     {
     }
 
@@ -307,6 +316,7 @@ public sealed class GalaxyDriver
     ///     Optional ack-credential seam (lmxopcua#749). Defaults to <see cref="GalaxyAckCredentialResolver"/>
     ///     over <see cref="GalaxyDriverOptions.AlarmAck"/> and <paramref name="secretResolver"/>.
     /// </param>
+    /// <param name="ackIdentityDirectory">Optional cached key → Galaxy-identity set (lmxopcua#765).</param>
     internal GalaxyDriver(
         string driverInstanceId,
         GalaxyDriverOptions options,
@@ -320,8 +330,10 @@ public sealed class GalaxyDriver
         ISecretResolver? secretResolver = null,
         IGalaxyDeployWatchSource? deployWatchSource = null,
         IGalaxyPlatformSource? platformSource = null,
-        IGalaxyAckCredentialResolver? ackCredentialResolver = null)
+        IGalaxyAckCredentialResolver? ackCredentialResolver = null,
+        IGalaxyAckIdentityDirectory? ackIdentityDirectory = null)
     {
+        _ackIdentityDirectory = ackIdentityDirectory;
         _deployWatchSource = deployWatchSource;
         _platformSource = platformSource;
         _driverInstanceId = !string.IsNullOrWhiteSpace(driverInstanceId)
@@ -1968,17 +1980,32 @@ public sealed class GalaxyDriver
 
     /// <inheritdoc />
     /// <remarks>
-    ///     Only the case decidable without I/O (lmxopcua#760): an anonymous session (no API key, so no key-mapped
-    ///     Galaxy identity can exist) on a driver with no <c>AlarmAck.FallbackUser</c>. A keyed session is let
-    ///     through — whether the key has an identity lives in the Secrets store, which must not be read under the
-    ///     SDK lock; <see cref="AcknowledgeAsync"/> still refuses it, and the host reverts the condition.
+    ///     Only cases decidable without I/O, on a driver with no <c>AlarmAck.FallbackUser</c>: an anonymous session
+    ///     (lmxopcua#760 — no API key, so no key-mapped Galaxy identity can exist), and a keyed session whose key the
+    ///     <see cref="IGalaxyAckIdentityDirectory"/> reports <see cref="GalaxyAckIdentityPresence.Absent"/>
+    ///     (lmxopcua#765 — a cached, periodically refreshed view of the Secrets store, because the store itself must
+    ///     not be read under the SDK lock). Anything the directory does not know for certain (no directory, no
+    ///     refresh yet, stale, a change pending) is let through: <see cref="AcknowledgeAsync"/> still refuses it,
+    ///     and the host reverts the condition. A refusal also asks the directory to refresh, so a retry after an
+    ///     identity was set out of band (the <c>secret</c> CLI) is not refused for a whole refresh interval.
     /// </remarks>
     public string? GetAcknowledgeRefusal(string? apiKeyId)
     {
-        if (!string.IsNullOrWhiteSpace(apiKeyId) || (_options.AlarmAck?.HasFallback ?? false)) return null;
-        return $"the session is anonymous (no API key) and Galaxy driver '{_driverInstanceId}' has no " +
-            "AlarmAck.FallbackUser configured, so the acknowledge cannot be sent to Galaxy. Connect with an API key " +
-            "that has a Galaxy ack identity (AdminUI /api-keys) or configure the driver's fallback ack account.";
+        if (_options.AlarmAck?.HasFallback ?? false) return null;
+        if (string.IsNullOrWhiteSpace(apiKeyId))
+        {
+            return $"the session is anonymous (no API key) and Galaxy driver '{_driverInstanceId}' has no " +
+                "AlarmAck.FallbackUser configured, so the acknowledge cannot be sent to Galaxy. Connect with an API key " +
+                "that has a Galaxy ack identity (AdminUI /api-keys) or configure the driver's fallback ack account.";
+        }
+
+        if (_ackIdentityDirectory is not { } directory
+            || directory.GetPresence(apiKeyId) != GalaxyAckIdentityPresence.Absent) return null;
+
+        directory.RequestRefresh();
+        return $"API key '{apiKeyId.Trim()}' has no Galaxy ack identity and Galaxy driver '{_driverInstanceId}' has no " +
+            "AlarmAck.FallbackUser configured, so the acknowledge cannot be sent to Galaxy. Map a Galaxy ack identity " +
+            "to the key (AdminUI /api-keys) or configure the driver's fallback ack account.";
     }
 
     private static string PrincipalOf(AlarmAcknowledgeRequest ack) =>

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriverFactoryExtensions.cs

@@ -4,6 +4,7 @@ using Microsoft.Extensions.Logging;
 using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 using ZB.MOM.WW.OtOpcUa.Core.Hosting;
 using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Config;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Runtime;
 using ZB.MOM.WW.Secrets.Abstractions;
 
 namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy;
@@ -31,14 +32,20 @@ public static class GalaxyDriverFactoryExtensions
     ///     each instance's <c>Gateway.ApiKeySecretRef</c>.
     /// </param>
     /// <param name="loggerFactory">The optional logger factory for creating drivers.</param>
+    /// <param name="ackIdentityDirectory">
+    ///     Optional node-wide cached set of API keys with a Galaxy ack identity (lmxopcua#765), shared by every
+    ///     Galaxy driver instance for the synchronous ack pre-check. Null = keyed sessions are never pre-refused.
+    /// </param>
     public static void Register(
         DriverFactoryRegistry registry,
         ISecretResolver secretResolver,
-        ILoggerFactory? loggerFactory = null)
+        ILoggerFactory? loggerFactory = null,
+        IGalaxyAckIdentityDirectory? ackIdentityDirectory = null)
     {
         ArgumentNullException.ThrowIfNull(registry);
         ArgumentNullException.ThrowIfNull(secretResolver);
-        registry.Register(DriverTypeName, (id, json) => CreateInstance(id, json, loggerFactory, secretResolver));
+        registry.Register(DriverTypeName,
+            (id, json) => CreateInstance(id, json, loggerFactory, secretResolver, ackIdentityDirectory));
     }
 
     /// <summary>
@@ -58,9 +65,11 @@ public static class GalaxyDriverFactoryExtensions
     /// <param name="driverConfigJson">The driver configuration in JSON format.</param>
     /// <param name="loggerFactory">The optional logger factory for creating drivers.</param>
     /// <param name="secretResolver">The secret resolver for the driver's <c>secret:</c> API-key arm.</param>
+    /// <param name="ackIdentityDirectory">Optional cached key → Galaxy-identity set (lmxopcua#765).</param>
     /// <returns>The constructed Galaxy driver instance.</returns>
     public static GalaxyDriver CreateInstance(
-        string driverInstanceId, string driverConfigJson, ILoggerFactory? loggerFactory, ISecretResolver secretResolver)
+        string driverInstanceId, string driverConfigJson, ILoggerFactory? loggerFactory, ISecretResolver secretResolver,
+        IGalaxyAckIdentityDirectory? ackIdentityDirectory = null)
     {
         ArgumentNullException.ThrowIfNull(secretResolver);
         ArgumentException.ThrowIfNullOrWhiteSpace(driverInstanceId);
@@ -108,7 +117,8 @@ public static class GalaxyDriverFactoryExtensions
                 FallbackPasswordSecretRef: NullIfBlank(dto.AlarmAck?.FallbackPasswordSecretRef)),
         };
 
-        return new GalaxyDriver(driverInstanceId, options, secretResolver, loggerFactory?.CreateLogger<GalaxyDriver>());
+        return new GalaxyDriver(
+            driverInstanceId, options, secretResolver, loggerFactory?.CreateLogger<GalaxyDriver>(), ackIdentityDirectory);
     }
 
     private static string? NullIfBlank(string? value) => string.IsNullOrWhiteSpace(value) ? null : value.Trim();

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/GalaxyAckIdentityDirectory.cs

@@ -0,0 +1,216 @@
+using System.Collections.Frozen;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Config;
+using ZB.MOM.WW.Secrets.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Runtime;
+
+/// <summary>What the <see cref="IGalaxyAckIdentityDirectory"/> knows about one API key's Galaxy ack identity.</summary>
+public enum GalaxyAckIdentityPresence
+{
+    /// <summary>
+    ///     Not known right now — no successful refresh yet, the last one is stale, a change is pending, or the key
+    ///     id is not one the directory can look up. Callers must treat it as "may have one" (fail open).
+    /// </summary>
+    Unknown = 0,
+
+    /// <summary>The key has a Galaxy ack user secret (it may still be refused at ack time, e.g. no password).</summary>
+    Present,
+
+    /// <summary>The key has no Galaxy ack user secret as of a fresh refresh.</summary>
+    Absent,
+}
+
+/// <summary>
+///     Lock-free answer to "does this API key have a Galaxy ack identity?" (lmxopcua#765), for the synchronous
+///     native-ack pre-check (<see cref="ZB.MOM.WW.OtOpcUa.Core.Abstractions.IAlarmAcknowledgePreflight"/>), which
+///     runs under the OPC UA SDK lock and must not read the Secrets store.
+/// </summary>
+public interface IGalaxyAckIdentityDirectory
+{
+    /// <summary>Looks the key up in the cached snapshot. No I/O, never throws.</summary>
+    /// <param name="apiKeyId">The session's API key id.</param>
+    /// <returns>The key's presence, or <see cref="GalaxyAckIdentityPresence.Unknown"/> when not known.</returns>
+    GalaxyAckIdentityPresence GetPresence(string? apiKeyId);
+
+    /// <summary>Asks for a refresh soon (coalesced; never blocks). Used after a refusal so a retry sees a new identity.</summary>
+    void RequestRefresh();
+}
+
+/// <summary>
+///     Production <see cref="IGalaxyAckIdentityDirectory"/>: a periodically refreshed set of the API key ids that
+///     have a Galaxy ack user secret (<see cref="GalaxyAckIdentitySecrets.UserSecretName"/>) in this node's
+///     Secrets store — the same store <see cref="GalaxyAckCredentialResolver"/> reads at ack time.
+/// </summary>
+/// <remarks>
+///     <para>
+///         Built from <see cref="ISecretStore.ListAsync"/> metadata only — nothing is decrypted, no password is
+///         read. A refresh runs every <c>refreshInterval</c> (<see cref="RunAsync"/>), and early on
+///         <see cref="NotifySecretChanged"/> / <see cref="RequestRefresh"/>.
+///     </para>
+///     <para>
+///         ⚠️ Fails OPEN: anything not certain answers <see cref="GalaxyAckIdentityPresence.Unknown"/>, which
+///         lets the ack through to today's path (the driver resolves at ack time and the host reverts a refusal).
+///         That covers no refresh yet, a store with nothing to list, a snapshot older than <c>staleAfter</c>
+///         (e.g. the store has been failing), and a change to an identity secret signalled since the snapshot
+///         was taken — so an identity just set through AdminUI or arriving by replication is never refused in
+///         the window before the next refresh. A change the process is not told about (the <c>secret</c> CLI
+///         writing the store directly) is picked up by the next timed refresh, or the refresh a refusal kicks.
+///     </para>
+/// </remarks>
+public sealed class GalaxyAckIdentityDirectory : IGalaxyAckIdentityDirectory, IDisposable
+{
+    private const string UserSuffix = "/galaxy-ack-user";
+
+    private readonly ISecretStore? _store;
+    private readonly TimeProvider _time;
+    private readonly ILogger _logger;
+    private readonly SemaphoreSlim _wake = new(0, 1);
+
+    // Bumped on every signalled change to an identity secret; a snapshot taken under an older value is not trusted.
+    private long _changeGeneration;
+    private volatile Snapshot? _snapshot;
+
+    /// <summary>Initializes a new instance of the <see cref="GalaxyAckIdentityDirectory"/> class.</summary>
+    /// <param name="store">This node's Secrets store; null = nothing to list, so every lookup is Unknown.</param>
+    /// <param name="refreshInterval">How often <see cref="RunAsync"/> refreshes.</param>
+    /// <param name="staleAfter">How old a snapshot may get before lookups answer Unknown.</param>
+    /// <param name="timeProvider">Clock for snapshot age; defaults to the system clock.</param>
+    /// <param name="logger">Logger; never receives a secret value.</param>
+    public GalaxyAckIdentityDirectory(
+        ISecretStore? store, TimeSpan refreshInterval, TimeSpan staleAfter,
+        TimeProvider? timeProvider = null, ILogger? logger = null)
+    {
+        if (refreshInterval <= TimeSpan.Zero)
+            throw new ArgumentOutOfRangeException(nameof(refreshInterval), refreshInterval, "Must be positive.");
+        if (staleAfter <= TimeSpan.Zero)
+            throw new ArgumentOutOfRangeException(nameof(staleAfter), staleAfter, "Must be positive.");
+        _store = store;
+        RefreshInterval = refreshInterval;
+        StaleAfter = staleAfter;
+        _time = timeProvider ?? TimeProvider.System;
+        _logger = logger ?? NullLogger.Instance;
+    }
+
+    /// <summary>Gets the refresh period of <see cref="RunAsync"/>.</summary>
+    public TimeSpan RefreshInterval { get; }
+
+    /// <summary>Gets the snapshot age after which every lookup answers Unknown.</summary>
+    public TimeSpan StaleAfter { get; }
+
+    /// <inheritdoc />
+    public GalaxyAckIdentityPresence GetPresence(string? apiKeyId)
+    {
+        var snapshot = _snapshot;
+        if (snapshot is null) return GalaxyAckIdentityPresence.Unknown;
+        if (snapshot.Generation != Interlocked.Read(ref _changeGeneration)) return GalaxyAckIdentityPresence.Unknown;
+        if (_time.GetUtcNow() - snapshot.RefreshedUtc > StaleAfter) return GalaxyAckIdentityPresence.Unknown;
+        if (TryNormalize(apiKeyId) is not { } keyId) return GalaxyAckIdentityPresence.Unknown;
+        return snapshot.KeyIds.Contains(keyId) ? GalaxyAckIdentityPresence.Present : GalaxyAckIdentityPresence.Absent;
+    }
+
+    /// <inheritdoc />
+    public void RequestRefresh()
+    {
+        try
+        {
+            _wake.Release();
+        }
+        catch (SemaphoreFullException)
+        {
+            // A refresh is already requested — coalesced.
+        }
+        catch (ObjectDisposedException)
+        {
+            // Shutting down.
+        }
+    }
+
+    /// <summary>
+    ///     Tells the directory a secret changed (wired from the Secrets cache-invalidation seam, which AdminUI
+    ///     writes, the Secrets UI and replication all call). A Galaxy ack identity secret marks the current
+    ///     snapshot untrusted until the next refresh, and requests one; any other name is ignored.
+    /// </summary>
+    /// <param name="name">The changed secret.</param>
+    public void NotifySecretChanged(SecretName name)
+    {
+        if (!name.Value.StartsWith(GalaxyAckIdentitySecrets.Prefix, StringComparison.Ordinal)) return;
+        Interlocked.Increment(ref _changeGeneration);
+        RequestRefresh();
+    }
+
+    /// <summary>Lists the store once and replaces the snapshot. Never throws except on cancellation.</summary>
+    /// <param name="cancellationToken">Cancels the listing.</param>
+    /// <returns>True when the snapshot was replaced; false on a failure (the old snapshot keeps ageing).</returns>
+    public async Task<bool> RefreshAsync(CancellationToken cancellationToken)
+    {
+        if (_store is null) return false;
+        // Read BEFORE listing: a change signalled while the listing runs leaves this snapshot untrusted.
+        var generation = Interlocked.Read(ref _changeGeneration);
+        try
+        {
+            var rows = await _store.ListAsync(includeDeleted: false, cancellationToken).ConfigureAwait(false);
+            var keyIds = new HashSet<string>(StringComparer.Ordinal);
+            foreach (var row in rows)
+            {
+                if (row.IsDeleted) continue;
+                var name = row.Name.Value;
+                if (!name.StartsWith(GalaxyAckIdentitySecrets.Prefix, StringComparison.Ordinal)
+                    || !name.EndsWith(UserSuffix, StringComparison.Ordinal)
+                    || name.Length <= GalaxyAckIdentitySecrets.Prefix.Length + UserSuffix.Length) continue;
+                if (TryNormalize(name[GalaxyAckIdentitySecrets.Prefix.Length..^UserSuffix.Length]) is { } keyId)
+                    keyIds.Add(keyId);
+            }
+
+            _snapshot = new Snapshot(keyIds.ToFrozenSet(StringComparer.Ordinal), _time.GetUtcNow(), generation);
+            return true;
+        }
+        catch (Exception ex) when (ex is not OperationCanceledException)
+        {
+            _logger.LogWarning(
+                "Galaxy ack identity directory: listing the Secrets store failed ({ExceptionType}); native acks from " +
+                "keyed sessions fall back to the ack-time check once the snapshot is older than {StaleAfter}",
+                ex.GetType().Name, StaleAfter);
+            return false;
+        }
+    }
+
+    /// <summary>Refreshes now, then every <see cref="RefreshInterval"/> or sooner when asked, until cancelled.</summary>
+    /// <param name="cancellationToken">Stops the loop.</param>
+    /// <returns>A task that completes when cancelled.</returns>
+    public async Task RunAsync(CancellationToken cancellationToken)
+    {
+        try
+        {
+            while (!cancellationToken.IsCancellationRequested)
+            {
+                await RefreshAsync(cancellationToken).ConfigureAwait(false);
+                await _wake.WaitAsync(RefreshInterval, cancellationToken).ConfigureAwait(false);
+            }
+        }
+        catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+        {
+            // Normal shutdown.
+        }
+    }
+
+    /// <inheritdoc />
+    public void Dispose() => _wake.Dispose();
+
+    // Same shape GalaxyAckIdentitySecrets accepts (trimmed, lower-case ASCII letters/digits); anything else is
+    // not a key id the resolver could look up, so the directory does not claim to know about it.
+    private static string? TryNormalize(string? apiKeyId)
+    {
+        if (string.IsNullOrWhiteSpace(apiKeyId)) return null;
+        var id = apiKeyId.Trim().ToLowerInvariant();
+        foreach (var c in id)
+        {
+            if (!char.IsAsciiLetterOrDigit(c)) return null;
+        }
+
+        return id;
+    }
+
+    private sealed record Snapshot(FrozenSet<string> KeyIds, DateTimeOffset RefreshedUtc, long Generation);
+}

src/Server/ZB.MOM.WW.OtOpcUa.Host/Configuration/GalaxyAckIdentityCacheOptions.cs

@@ -0,0 +1,72 @@
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using ZB.MOM.WW.Configuration;
+
+namespace ZB.MOM.WW.OtOpcUa.Host.Configuration;
+
+/// <summary>
+///     Node-level settings for the cached set of API keys that have a Galaxy ack identity (lmxopcua#765), bound
+///     from <c>Drivers:Galaxy:AckIdentityCache</c>. The set lets the synchronous native-ack pre-check refuse a
+///     keyed session without an identity (<c>BadUserAccessDenied</c>) instead of returning Good and reverting.
+///     Overridable by environment variables <c>Drivers__Galaxy__AckIdentityCache__RefreshIntervalSeconds</c> /
+///     <c>…__StaleAfterSeconds</c>.
+/// </summary>
+public sealed class GalaxyAckIdentityCacheOptions
+{
+    /// <summary>The configuration section path.</summary>
+    public const string SectionName = "Drivers:Galaxy:AckIdentityCache";
+
+    /// <summary>How often the set is re-read from the Secrets store, in seconds (1..3600). Default 30.</summary>
+    public int RefreshIntervalSeconds { get; set; } = 30;
+
+    /// <summary>
+    ///     How old the set may get, in seconds, before keyed acks fall back to the pre-#765 Good-then-revert path
+    ///     (fail open, e.g. while the store cannot be listed). 0 (the default) = three refresh intervals. When set,
+    ///     must be greater than <see cref="RefreshIntervalSeconds"/>.
+    /// </summary>
+    public int StaleAfterSeconds { get; set; }
+
+    /// <summary>Gets the refresh period.</summary>
+    public TimeSpan RefreshInterval => TimeSpan.FromSeconds(RefreshIntervalSeconds);
+
+    /// <summary>Gets the staleness bound (explicit, or three refresh intervals).</summary>
+    public TimeSpan StaleAfter => StaleAfterSeconds > 0
+        ? TimeSpan.FromSeconds(StaleAfterSeconds)
+        : TimeSpan.FromSeconds(RefreshIntervalSeconds * 3L);
+}
+
+/// <summary>Fail-fast startup validator for <see cref="GalaxyAckIdentityCacheOptions"/>.</summary>
+public sealed class GalaxyAckIdentityCacheOptionsValidator : OptionsValidatorBase<GalaxyAckIdentityCacheOptions>
+{
+    /// <inheritdoc />
+    protected override void Validate(ValidationBuilder builder, GalaxyAckIdentityCacheOptions options)
+    {
+        builder.RequireThat(
+            options.RefreshIntervalSeconds is >= 1 and <= 3600,
+            $"{GalaxyAckIdentityCacheOptions.SectionName}:RefreshIntervalSeconds is {options.RefreshIntervalSeconds} — " +
+            "expected 1..3600.");
+        builder.RequireThat(
+            options.StaleAfterSeconds == 0 || options.StaleAfterSeconds > options.RefreshIntervalSeconds,
+            $"{GalaxyAckIdentityCacheOptions.SectionName}:StaleAfterSeconds is {options.StaleAfterSeconds} — expected 0 " +
+            $"(three refresh intervals) or more than RefreshIntervalSeconds ({options.RefreshIntervalSeconds}); otherwise " +
+            "the set would go stale between refreshes and never be used.");
+    }
+}
+
+/// <summary>Composition-root registration of <see cref="GalaxyAckIdentityCacheOptions"/>.</summary>
+public static class GalaxyAckIdentityCacheRegistration
+{
+    /// <summary>
+    ///     Binds + validates <c>Drivers:Galaxy:AckIdentityCache</c>. A bad value fails host start. Without this call
+    ///     the driver factories use the compiled defaults.
+    /// </summary>
+    /// <param name="services">The service collection.</param>
+    /// <param name="configuration">The host configuration.</param>
+    /// <returns>The same collection.</returns>
+    public static IServiceCollection AddGalaxyAckIdentityCache(this IServiceCollection services, IConfiguration configuration)
+    {
+        services.AddValidatedOptions<GalaxyAckIdentityCacheOptions, GalaxyAckIdentityCacheOptionsValidator>(
+            configuration, GalaxyAckIdentityCacheOptions.SectionName);
+        return services;
+    }
+}

src/Server/ZB.MOM.WW.OtOpcUa.Host/Configuration/SecretChangeSignallingInvalidator.cs

@@ -0,0 +1,26 @@
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Runtime;
+using ZB.MOM.WW.Secrets.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Host.Configuration;
+
+/// <summary>
+///     Decorates the Secrets library's <see cref="ISecretCacheInvalidator"/> so a secret change also reaches the
+///     <see cref="GalaxyAckIdentityDirectory"/> (lmxopcua#765). The library has no change event; this seam is the
+///     one every in-process writer already calls — the AdminUI <c>/api-keys</c> ack-identity service, the
+///     <c>/admin/secrets</c> editor and delete modal, and the replication appliers (Akka / SQL Server).
+/// </summary>
+/// <param name="inner">The library's invalidator (the resolver cache).</param>
+/// <param name="directory">The node's directory, or null on a node that runs no drivers.</param>
+internal sealed class SecretChangeSignallingInvalidator(
+    ISecretCacheInvalidator inner, GalaxyAckIdentityDirectory? directory) : ISecretCacheInvalidator
+{
+    /// <summary>Gets the decorated invalidator (test seam).</summary>
+    internal ISecretCacheInvalidator Inner => inner;
+
+    /// <inheritdoc />
+    public void Invalidate(SecretName name)
+    {
+        inner.Invalidate(name);
+        directory?.NotifySecretChanged(name);
+    }
+}

src/Server/ZB.MOM.WW.OtOpcUa.Host/Configuration/SecretsRegistration.cs

@@ -1,6 +1,8 @@
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Runtime;
+using ZB.MOM.WW.Secrets.Abstractions;
 using ZB.MOM.WW.Secrets.DependencyInjection;
 using ZB.MOM.WW.Secrets.Replicator.AkkaDotNet.DependencyInjection;
 
@@ -67,6 +69,7 @@ public static class SecretsRegistration
         if (!IsReplicationEnabled(configuration))
         {
             services.AddZbSecrets(configuration, SecretsSectionPath);
+            DecorateCacheInvalidator(services);
             return services;
         }
 
@@ -77,6 +80,27 @@ public static class SecretsRegistration
         services.AddSingleton<SecretReplicationStarter>();
         services.AddHostedService(sp => sp.GetRequiredService<SecretReplicationStarter>());
 
+        DecorateCacheInvalidator(services);
         return services;
     }
+
+    /// <summary>
+    ///     Wraps the library's <see cref="ISecretCacheInvalidator"/> registration in
+    ///     <see cref="SecretChangeSignallingInvalidator"/>, so every in-process secret write and replicated apply
+    ///     also tells the Galaxy ack identity directory (lmxopcua#765). Every consumer resolves the invalidator
+    ///     through the interface (the replication appliers lazily), so replacing the descriptor reaches them all.
+    ///     The directory is resolved optionally — admin-only nodes register none and get a pass-through.
+    /// </summary>
+    /// <param name="services">The service collection, after <c>AddZbSecrets</c>.</param>
+    internal static void DecorateCacheInvalidator(IServiceCollection services)
+    {
+        var existing = services.LastOrDefault(d => d.ServiceType == typeof(ISecretCacheInvalidator) && !d.IsKeyedService);
+        if (existing is null) return;
+        services.Remove(existing);
+        services.AddSingleton<ISecretCacheInvalidator>(sp => new SecretChangeSignallingInvalidator(
+            (ISecretCacheInvalidator)(existing.ImplementationInstance
+                ?? existing.ImplementationFactory?.Invoke(sp)
+                ?? ActivatorUtilities.CreateInstance(sp, existing.ImplementationType!)),
+            sp.GetService<GalaxyAckIdentityDirectory>()));
+    }
 }

src/Server/ZB.MOM.WW.OtOpcUa.Host/Drivers/DriverFactoryBootstrap.cs

@@ -61,6 +61,22 @@ public static class DriverFactoryBootstrap
         services.TryAddSingleton<RedundancyRoleView>();
         services.TryAddSingleton<IDriverPrimaryRoleSource>(sp => sp.GetRequiredService<RedundancyRoleView>());
 
+        // lmxopcua#765: the node-wide cached set of API keys with a Galaxy ack identity, read without I/O by every
+        // Galaxy driver's synchronous ack pre-check. Listed from this node's Secrets store (optional here, so a
+        // test graph without one gets a directory that always answers Unknown = fail open), refreshed by the
+        // hosted loop below and early on identity-secret changes (SecretsRegistration decorates the invalidator).
+        services.TryAddSingleton(sp =>
+        {
+            var options = sp.GetService<IOptions<GalaxyAckIdentityCacheOptions>>()?.Value ?? new GalaxyAckIdentityCacheOptions();
+            return new Driver.Galaxy.Runtime.GalaxyAckIdentityDirectory(
+                sp.GetService<ISecretStore>(), options.RefreshInterval, options.StaleAfter,
+                sp.GetService<TimeProvider>(),
+                sp.GetService<ILoggerFactory>()?.CreateLogger<Driver.Galaxy.Runtime.GalaxyAckIdentityDirectory>());
+        });
+        services.TryAddSingleton<Driver.Galaxy.Runtime.IGalaxyAckIdentityDirectory>(
+            sp => sp.GetRequiredService<Driver.Galaxy.Runtime.GalaxyAckIdentityDirectory>());
+        services.AddHostedService<GalaxyAckIdentityRefreshService>();
+
         services.AddSingleton<DriverFactoryRegistry>(sp =>
         {
             var registry = new DriverFactoryRegistry();
@@ -86,7 +102,7 @@ public static class DriverFactoryBootstrap
             // failures; without this it writes to NullLogger in production.
             ConfigureAbTransportLogging(sp);
             Register(registry, loggerFactory, secretResolver, primaryRoleSource, ResolveAbCipDefaultTransport(sp),
-                scriptRoot, scripting);
+                sp.GetRequiredService<Driver.Galaxy.Runtime.IGalaxyAckIdentityDirectory>(), scriptRoot, scripting);
             return registry;
         });
         services.AddSingleton<IDriverFactory>(sp =>
@@ -215,6 +231,7 @@ public static class DriverFactoryBootstrap
         ISecretResolver secretResolver,
         IDriverPrimaryRoleSource primaryRoleSource,
         Driver.AbCip.AbCipTransport abCipDefaultTransport,
+        Driver.Galaxy.Runtime.IGalaxyAckIdentityDirectory galaxyAckIdentities,
         ZB.MOM.WW.OtOpcUa.Core.Scripting.ScriptRootLogger? scriptRoot = null,
         ZB.MOM.WW.OtOpcUa.Core.Scripting.ScriptingOptions? scripting = null)
     {
@@ -222,7 +239,7 @@ public static class DriverFactoryBootstrap
         Driver.AbLegacy.AbLegacyDriverFactoryExtensions.Register(registry, loggerFactory);
         Driver.Calculation.CalculationDriverFactoryExtensions.Register(registry, loggerFactory, scriptRoot, scripting);
         Driver.FOCAS.FocasDriverFactoryExtensions.Register(registry, loggerFactory);
-        Driver.Galaxy.GalaxyDriverFactoryExtensions.Register(registry, secretResolver, loggerFactory);
+        Driver.Galaxy.GalaxyDriverFactoryExtensions.Register(registry, secretResolver, loggerFactory, galaxyAckIdentities);
         Driver.Modbus.ModbusDriverFactoryExtensions.Register(registry, loggerFactory);
         Driver.Mqtt.MqttDriverFactoryExtensions.Register(registry, loggerFactory, primaryRoleSource);
         // Tier A (the Register default): fully managed — HttpClient + System.Xml.Linq, no native SDK,

src/Server/ZB.MOM.WW.OtOpcUa.Host/Drivers/GalaxyAckIdentityRefreshService.cs

@@ -0,0 +1,16 @@
+using Microsoft.Extensions.Hosting;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Runtime;
+
+namespace ZB.MOM.WW.OtOpcUa.Host.Drivers;
+
+/// <summary>
+///     Runs the <see cref="GalaxyAckIdentityDirectory"/> refresh loop on driver nodes (lmxopcua#765): an initial
+///     refresh at start, then every <c>Drivers:Galaxy:AckIdentityCache:RefreshIntervalSeconds</c>, and early when
+///     an identity secret changes or a refusal asks for one.
+/// </summary>
+/// <param name="directory">The node's directory.</param>
+public sealed class GalaxyAckIdentityRefreshService(GalaxyAckIdentityDirectory directory) : BackgroundService
+{
+    /// <inheritdoc />
+    protected override Task ExecuteAsync(CancellationToken stoppingToken) => directory.RunAsync(stoppingToken);
+}

src/Server/ZB.MOM.WW.OtOpcUa.Host/Program.cs

@@ -194,6 +194,8 @@ builder.Services.AddValidatedOptions<LdapOptions, LdapOptionsValidator>(builder.
 // Test Connect probe (admin role, which must probe the stack the driver would run). A bad value fails host
 // start here rather than silently falling back.
 builder.Services.AddAbCipDriverDefaults(builder.Configuration);
+// lmxopcua#765: refresh/staleness of the cached API-key → Galaxy ack identity set (driver nodes consume it).
+builder.Services.AddGalaxyAckIdentityCache(builder.Configuration);
 
 if (hasDriver)
 {

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/GalaxyAckIdentityDirectoryTests.cs

@@ -0,0 +1,356 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Config;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Runtime;
+using ZB.MOM.WW.Secrets.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests.Runtime;
+
+/// <summary>
+///     lmxopcua#765 item 3 — a keyed session whose API key has no Galaxy ack identity is refused synchronously
+///     (<c>BadUserAccessDenied</c> via <see cref="ZB.MOM.WW.OtOpcUa.Core.Abstractions.IAlarmAcknowledgePreflight"/>)
+///     from a cached, periodically refreshed set read without I/O, and everything the set is not certain about
+///     fails open to the Good-then-revert path.
+/// </summary>
+public sealed class GalaxyAckIdentityDirectoryTests
+{
+    private const string WithIdentity = "0a1b2c3d4e5f6071";
+    private const string WithoutIdentity = "ffeeddccbbaa0099";
+    private static readonly TimeSpan Interval = TimeSpan.FromSeconds(30);
+    private static readonly TimeSpan StaleAfter = TimeSpan.FromSeconds(90);
+
+    // ------------------------------------------------------------------ driver pre-check
+
+    [Fact]
+    public async Task Keyed_session_without_an_identity_is_refused_synchronously()
+    {
+        var (directory, _, _) = await RefreshedDirectoryAsync();
+        using var driver = NewDriver(directory);
+
+        var refusal = driver.GetAcknowledgeRefusal(WithoutIdentity);
+
+        refusal.ShouldNotBeNull();
+        refusal.ShouldContain(WithoutIdentity);
+        refusal.ShouldContain("no Galaxy ack identity");
+        refusal.ShouldContain("drv");
+    }
+
+    [Fact]
+    public async Task Keyed_session_with_an_identity_proceeds()
+    {
+        var (directory, _, _) = await RefreshedDirectoryAsync();
+        using var driver = NewDriver(directory);
+
+        driver.GetAcknowledgeRefusal(WithIdentity).ShouldBeNull();
+        driver.GetAcknowledgeRefusal(WithIdentity.ToUpperInvariant()).ShouldBeNull("key ids are case-insensitive");
+    }
+
+    [Fact]
+    public async Task A_fallback_account_lets_a_keyed_session_without_an_identity_through()
+    {
+        var (directory, _, _) = await RefreshedDirectoryAsync();
+        using var driver = NewDriver(directory, Opts() with
+        {
+            AlarmAck = new GalaxyAlarmAckOptions("ack-svc", "PLANT", "secret:otopcua/galaxy/ack-fallback"),
+        });
+
+        driver.GetAcknowledgeRefusal(WithoutIdentity).ShouldBeNull();
+    }
+
+    [Fact]
+    public void Without_a_directory_a_keyed_session_is_never_pre_refused()
+    {
+        using var driver = NewDriver(directory: null);
+        driver.GetAcknowledgeRefusal(WithoutIdentity).ShouldBeNull();
+        driver.GetAcknowledgeRefusal(apiKeyId: null).ShouldNotBeNull("the #760 anonymous refusal is unchanged");
+    }
+
+    [Fact]
+    public async Task A_refusal_asks_for_a_refresh_so_a_retry_sees_an_identity_set_out_of_band()
+    {
+        var (directory, store, _) = await RefreshedDirectoryAsync(TimeSpan.FromHours(1));
+        using var driver = NewDriver(directory);
+        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(30));
+        var loop = directory.RunAsync(cts.Token);
+        await WaitForAsync(() => store.ListCalls >= 2, cts.Token); // RunAsync's own initial refresh
+
+        // Set by the `secret` CLI: nothing in this process is told.
+        store.AddIdentity(WithoutIdentity);
+        driver.GetAcknowledgeRefusal(WithoutIdentity).ShouldNotBeNull();
+
+        // The refusal kicked a refresh well inside the one-hour interval.
+        await WaitForAsync(() => driver.GetAcknowledgeRefusal(WithoutIdentity) is null, cts.Token);
+
+        await cts.CancelAsync();
+        await loop;
+    }
+
+    // ------------------------------------------------------------------ fail open
+
+    [Fact]
+    public void Before_the_first_refresh_every_key_is_Unknown_and_proceeds()
+    {
+        using var directory = new GalaxyAckIdentityDirectory(new FakeSecretStore(), Interval, StaleAfter);
+        using var driver = NewDriver(directory);
+
+        directory.GetPresence(WithoutIdentity).ShouldBe(GalaxyAckIdentityPresence.Unknown);
+        driver.GetAcknowledgeRefusal(WithoutIdentity).ShouldBeNull();
+    }
+
+    [Fact]
+    public async Task A_stale_snapshot_is_Unknown_and_proceeds()
+    {
+        var (directory, store, clock) = await RefreshedDirectoryAsync();
+        using var driver = NewDriver(directory);
+        driver.GetAcknowledgeRefusal(WithoutIdentity).ShouldNotBeNull("positive control: fresh snapshot refuses");
+
+        // The store starts failing: the old snapshot is kept, and ages out.
+        store.Fail = true;
+        (await directory.RefreshAsync(CancellationToken.None)).ShouldBeFalse();
+        clock.Advance(StaleAfter - TimeSpan.FromSeconds(1));
+        directory.GetPresence(WithoutIdentity).ShouldBe(GalaxyAckIdentityPresence.Absent);
+
+        clock.Advance(TimeSpan.FromSeconds(2));
+        directory.GetPresence(WithoutIdentity).ShouldBe(GalaxyAckIdentityPresence.Unknown);
+        directory.GetPresence(WithIdentity).ShouldBe(GalaxyAckIdentityPresence.Unknown);
+        driver.GetAcknowledgeRefusal(WithoutIdentity).ShouldBeNull();
+    }
+
+    [Fact]
+    public async Task A_signalled_identity_change_makes_the_snapshot_Unknown_until_the_next_refresh()
+    {
+        var (directory, store, _) = await RefreshedDirectoryAsync();
+        using var driver = NewDriver(directory);
+        driver.GetAcknowledgeRefusal(WithoutIdentity).ShouldNotBeNull("positive control");
+
+        // AdminUI sets the identity: the write invalidates the secret, the directory is told.
+        store.AddIdentity(WithoutIdentity);
+        directory.NotifySecretChanged(new SecretName(GalaxyAckIdentitySecrets.UserSecretName(WithoutIdentity)));
+
+        directory.GetPresence(WithoutIdentity).ShouldBe(GalaxyAckIdentityPresence.Unknown);
+        driver.GetAcknowledgeRefusal(WithoutIdentity).ShouldBeNull("never refuse in the window before the refresh");
+
+        (await directory.RefreshAsync(CancellationToken.None)).ShouldBeTrue();
+        directory.GetPresence(WithoutIdentity).ShouldBe(GalaxyAckIdentityPresence.Present);
+    }
+
+    [Fact]
+    public async Task A_change_signalled_during_a_refresh_leaves_that_snapshot_untrusted()
+    {
+        var (directory, store, _) = await RefreshedDirectoryAsync();
+        store.OnList = () => directory.NotifySecretChanged(
+            new SecretName(GalaxyAckIdentitySecrets.PasswordSecretName(WithoutIdentity)));
+
+        (await directory.RefreshAsync(CancellationToken.None)).ShouldBeTrue();
+
+        directory.GetPresence(WithoutIdentity).ShouldBe(GalaxyAckIdentityPresence.Unknown);
+    }
+
+    [Fact]
+    public async Task An_unrelated_secret_change_does_not_untrust_the_snapshot()
+    {
+        var (directory, _, _) = await RefreshedDirectoryAsync();
+        directory.NotifySecretChanged(new SecretName("otopcua/galaxy/ack-fallback"));
+        directory.GetPresence(WithoutIdentity).ShouldBe(GalaxyAckIdentityPresence.Absent);
+    }
+
+    [Fact]
+    public async Task A_directory_without_a_store_is_always_Unknown()
+    {
+        using var directory = new GalaxyAckIdentityDirectory(store: null, Interval, StaleAfter);
+        (await directory.RefreshAsync(CancellationToken.None)).ShouldBeFalse();
+        directory.GetPresence(WithoutIdentity).ShouldBe(GalaxyAckIdentityPresence.Unknown);
+    }
+
+    [Fact]
+    public async Task A_key_id_the_resolver_could_not_look_up_is_Unknown()
+    {
+        var (directory, _, _) = await RefreshedDirectoryAsync();
+        directory.GetPresence("../etc").ShouldBe(GalaxyAckIdentityPresence.Unknown);
+        directory.GetPresence(" ").ShouldBe(GalaxyAckIdentityPresence.Unknown);
+        directory.GetPresence(null).ShouldBe(GalaxyAckIdentityPresence.Unknown);
+    }
+
+    // ------------------------------------------------------------------ refresh
+
+    [Fact]
+    public async Task Refresh_picks_up_added_and_removed_identities()
+    {
+        var (directory, store, _) = await RefreshedDirectoryAsync();
+        directory.GetPresence(WithoutIdentity).ShouldBe(GalaxyAckIdentityPresence.Absent);
+
+        store.AddIdentity(WithoutIdentity);
+        directory.GetPresence(WithoutIdentity).ShouldBe(GalaxyAckIdentityPresence.Absent, "a cache, until refreshed");
+        (await directory.RefreshAsync(CancellationToken.None)).ShouldBeTrue();
+        directory.GetPresence(WithoutIdentity).ShouldBe(GalaxyAckIdentityPresence.Present);
+
+        store.RemoveIdentity(WithIdentity, softDelete: true);
+        (await directory.RefreshAsync(CancellationToken.None)).ShouldBeTrue();
+        directory.GetPresence(WithIdentity).ShouldBe(GalaxyAckIdentityPresence.Absent, "a deleted user secret is no identity");
+    }
+
+    [Fact]
+    public async Task Only_user_secrets_count_as_an_identity()
+    {
+        var store = new FakeSecretStore();
+        store.Add(GalaxyAckIdentitySecrets.PasswordSecretName(WithoutIdentity)); // password without user
+        store.Add("otopcua/galaxy/ack-fallback");
+        using var directory = new GalaxyAckIdentityDirectory(store, Interval, StaleAfter);
+
+        (await directory.RefreshAsync(CancellationToken.None)).ShouldBeTrue();
+
+        directory.GetPresence(WithoutIdentity).ShouldBe(GalaxyAckIdentityPresence.Absent);
+    }
+
+    [Fact]
+    public async Task The_loop_refreshes_on_its_interval()
+    {
+        var store = new FakeSecretStore();
+        using var directory = new GalaxyAckIdentityDirectory(store, TimeSpan.FromMilliseconds(50), TimeSpan.FromMinutes(1));
+        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(30));
+        var loop = directory.RunAsync(cts.Token);
+
+        await WaitForAsync(() => directory.GetPresence(WithIdentity) == GalaxyAckIdentityPresence.Absent, cts.Token);
+        store.AddIdentity(WithIdentity);
+        await WaitForAsync(() => directory.GetPresence(WithIdentity) == GalaxyAckIdentityPresence.Present, cts.Token);
+
+        await cts.CancelAsync();
+        await loop; // completes cleanly on cancellation
+    }
+
+    [Fact]
+    public async Task The_loop_refreshes_early_on_a_signalled_change()
+    {
+        var store = new FakeSecretStore();
+        using var directory = new GalaxyAckIdentityDirectory(store, TimeSpan.FromHours(1), TimeSpan.FromHours(3));
+        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(30));
+        var loop = directory.RunAsync(cts.Token);
+        await WaitForAsync(() => directory.GetPresence(WithIdentity) == GalaxyAckIdentityPresence.Absent, cts.Token);
+
+        store.AddIdentity(WithIdentity);
+        directory.NotifySecretChanged(new SecretName(GalaxyAckIdentitySecrets.UserSecretName(WithIdentity)));
+
+        await WaitForAsync(() => directory.GetPresence(WithIdentity) == GalaxyAckIdentityPresence.Present, cts.Token);
+        await cts.CancelAsync();
+        await loop;
+    }
+
+    // ------------------------------------------------------------------ helpers
+
+    private static async Task<(GalaxyAckIdentityDirectory Directory, FakeSecretStore Store, ManualClock Clock)>
+        RefreshedDirectoryAsync(TimeSpan? interval = null)
+    {
+        var store = new FakeSecretStore();
+        store.AddIdentity(WithIdentity);
+        var clock = new ManualClock();
+        var directory = new GalaxyAckIdentityDirectory(
+            store, interval ?? Interval, interval is { } i ? i * 3 : StaleAfter, clock);
+        (await directory.RefreshAsync(CancellationToken.None)).ShouldBeTrue();
+        return (directory, store, clock);
+    }
+
+    private static async Task WaitForAsync(Func<bool> condition, CancellationToken ct)
+    {
+        while (!condition())
+        {
+            ct.ThrowIfCancellationRequested();
+            await Task.Delay(10, ct);
+        }
+    }
+
+    private static GalaxyDriverOptions Opts() => new(
+        new GalaxyGatewayOptions("https://mxgw.test:5001", "dev:key"),
+        new GalaxyMxAccessOptions("OtOpcUa-A"),
+        new GalaxyRepositoryOptions(),
+        new GalaxyReconnectOptions());
+
+    private static GalaxyDriver NewDriver(IGalaxyAckIdentityDirectory? directory, GalaxyDriverOptions? options = null) =>
+        new("drv", options ?? Opts(), hierarchySource: null, ackIdentityDirectory: directory);
+
+    private sealed class ManualClock : TimeProvider
+    {
+        private DateTimeOffset _now = new(2026, 11, 9, 12, 0, 0, TimeSpan.Zero);
+
+        public void Advance(TimeSpan by) => _now += by;
+
+        public override DateTimeOffset GetUtcNow() => _now;
+    }
+
+    /// <summary>Metadata-only store: <see cref="ListAsync"/> is all the directory may call.</summary>
+    private sealed class FakeSecretStore : ISecretStore
+    {
+        private readonly Dictionary<string, bool> _rows = new(StringComparer.Ordinal); // name → deleted
+        private readonly Lock _lock = new();
+        private int _listCalls;
+
+        public bool Fail { get; set; }
+
+        public Action? OnList { get; set; }
+
+        public int ListCalls => Volatile.Read(ref _listCalls);
+
+        public void Add(string name)
+        {
+            lock (_lock) _rows[name] = false;
+        }
+
+        public void AddIdentity(string keyId)
+        {
+            Add(GalaxyAckIdentitySecrets.PasswordSecretName(keyId));
+            Add(GalaxyAckIdentitySecrets.UserSecretName(keyId));
+        }
+
+        public void RemoveIdentity(string keyId, bool softDelete)
+        {
+            lock (_lock)
+            {
+                foreach (var name in new[]
+                         {
+                             GalaxyAckIdentitySecrets.UserSecretName(keyId), GalaxyAckIdentitySecrets.PasswordSecretName(keyId),
+                         })
+                {
+                    if (softDelete) _rows[name] = true;
+                    else _rows.Remove(name);
+                }
+            }
+        }
+
+        public Task<IReadOnlyList<SecretMetadata>> ListAsync(bool includeDeleted, CancellationToken ct)
+        {
+            Interlocked.Increment(ref _listCalls);
+            OnList?.Invoke();
+            if (Fail) throw new InvalidOperationException("store unavailable");
+            lock (_lock)
+            {
+                // Deleted rows are returned regardless, to prove the directory filters them itself.
+                IReadOnlyList<SecretMetadata> rows = _rows.Select(r => new SecretMetadata
+                {
+                    Name = new SecretName(r.Key),
+                    ContentType = SecretContentType.Text,
+                    KekId = "k",
+                    Revision = 1,
+                    IsDeleted = r.Value,
+                    CreatedUtc = DateTimeOffset.UnixEpoch,
+                    UpdatedUtc = DateTimeOffset.UnixEpoch,
+                }).ToList();
+                return Task.FromResult(rows);
+            }
+        }
+
+        public Task<StoredSecret?> GetAsync(SecretName name, CancellationToken ct) => throw Unexpected();
+
+        public Task UpsertAsync(StoredSecret row, CancellationToken ct) => throw Unexpected();
+
+        public Task<bool> DeleteAsync(SecretName name, string? actor, CancellationToken ct) => throw Unexpected();
+
+        public Task<IReadOnlyList<SecretManifestEntry>> GetManifestAsync(CancellationToken ct) => throw Unexpected();
+
+        public Task ApplyReplicatedAsync(StoredSecret row, CancellationToken ct) => throw Unexpected();
+
+        public Task<bool> ApplyRewrapAsync(StoredSecret rewrappedRow, byte[] expectedCurrentWrappedDek, CancellationToken ct) =>
+            throw Unexpected();
+
+        // The directory must read metadata only — never a row (that would decrypt-adjacent I/O per key).
+        private static InvalidOperationException Unexpected() => new("the directory may only list metadata");
+    }
+}

tests/Server/ZB.MOM.WW.OtOpcUa.Host.Tests/Configuration/GalaxyAckIdentityCacheWiringTests.cs

@@ -0,0 +1,208 @@
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Options;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Core.Hosting;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Config;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Runtime;
+using ZB.MOM.WW.OtOpcUa.Host.Configuration;
+using ZB.MOM.WW.OtOpcUa.Host.Drivers;
+using ZB.MOM.WW.Secrets;
+using ZB.MOM.WW.Secrets.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Host.Tests.Configuration;
+
+/// <summary>
+///     lmxopcua#765 item 3 — the host wiring of the cached API-key → Galaxy ack identity set: the driver registry
+///     hands every Galaxy driver the node's directory (so the synchronous pre-check can refuse a keyed session
+///     without an identity), a refresh loop runs on driver nodes, secret writes signal it through the decorated
+///     invalidator, and the refresh interval is configurable.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class GalaxyAckIdentityCacheWiringTests
+{
+    private const string WithIdentity = "0a1b2c3d4e5f6071";
+    private const string WithoutIdentity = "ffeeddccbbaa0099";
+
+    private const string GalaxyConfig = """
+        {
+          "Gateway": { "Endpoint": "https://mxgw.test:5001", "ApiKeySecretRef": "dev:key" },
+          "MxAccess": { "ClientName": "OtOpcUa-A" }
+        }
+        """;
+
+    [Fact]
+    public async Task A_registry_built_Galaxy_driver_refuses_a_keyed_session_without_an_identity()
+    {
+        var store = new ListOnlySecretStore(GalaxyAckIdentitySecrets.UserSecretName(WithIdentity));
+        using var sp = DriverServices(store).BuildServiceProvider();
+        var directory = sp.GetRequiredService<GalaxyAckIdentityDirectory>();
+        (await directory.RefreshAsync(CancellationToken.None)).ShouldBeTrue();
+
+        var driver = sp.GetRequiredService<DriverFactoryRegistry>()
+            .TryGet(GalaxyDriverFactoryExtensions.DriverTypeName)!.Invoke("galaxy-1", GalaxyConfig);
+        var preflight = driver.ShouldBeAssignableTo<IAlarmAcknowledgePreflight>()!;
+
+        preflight.GetAcknowledgeRefusal(WithoutIdentity).ShouldNotBeNull();
+        preflight.GetAcknowledgeRefusal(WithIdentity).ShouldBeNull();
+        (driver as IDisposable)?.Dispose();
+    }
+
+    [Fact]
+    public void Driver_nodes_run_the_refresh_loop_and_share_one_directory()
+    {
+        var services = DriverServices(new ListOnlySecretStore());
+        services.AddOtOpcUaDriverFactories(); // a fused admin,driver node may reach it twice
+        // By descriptor: resolving every IHostedService would need the resilience publisher's whole graph.
+        services.Count(d => d.ServiceType == typeof(IHostedService)
+            && d.ImplementationType == typeof(GalaxyAckIdentityRefreshService)).ShouldBe(1);
+        using var sp = services.BuildServiceProvider();
+        sp.GetRequiredService<IGalaxyAckIdentityDirectory>()
+            .ShouldBeSameAs(sp.GetRequiredService<GalaxyAckIdentityDirectory>());
+    }
+
+    [Fact]
+    public void The_refresh_interval_and_staleness_are_configurable()
+    {
+        var services = DriverServices(new ListOnlySecretStore());
+        services.AddGalaxyAckIdentityCache(Config(new()
+        {
+            ["Drivers:Galaxy:AckIdentityCache:RefreshIntervalSeconds"] = "7",
+            ["Drivers:Galaxy:AckIdentityCache:StaleAfterSeconds"] = "50",
+        }));
+        using var sp = services.BuildServiceProvider();
+
+        var directory = sp.GetRequiredService<GalaxyAckIdentityDirectory>();
+        directory.RefreshInterval.ShouldBe(TimeSpan.FromSeconds(7));
+        directory.StaleAfter.ShouldBe(TimeSpan.FromSeconds(50));
+    }
+
+    [Fact]
+    public void Defaults_are_30s_and_three_intervals()
+    {
+        using var sp = DriverServices(new ListOnlySecretStore()).BuildServiceProvider();
+        var directory = sp.GetRequiredService<GalaxyAckIdentityDirectory>();
+        directory.RefreshInterval.ShouldBe(TimeSpan.FromSeconds(30));
+        directory.StaleAfter.ShouldBe(TimeSpan.FromSeconds(90));
+    }
+
+    [Theory]
+    [InlineData("0", "0")]
+    [InlineData("3601", "0")]
+    [InlineData("30", "30")]
+    public void Bad_cache_settings_fail_validation(string interval, string staleAfter)
+    {
+        var services = new ServiceCollection();
+        services.AddGalaxyAckIdentityCache(Config(new()
+        {
+            ["Drivers:Galaxy:AckIdentityCache:RefreshIntervalSeconds"] = interval,
+            ["Drivers:Galaxy:AckIdentityCache:StaleAfterSeconds"] = staleAfter,
+        }));
+        using var sp = services.BuildServiceProvider();
+
+        Should.Throw<OptionsValidationException>(() => sp.GetRequiredService<IOptions<GalaxyAckIdentityCacheOptions>>().Value);
+    }
+
+    [Fact]
+    public async Task AddOtOpcUaSecrets_decorates_the_invalidator_so_an_identity_write_untrusts_the_directory()
+    {
+        // Registered before AddOtOpcUaSecrets, so the library's TryAdd keeps this store for the directory's listing.
+        var services = DriverServices(new ListOnlySecretStore());
+        services.AddLogging();
+        services.AddOtOpcUaSecrets(Config(new()
+        {
+            // Resolving the invalidator never opens the store; the path only has to pass validation.
+            ["Secrets:SqlitePath"] = Path.Combine(Path.GetTempPath(), $"otopcua-secrets-{Guid.NewGuid():N}.db"),
+            ["Secrets:MasterKey:Source"] = "Environment",
+            ["Secrets:MasterKey:EnvVarName"] = "ZB_SECRETS_MASTER_KEY",
+            ["Secrets:RunMigrationsOnStartup"] = "false",
+        }));
+        using var sp = services.BuildServiceProvider();
+
+        var invalidator = sp.GetRequiredService<ISecretCacheInvalidator>();
+        var decorated = invalidator.ShouldBeOfType<SecretChangeSignallingInvalidator>();
+        decorated.Inner.ShouldBeOfType<DefaultSecretResolver>();
+        decorated.Inner.ShouldBeSameAs(sp.GetRequiredService<DefaultSecretResolver>(), "the resolver cache is still invalidated");
+
+        var directory = sp.GetRequiredService<GalaxyAckIdentityDirectory>();
+        (await directory.RefreshAsync(CancellationToken.None)).ShouldBeTrue();
+        directory.GetPresence(WithoutIdentity).ShouldBe(GalaxyAckIdentityPresence.Absent);
+
+        // What ApiKeyGalaxyAckIdentityService.SetAsync does after writing the user secret.
+        invalidator.Invalidate(new SecretName(GalaxyAckIdentitySecrets.UserSecretName(WithoutIdentity)));
+
+        directory.GetPresence(WithoutIdentity).ShouldBe(GalaxyAckIdentityPresence.Unknown);
+    }
+
+    [Fact]
+    public void Without_a_directory_the_decorated_invalidator_is_a_pass_through()
+    {
+        var inner = new RecordingInvalidator();
+        var services = new ServiceCollection();
+        services.AddSingleton<ISecretCacheInvalidator>(_ => inner);
+        SecretsRegistration.DecorateCacheInvalidator(services);
+        using var sp = services.BuildServiceProvider();
+
+        var name = new SecretName("otopcua/apikeys/abc/galaxy-ack-user");
+        sp.GetRequiredService<ISecretCacheInvalidator>().Invalidate(name);
+
+        inner.Names.ShouldBe([name]);
+    }
+
+    // ------------------------------------------------------------------ helpers
+
+    private static IServiceCollection DriverServices(ISecretStore store)
+    {
+        var services = new ServiceCollection();
+        services.AddSingleton<ISecretResolver>(new NullResolver());
+        services.AddSingleton(store);
+        services.AddOtOpcUaDriverFactories();
+        return services;
+    }
+
+    private static IConfiguration Config(Dictionary<string, string?> values) =>
+        new ConfigurationBuilder().AddInMemoryCollection(values).Build();
+
+    private sealed class NullResolver : ISecretResolver
+    {
+        public Task<string?> GetAsync(SecretName name, CancellationToken ct) => Task.FromResult<string?>(null);
+    }
+
+    private sealed class RecordingInvalidator : ISecretCacheInvalidator
+    {
+        public List<SecretName> Names { get; } = [];
+
+        public void Invalidate(SecretName name) => Names.Add(name);
+    }
+
+    private sealed class ListOnlySecretStore(params string[] names) : ISecretStore
+    {
+        public Task<IReadOnlyList<SecretMetadata>> ListAsync(bool includeDeleted, CancellationToken ct) =>
+            Task.FromResult<IReadOnlyList<SecretMetadata>>(names.Select(n => new SecretMetadata
+            {
+                Name = new SecretName(n),
+                ContentType = SecretContentType.Text,
+                KekId = "k",
+                Revision = 1,
+                CreatedUtc = DateTimeOffset.UnixEpoch,
+                UpdatedUtc = DateTimeOffset.UnixEpoch,
+            }).ToList());
+
+        public Task<StoredSecret?> GetAsync(SecretName name, CancellationToken ct) => throw new NotSupportedException();
+
+        public Task UpsertAsync(StoredSecret row, CancellationToken ct) => throw new NotSupportedException();
+
+        public Task<bool> DeleteAsync(SecretName name, string? actor, CancellationToken ct) => throw new NotSupportedException();
+
+        public Task<IReadOnlyList<SecretManifestEntry>> GetManifestAsync(CancellationToken ct) => throw new NotSupportedException();
+
+        public Task ApplyReplicatedAsync(StoredSecret row, CancellationToken ct) => throw new NotSupportedException();
+
+        public Task<bool> ApplyRewrapAsync(StoredSecret rewrappedRow, byte[] expectedCurrentWrappedDek, CancellationToken ct) =>
+            throw new NotSupportedException();
+    }
+}