fix(host): follow an in-place site PSK rotation without a central restart (ScadaBridge#96)

SitePskProvider cached each site's gRPC preshared key forever; only site removal
invalidated it, so rotating SB-GRPC-PSK-{site} broke the central<->site link on
both central nodes until they restarted. The cached key is now dropped when:

- the secret is written through /admin/secrets: central replaces the secrets
  library's ISecretCacheInvalidator with SitePskRotationInvalidator, which also
  publishes SitePskInvalidated on a new DistributedPubSub topic
  (site-psk-invalidation) so the peer central node drops its copy and its
  resolver cache entry too (SitePskInvalidationSubscriber);
- a site rejects a central->site call with PermissionDenied/Unauthenticated:
  WithSiteCredentials wraps every per-site channel's handler in
  PskRejectionRefreshHandler, so the next call re-reads the key (covers
  out-of-process writes such as the secret CLI);
- a site->central call presents a token that does not match the cached key:
  CentralControlAuthInterceptor invalidates and re-reads once before refusing;
- ScadaBridge:Communication:SitePsks reloads.

Invalidation is case-insensitive because SecretName lowercases names. Rotation
runbook (topology-guide), Component-Communication/Host and the #85 cache audit
(D2) updated.

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

docs/deployment/topology-guide.md

@@ -557,8 +557,14 @@ One key **per site**, never one for the fleet: a compromised site must not yield
 key. And never share it with `LocalDb:Replication:ApiKey` — that authenticates the *pair partner*
 for database replication, a different trust relationship on the same listener.
 
-**Rotation:** set the new value on both sides, then restart the pair (pairs restart together
+**Rotation:** set the new value on both sides, then restart the site pair (pairs restart together
-anyway — see above). **Upgrading to a build that has this gate requires seeding the key first**,
+anyway — see above; the site reads its key once, at boot). **Central needs no restart** (Gitea #96):
+writing `SB-GRPC-PSK-<siteId>` through `/admin/secrets` on either central node drops the cached key
+on both central nodes at once, and a key changed any other way (the `secret` CLI, a `SitePsks`
+edit) is picked up on the first call the site rejects, or the first site call central's own gate
+cannot match — within `Secrets:ResolveCacheTtl` (30 s) of
+the change. Expect the link to that site to fail between the two halves of the change; doing
+central first and the site straight after keeps that window short. **Upgrading to a build that has this gate requires seeding the key first**,
 including in the on-host `deploy/` overlays.
 
 The key is a bearer token over plaintext h2c, so it is readable and replayable by anyone on the

docs/plans/2026-11-09-central-cache-staleness-audit.md

@@ -28,7 +28,7 @@ node after a change is made through the other one. Same defect class as #83
 | `PendingUpdatesCache` (DeploymentManager, #91) | Fleet pending-updates snapshot | Deployment finished on this node | `DeploymentManager:PendingUpdatesCacheTtl`, default **60 s** | OK — display only |
 | `CentralCommunicationActor` site-address cache (+ `GrpcSiteTransport` channel pairs) | Site gRPC endpoints | Site create/update/delete → `RefreshSiteAddresses` | Periodic DB reload, **60 s** (hard-coded) | OK — rare edit, self-heals |
 | `SiteStreamGrpcClientFactory` clients | Per (site, endpoint) stream clients | Site delete → `RemoveSiteAsync` (local only) | Not stale data: a removed site's client on the peer is reclaimed by idle eviction, `Communication:GrpcClientIdleTtl` (**30 min**) × 1.5 after last use | OK — resource, not correctness |
-| `SitePskProvider` (Host) | Per-site gRPC PSK from config / secret store | Site removal (via either gRPC client path, on every node within the 60 s reload) | **Unbounded** for an in-place key rotation — on **both** nodes, not only the peer | Owner decision — see D2 |
+| `SitePskProvider` (Host) | Per-site gRPC PSK from config / secret store | Site removal; `SB-GRPC-PSK-{site}` write via `/admin/secrets`; site auth rejection; `SitePsks` reload (#96) | Write via `/admin/secrets`: **none** (peer told over `DistributedPubSub` topic `site-psk-invalidation`); other writes: first rejected call after `Secrets:ResolveCacheTtl` | Fixed — #96 (see D2) |
 | `DefaultSecretResolver` (shared `ZB.MOM.WW.Secrets`) | Decrypted secret values | Lib's own invalidator (UI writes, sync service) | `Secrets:ResolveCacheTtl`, **30 s** | OK — owned by the shared lib |
 | `KpiSnapshotCache` (CentralUI) | KPI query results (operational data) | — | **8 s** | OK — not config |
 | `SharedAlarmSummaryService` (CentralUI) | Per-site alarm summary fan-out | — | **12 s** cold-cache window | OK — not config |
@@ -69,7 +69,9 @@ broadcast was added.
   new key) leaves **both** central nodes dialling with the old key until they restart. Options: make
   central restart part of the rotation runbook; give the cache a TTL; or invalidate on a
   `PermissionDenied` from the site. Not peer-specific, so it is out of #85's fix scope — file it
-  separately.
+  separately. **Resolved by #96:** all three — invalidate on the rotation write (and the peer, via
+  the new `site-psk-invalidation` topic; the first config-invalidation broadcast topic D1/D3 could
+  join), refresh on a site's auth rejection, and drop the cache on a `SitePsks` reload.
 - **D3 — `ExternalDefinitionLookupCache` peer bound (10 s).** Already tracked by #83; the
   recommendation there (accept and document) still stands. If D1 gets a broadcast topic, this cache
   and `ApiMethodCache` should join it rather than add a second mechanism.

docs/requirements/Component-Communication.md

@@ -204,6 +204,18 @@ because sites are created at runtime and cannot be enumerated in configuration a
 `ControlPlaneCredentials` binds the key to the channel as `CallCredentials`, so no individual call
 site can omit it.
 
+Central caches each resolved key, and drops the entry (Gitea #96) when: the site is removed; the
+secret `SB-GRPC-PSK-<siteId>` is written through central's secrets write path (`/admin/secrets`) —
+`SitePskRotationInvalidator` wraps the secrets library's `ISecretCacheInvalidator` and also
+broadcasts a `SitePskInvalidated` on the central `DistributedPubSub` topic `site-psk-invalidation`
+so the peer central node drops its copy too; the site rejects a call with `PermissionDenied` /
+`Unauthenticated` (`PskRejectionRefreshHandler`, wrapped around every per-site channel by
+`WithSiteCredentials`, so the next call re-reads the key — this covers keys rotated outside the
+process, e.g. by the `secret` CLI, after at most the resolver's `Secrets:ResolveCacheTtl`); a
+site's inbound call to central presents a token that does not match the cached key
+(`CentralControlAuthInterceptor` drops the entry and re-reads once before refusing); or
+`ScadaBridge:Communication:SitePsks` reloads. An in-place rotation therefore needs no central restart.
+
 This is distinct from `LocalDb:Replication:ApiKey`, which gates
 `/localdb_sync.v1.LocalDbSync/` on the same listener via its own interceptor: that key
 authenticates the *pair partner* for database replication, a different trust relationship, and the

docs/requirements/Component-Host.md

@@ -351,4 +351,5 @@ built)* section for where the shipped code deviates from the original design).
 - **Port options come from the bound `NodeOptions`.** `Program.cs` binds `ScadaBridge:Node` once and reads `GrpcPort` / `CentralGrpcPort` / `MetricsPort` from it; the 8083/8084 defaults live only in `NodeOptions`. Central's dedicated metrics listener remains an explicit *presence* check on the raw key (a defaulted read would bind a new port on every existing central node).
 - **Inbound method warm-up is a hosted service** (`InboundMethodWarmupService`, registered after `AkkaHostedService`), not a pre-`RunAsync` loop — see Component-InboundAPI.md.
 - **`SitePskProvider` distinguishes a secret-store fault from a missing secret.** A store fault logs a Warning with the exception and throws `SecretStoreUnavailableException` (not cached); only a genuinely absent/empty/tombstoned secret produces the "seed `SB-GRPC-PSK-{site}`" message.
+- **`SitePskProvider`'s cache follows an in-place rotation (Gitea #96).** Central replaces the secrets library's `ISecretCacheInvalidator` with `SitePskRotationInvalidator` (`AddSitePskRotationInvalidation`), which drops the site's cached key when `SB-GRPC-PSK-{site}` is written and publishes `SitePskInvalidated` on the `site-psk-invalidation` `DistributedPubSub` topic; `AkkaHostedService.RegisterCentralActors` starts the `SitePskInvalidationSubscriber` that applies the peer's broadcasts. A site's auth rejection and a `SitePsks` reload also drop the cache — see Component-Communication.md → *Authentication*.
 - Central registers `ExternalSystemGatewayOptions.DefinitionCacheTtl = 10 s` ahead of `AddExternalSystemGateway()` so an explicit config value wins; sites leave it off.

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/ISitePskProvider.cs

@@ -110,6 +110,12 @@ public static class ControlPlaneCredentials
     /// A null provider leaves the options untouched — the shape used by test-only and
     /// default constructors that never dial a gated endpoint.
     /// </summary>
+    /// <remarks>
+    /// Also wraps the channel's HTTP handler in <see cref="PskRejectionRefreshHandler"/>, so a
+    /// call the peer rejects as unauthenticated drops the cached key and the NEXT call re-reads
+    /// it (Gitea #96). The rejected call itself is not retried; every caller already retries
+    /// (heartbeats, stream reconnects, the next pull tick).
+    /// </remarks>
     /// <param name="options">Channel options being built.</param>
     /// <param name="provider">Key provider, or null to leave the channel unauthenticated.</param>
     /// <param name="siteId">The site this channel talks to.</param>
@@ -127,6 +133,78 @@ public static class ControlPlaneCredentials
         options.Credentials = ChannelCredentials.Create(
             ChannelCredentials.Insecure, ForSite(provider, siteId));
         options.UnsafeUseInsecureChannelCallCredentials = true;
+        if (options.HttpHandler is not PskRejectionRefreshHandler)
+        {
+            // Null means "let GrpcChannel build its default", which is this same handler shape.
+            options.HttpHandler = new PskRejectionRefreshHandler(
+                provider,
+                siteId,
+                options.HttpHandler ?? new SocketsHttpHandler { EnableMultipleHttp2Connections = true });
+        }
+
         return options;
     }
 }
+
+/// <summary>
+/// Drops a site's cached preshared key when the peer rejects a call as unauthenticated, so a key
+/// rotated in place is re-read on the next call instead of being presented until a restart
+/// (Gitea #96).
+/// </summary>
+/// <remarks>
+/// <para>
+/// <b>Why an HTTP handler.</b> Every per-site channel is built through
+/// <see cref="ControlPlaneCredentials.WithSiteCredentials"/>, which can see the channel's HTTP
+/// handler but not its call invoker. The control-plane interceptors reject before the service
+/// runs, which gRPC sends as a trailers-only response — the <c>grpc-status</c> rides the response
+/// headers, where this handler can read it without touching the body. That covers unary and
+/// streaming calls alike.
+/// </para>
+/// <para>
+/// <b>No throttle.</b> An invalidation only costs the next call a re-read, which the secret
+/// resolver serves from its own short-TTL cache; a site with a genuinely wrong key costs at most
+/// one store read per resolver TTL.
+/// </para>
+/// </remarks>
+public sealed class PskRejectionRefreshHandler : DelegatingHandler
+{
+    private readonly ISitePskProvider _provider;
+    private readonly string _siteId;
+
+    /// <summary>Creates the handler.</summary>
+    /// <param name="provider">The key cache to invalidate.</param>
+    /// <param name="siteId">The site the channel talks to.</param>
+    /// <param name="innerHandler">The transport handler.</param>
+    public PskRejectionRefreshHandler(
+        ISitePskProvider provider, string siteId, HttpMessageHandler innerHandler)
+        : base(innerHandler)
+    {
+        ArgumentNullException.ThrowIfNull(provider);
+        ArgumentException.ThrowIfNullOrWhiteSpace(siteId);
+        _provider = provider;
+        _siteId = siteId;
+    }
+
+    /// <inheritdoc />
+    protected override async Task<HttpResponseMessage> SendAsync(
+        HttpRequestMessage request, CancellationToken cancellationToken)
+    {
+        var response = await base.SendAsync(request, cancellationToken).ConfigureAwait(false);
+        if (IsAuthRejection(response))
+        {
+            _provider.Invalidate(_siteId);
+        }
+
+        return response;
+    }
+
+    /// <summary>
+    /// True when the response is a trailers-only gRPC <c>PermissionDenied</c> (7) or
+    /// <c>Unauthenticated</c> (16) — the statuses the control-plane interceptors reject with.
+    /// </summary>
+    /// <param name="response">The HTTP response.</param>
+    /// <returns>Whether the call was rejected as unauthenticated.</returns>
+    internal static bool IsAuthRejection(HttpResponseMessage response) =>
+        response.Headers.TryGetValues("grpc-status", out var values)
+        && values.FirstOrDefault() is "7" or "16";
+}

src/ZB.MOM.WW.ScadaBridge.Host/Actors/AkkaHostedService.cs

@@ -463,6 +463,18 @@ akka {{
             "CentralControlGrpcService readiness set (service bound: {Bound})",
             centralControlGrpc is not null);
 
+        // Peer half of site-PSK rotation invalidation (Gitea #96): subscribe to the topic, and give
+        // the invalidator the mediator so a rotation written on this node reaches the other one.
+        var pskRotation = _serviceProvider.GetService<SitePskRotationInvalidator>();
+        if (pskRotation is not null)
+        {
+            _actorSystem!.ActorOf(
+                Props.Create(() => new SitePskInvalidationSubscriber(pskRotation)),
+                "site-psk-invalidation");
+            pskRotation.SetMediator(
+                Akka.Cluster.Tools.PublishSubscribe.DistributedPubSub.Get(_actorSystem!).Mediator);
+        }
+
         // Wire up the CommunicationService with the actor reference
         var commService = _serviceProvider.GetService<CommunicationService>();
         commService?.SetCommunicationActor(centralCommActor);

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

@@ -158,10 +158,35 @@ public sealed class CentralControlAuthInterceptor : Interceptor
             throw Denied("missing site identity header");
         }
 
-        string expected;
+        var expected = await ResolveExpectedAsync(siteId, context).ConfigureAwait(false);
+        var presented = ExtractBearerToken(context.RequestHeaders);
+
+        if (presented is not null && !FixedTimeEquals(presented, expected))
+        {
+            // Gitea #96: the cached key may predate an in-place rotation the site already runs
+            // on. Drop it and re-read once; the secret resolver's own short-TTL cache bounds what
+            // a stream of wrong tokens can cost the store.
+            _pskProvider.Invalidate(siteId);
+            expected = await ResolveExpectedAsync(siteId, context).ConfigureAwait(false);
+        }
+
+        if (presented is null || !FixedTimeEquals(presented, expected))
+        {
+            _logger.LogWarning(
+                "Rejected a central control-plane call to {Method} from site {SiteId}: {Reason}.",
+                context.Method, siteId,
+                presented is null ? "no bearer token presented" : "bearer token did not match");
+            throw Denied("control plane authentication failed");
+        }
+
+        CentralControlSiteIdentity.SetVerified(context, siteId);
+    }
+
+    private async Task<string> ResolveExpectedAsync(string siteId, ServerCallContext context)
+    {
         try
         {
-            expected = await _pskProvider.GetAsync(siteId, context.CancellationToken)
+            return await _pskProvider.GetAsync(siteId, context.CancellationToken)
                 .ConfigureAwait(false);
         }
         catch (OperationCanceledException)
@@ -178,18 +203,6 @@ public sealed class CentralControlAuthInterceptor : Interceptor
                 context.Method, siteId);
             throw Denied("no key configured for the presented site");
         }
-
-        var presented = ExtractBearerToken(context.RequestHeaders);
-        if (presented is null || !FixedTimeEquals(presented, expected))
-        {
-            _logger.LogWarning(
-                "Rejected a central control-plane call to {Method} from site {SiteId}: {Reason}.",
-                context.Method, siteId,
-                presented is null ? "no bearer token presented" : "bearer token did not match");
-            throw Denied("control plane authentication failed");
-        }
-
-        CentralControlSiteIdentity.SetVerified(context, siteId);
     }
 
     private static RpcException Denied(string reason)

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

@@ -411,6 +411,10 @@ try
         // options validation) and why the connection string can never itself be a ${secret:}
         // reference.
         builder.Services.AddScadaBridgeSecrets(builder.Configuration, SecretsNodeRole.Central);
+        // An in-place rotation of a site's SB-GRPC-PSK-{siteId} through /admin/secrets must drop
+        // central's cached copy of that key on BOTH central nodes (Gitea #96). Wraps the library's
+        // ISecretCacheInvalidator; the peer half is wired in AkkaHostedService.RegisterCentralActors.
+        builder.Services.AddSitePskRotationInvalidation();
         // Secrets UI authorization: adds the named policies secrets:manage + secrets:reveal
         // (role-based) consumed by the /admin/secrets page. AddSecretsAuthorization only ADDS
         // these two policies via Configure<AuthorizationOptions> — it composes additively with

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

@@ -22,19 +22,31 @@ namespace ZB.MOM.WW.ScadaBridge.Host;
 /// connection already uses for its per-connection API keys.
 /// </para>
 /// <para>
-/// <b>Caching.</b> Successful resolves are cached indefinitely; a site's key changes only on
+/// <b>Caching.</b> Successful resolves are cached until something says the key may have changed.
-/// rotation, and rotation restarts the pair. Failures are deliberately NOT cached, so a key
+/// Failures are deliberately NOT cached, so a key seeded after the first (failed) dial is picked
-/// seeded after the first (failed) dial is picked up on the next attempt instead of requiring a
+/// up on the next attempt instead of requiring a restart. <see cref="Invalidate"/> drops an entry,
-/// restart. <see cref="Invalidate"/> drops an entry when a site is removed or a key is rotated
+/// and four things call it (Gitea #96 — before that only site removal did, so an in-place
-/// in place.
+/// rotation left both central nodes dialling with the old key until they restarted):
 /// </para>
+/// <list type="bullet">
+/// <item>site removal (the gRPC client paths);</item>
+/// <item>a write to <c>SB-GRPC-PSK-{siteId}</c> through this node's secret write path
+/// (<c>/admin/secrets</c>) — <see cref="SitePskRotationInvalidator"/>, which also tells the peer
+/// central node over <c>DistributedPubSub</c>;</item>
+/// <item>a site rejecting a call with <c>PermissionDenied</c>/<c>Unauthenticated</c>
+/// (<c>ControlPlaneCredentials.WithSiteCredentials</c>), so a key rotated out of band (the
+/// <c>secret</c> CLI, a direct SQL write) is re-read on the next call — after the secret
+/// resolver's own <c>Secrets:ResolveCacheTtl</c> at most;</item>
+/// <item>a reload of <c>ScadaBridge:Communication:SitePsks</c>, which clears the whole cache.</item>
+/// </list>
+/// <para>
 /// <para>
 /// <b>Fail-closed.</b> A key absent from BOTH sources throws, and the dial that needed it fails.
 /// It never degrades to an unauthenticated call. The message names the exact config key and the
 /// exact secret name, so the fix is one setting or one <c>secret</c> CLI seed.
 /// </para>
 /// </remarks>
-public sealed class SitePskProvider : ISitePskProvider
+public sealed class SitePskProvider : ISitePskProvider, IDisposable
 {
     /// <summary>Prefix of the per-site secret name; the site identifier is appended verbatim.</summary>
     public const string SecretNamePrefix = "SB-GRPC-PSK-";
@@ -43,6 +55,7 @@ public sealed class SitePskProvider : ISitePskProvider
     private readonly ISecretResolver _resolver;
     private readonly IOptionsMonitor<CommunicationOptions> _options;
     private readonly ILogger<SitePskProvider> _logger;
+    private readonly IDisposable? _optionsChange;
 
     /// <summary>Creates the provider over the host's secret resolver.</summary>
     /// <param name="resolver">Runtime secret resolver from the host container.</param>
@@ -64,6 +77,11 @@ public sealed class SitePskProvider : ISitePskProvider
         _resolver = resolver;
         _options = options;
         _logger = logger;
+
+        // A reloaded SitePsks map may carry a rotated key for any site; drop everything so the
+        // next dial to each site re-reads its source. Re-reading is cheap (a dictionary lookup,
+        // or the secret resolver's own cache), so clearing all is simpler than diffing.
+        _optionsChange = options.OnChange((_, _) => _cache.Clear());
     }
 
     /// <summary>Builds the secret name for a site.</summary>
@@ -142,13 +160,42 @@ public sealed class SitePskProvider : ISitePskProvider
     }
 
     /// <inheritdoc />
+    /// <remarks>
+    /// Case-insensitive: the rotation hook derives the site id from a <c>SecretName</c>, which the
+    /// secrets library lowercases, and a secret name addresses the same secret in any case.
+    /// </remarks>
     public void Invalidate(string siteId)
     {
-        if (!string.IsNullOrWhiteSpace(siteId))
+        if (string.IsNullOrWhiteSpace(siteId))
         {
-            _cache.TryRemove(siteId, out _);
+            return;
+        }
+
+        foreach (var key in _cache.Keys)
+        {
+            if (string.Equals(key, siteId, StringComparison.OrdinalIgnoreCase))
+            {
+                _cache.TryRemove(key, out _);
             }
         }
+    }
+
+    /// <summary>
+    /// Returns the site identifier a secret name is the PSK of, or <c>null</c> when the name is
+    /// not a <see cref="SecretNamePrefix"/> name. The prefix matches in any case, because the
+    /// secrets library stores names lowercased.
+    /// </summary>
+    /// <param name="secretName">A secret name.</param>
+    /// <returns>The site identifier, or <c>null</c>.</returns>
+    public static string? SiteIdFromSecretName(string secretName) =>
+        secretName is not null
+        && secretName.Length > SecretNamePrefix.Length
+        && secretName.StartsWith(SecretNamePrefix, StringComparison.OrdinalIgnoreCase)
+            ? secretName[SecretNamePrefix.Length..]
+            : null;
+
+    /// <inheritdoc />
+    public void Dispose() => _optionsChange?.Dispose();
 }
 
 /// <summary>

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

@@ -0,0 +1,168 @@
+using Akka.Actor;
+using Akka.Cluster.Tools.PublishSubscribe;
+using Akka.Event;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
+using Microsoft.Extensions.Logging;
+using ZB.MOM.WW.ScadaBridge.Communication.Grpc;
+using ZB.MOM.WW.Secrets;
+using ZB.MOM.WW.Secrets.Abstractions;
+
+namespace ZB.MOM.WW.ScadaBridge.Host;
+
+/// <summary>
+/// Central's <see cref="ISecretCacheInvalidator"/>: forwards every eviction to the secret
+/// resolver's cache as before, and when the evicted secret is a site's gRPC preshared key
+/// (<c>SB-GRPC-PSK-{siteId}</c>) also drops that site's entry from <see cref="ISitePskProvider"/>
+/// and tells the peer central node to do the same (Gitea #96).
+/// </summary>
+/// <remarks>
+/// <para>
+/// <b>Why the invalidator seam.</b> Every <c>ZB.MOM.WW.Secrets</c> write path — the
+/// <c>/admin/secrets</c> editor and delete modal — calls <see cref="ISecretCacheInvalidator"/>
+/// right after a rotate or delete. Hooking that seam catches an in-place PSK rotation at the
+/// moment it is written, without ScadaBridge owning a write path of its own.
+/// </para>
+/// <para>
+/// <b>Why the peer needs telling.</b> Both central nodes cache the key in their own
+/// <see cref="SitePskProvider"/> (and their own resolver cache), and the write only ran on one of
+/// them. The peer is told over the central <c>DistributedPubSub</c> mediator on
+/// <see cref="Topic"/> — the same mediator the peer health replication uses. A node receives its
+/// own broadcast too; applying it twice is harmless, so there is no origin filtering.
+/// </para>
+/// <para>
+/// Writes made outside this process (the <c>secret</c> CLI, a direct SQL write) never reach this
+/// seam; the site's rejection of the stale key covers those (see
+/// <c>ControlPlaneCredentials.WithSiteCredentials</c>).
+/// </para>
+/// </remarks>
+public sealed class SitePskRotationInvalidator : ISecretCacheInvalidator
+{
+    /// <summary>The <c>DistributedPubSub</c> topic the central nodes exchange PSK invalidations on.</summary>
+    public const string Topic = "site-psk-invalidation";
+
+    private readonly ISecretCacheInvalidator _inner;
+    private readonly ISitePskProvider _pskProvider;
+    private readonly ILogger<SitePskRotationInvalidator> _logger;
+    private volatile IActorRef? _mediator;
+
+    /// <summary>Creates the invalidator.</summary>
+    /// <param name="inner">The library's own invalidator (the secret resolver's cache).</param>
+    /// <param name="pskProvider">Central's per-site PSK cache.</param>
+    /// <param name="logger">Logger.</param>
+    public SitePskRotationInvalidator(
+        ISecretCacheInvalidator inner,
+        ISitePskProvider pskProvider,
+        ILogger<SitePskRotationInvalidator> logger)
+    {
+        ArgumentNullException.ThrowIfNull(inner);
+        ArgumentNullException.ThrowIfNull(pskProvider);
+        ArgumentNullException.ThrowIfNull(logger);
+        _inner = inner;
+        _pskProvider = pskProvider;
+        _logger = logger;
+    }
+
+    /// <summary>
+    /// Supplies the <c>DistributedPubSub</c> mediator once the actor system exists. Until then
+    /// (and on a host without one) evictions stay local.
+    /// </summary>
+    /// <param name="mediator">The mediator to publish peer invalidations to.</param>
+    public void SetMediator(IActorRef mediator) => _mediator = mediator;
+
+    /// <inheritdoc />
+    public void Invalidate(SecretName name)
+    {
+        _inner.Invalidate(name);
+
+        var siteId = SitePskProvider.SiteIdFromSecretName(name.Value);
+        if (siteId is null)
+        {
+            return;
+        }
+
+        _pskProvider.Invalidate(siteId);
+        _logger.LogInformation(
+            "gRPC preshared key secret '{SecretName}' was written; dropped the cached key for site "
+            + "{SiteId} and notified the peer central node.", name.Value, siteId);
+        _mediator?.Tell(new Publish(Topic, new SitePskInvalidated(siteId)));
+    }
+
+    /// <summary>
+    /// Applies an invalidation received from a central node (the peer, or this node's own
+    /// broadcast): evicts the resolver's cached secret and the PSK cache entry. Never re-publishes.
+    /// </summary>
+    /// <param name="siteId">The site whose key was rotated.</param>
+    public void ApplyPeerInvalidation(string siteId)
+    {
+        if (string.IsNullOrWhiteSpace(siteId))
+        {
+            return;
+        }
+
+        _inner.Invalidate(new SecretName(SitePskProvider.SecretNameFor(siteId)));
+        _pskProvider.Invalidate(siteId);
+    }
+}
+
+/// <summary>
+/// Peer message: the gRPC preshared key of <paramref name="SiteId"/> was rotated on a central node.
+/// </summary>
+/// <param name="SiteId">The site whose key changed.</param>
+public sealed record SitePskInvalidated(string SiteId);
+
+/// <summary>
+/// Central actor that subscribes to <see cref="SitePskRotationInvalidator.Topic"/> and applies
+/// each <see cref="SitePskInvalidated"/> to this node's caches.
+/// </summary>
+public sealed class SitePskInvalidationSubscriber : ReceiveActor
+{
+    private readonly SitePskRotationInvalidator _invalidator;
+
+    /// <summary>Creates the subscriber.</summary>
+    /// <param name="invalidator">This node's invalidator.</param>
+    public SitePskInvalidationSubscriber(SitePskRotationInvalidator invalidator)
+    {
+        _invalidator = invalidator;
+        Receive<SitePskInvalidated>(m => _invalidator.ApplyPeerInvalidation(m.SiteId));
+        Receive<SubscribeAck>(_ => { });
+    }
+
+    /// <inheritdoc />
+    protected override void PreStart()
+    {
+        try
+        {
+            DistributedPubSub.Get(Context.System).Mediator.Tell(
+                new Subscribe(SitePskRotationInvalidator.Topic, Self));
+        }
+        catch (Exception ex)
+        {
+            // Non-clustered host (TestKit): peer invalidation is simply unavailable.
+            Context.GetLogger().Debug("DistributedPubSub not available — peer PSK invalidation disabled: {0}", ex.Message);
+        }
+    }
+}
+
+/// <summary>DI wiring for <see cref="SitePskRotationInvalidator"/>.</summary>
+public static class SitePskRotationServiceCollectionExtensions
+{
+    /// <summary>
+    /// Replaces the secrets library's <see cref="ISecretCacheInvalidator"/> with
+    /// <see cref="SitePskRotationInvalidator"/> wrapping the library's resolver. Central only;
+    /// call after <c>AddScadaBridgeSecrets</c> and after <see cref="ISitePskProvider"/> is
+    /// registered.
+    /// </summary>
+    /// <param name="services">The service collection.</param>
+    /// <returns>The same collection.</returns>
+    public static IServiceCollection AddSitePskRotationInvalidation(this IServiceCollection services)
+    {
+        services.TryAddSingleton(sp => new SitePskRotationInvalidator(
+            sp.GetRequiredService<DefaultSecretResolver>(),
+            sp.GetRequiredService<ISitePskProvider>(),
+            sp.GetRequiredService<ILogger<SitePskRotationInvalidator>>()));
+        services.Replace(ServiceDescriptor.Singleton<ISecretCacheInvalidator>(
+            sp => sp.GetRequiredService<SitePskRotationInvalidator>()));
+        return services;
+    }
+}

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

@@ -274,4 +274,58 @@ public class CentralControlAuthInterceptorTests
             CentralControlAuthInterceptor.DefaultGatedPrefixes,
             p => p == $"/{method}/");
     }
+    /// <summary>A provider whose cached key is stale until <see cref="Invalidate"/> runs.</summary>
+    private sealed class RotatingPskProvider(string cached, string rotated) : ISitePskProvider
+    {
+        private string _current = cached;
+
+        /// <summary>Gets the number of invalidations.</summary>
+        public int Invalidations { get; private set; }
+
+        /// <inheritdoc />
+        public ValueTask<string> GetAsync(string siteId, CancellationToken ct) => new(_current);
+
+        /// <inheritdoc />
+        public void Invalidate(string siteId)
+        {
+            Invalidations++;
+            _current = rotated;
+        }
+    }
+
+    /// <summary>
+    /// Gitea #96: a site already on its rotated key is accepted even while central's cached copy
+    /// is stale — the mismatch drops the cache and re-reads once.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task StaleCachedKey_IsReReadOnce_SoARotatedSiteIsAccepted()
+    {
+        var provider = new RotatingPskProvider("pre-rotation", "rotated");
+        var interceptor = new CentralControlAuthInterceptor(
+            provider, NullLogger<CentralControlAuthInterceptor>.Instance);
+
+        var result = await Invoke(interceptor, CreateContext(ControlMethod, SiteA, "Bearer rotated"));
+
+        Assert.Equal("ok", result);
+        Assert.Equal(1, provider.Invalidations);
+    }
+
+    /// <summary>Negative control: a wrong token is still refused after the one re-read, and a right one never invalidates.</summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task WrongToken_IsStillRefused_AfterTheReRead_AndAMatchNeverInvalidates()
+    {
+        var provider = new RotatingPskProvider("k", "k");
+        var interceptor = new CentralControlAuthInterceptor(
+            provider, NullLogger<CentralControlAuthInterceptor>.Instance);
+
+        Assert.Equal("ok", await Invoke(interceptor, CreateContext(ControlMethod, SiteA, "Bearer k")));
+        Assert.Equal(0, provider.Invalidations);
+
+        var ex = await Assert.ThrowsAsync<RpcException>(
+            () => Invoke(interceptor, CreateContext(ControlMethod, SiteA, "Bearer wrong")));
+        Assert.Equal(StatusCode.PermissionDenied, ex.StatusCode);
+        Assert.Equal(1, provider.Invalidations);
+    }
 }

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

@@ -407,6 +407,18 @@ public class CentralCompositionRootTests : IDisposable
         Assert.Null(_factory.Services.GetService<IInboundApiKeyStoreImporter>());
     }
 
+    /// <summary>
+    /// Gitea #96: the secrets write path's <see cref="ZB.MOM.WW.Secrets.Abstractions.ISecretCacheInvalidator"/>
+    /// must be the PSK-aware wrapper on central, or an in-place site-PSK rotation through
+    /// /admin/secrets never reaches the PSK cache.
+    /// </summary>
+    [Fact]
+    public void Central_SecretCacheInvalidator_IsTheSitePskRotationInvalidator()
+    {
+        Assert.IsType<SitePskRotationInvalidator>(
+            _factory.Services.GetService<ZB.MOM.WW.Secrets.Abstractions.ISecretCacheInvalidator>());
+    }
+
     /// <summary>Verifies that the shared audit seam resolves to the central direct-write bridge.</summary>
     [Fact]
     public void Central_SharedAuditSeam_IsTheCentralDirectWriteBridge()

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

@@ -7,8 +7,10 @@ using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
 using Microsoft.Extensions.Logging.Abstractions;
 using Microsoft.Extensions.Options;
+using NSubstitute;
 using ZB.MOM.WW.ScadaBridge.Communication;
 using ZB.MOM.WW.ScadaBridge.Communication.Grpc;
+using ZB.MOM.WW.Secrets.Abstractions;
 
 namespace ZB.MOM.WW.ScadaBridge.Host.Tests;
 
@@ -267,4 +269,87 @@ public class ControlPlaneAuthEndToEndTests : IAsyncLifetime
         var service = _host.Services.GetRequiredService<EchoSiteStreamService>();
         Assert.Equal("site-a", service.LastSiteHeader);
     }
+    // --- Gitea #96: a site's rejection drops central's cached key ---------------------------
+
+    /// <summary>
+    /// Central's real <see cref="SitePskProvider"/> over a store that served a stale key first and
+    /// the rotated (site-accepted) key after: the in-place rotation scenario.
+    /// </summary>
+    private static (SitePskProvider Provider, ISecretResolver Resolver) RotatedProvider()
+    {
+        var resolver = Substitute.For<ISecretResolver>();
+        resolver.GetAsync(Arg.Any<SecretName>(), Arg.Any<CancellationToken>())
+            .Returns("the-pre-rotation-key", SiteKey);
+        var options = Substitute.For<IOptionsMonitor<CommunicationOptions>>();
+        options.CurrentValue.Returns(new CommunicationOptions());
+        return (new SitePskProvider(resolver, options, NullLogger<SitePskProvider>.Instance), resolver);
+    }
+
+    private GrpcChannel Channel(ISitePskProvider provider) =>
+        GrpcChannel.ForAddress(_server.BaseAddress, new GrpcChannelOptions { HttpHandler = _server.CreateHandler() }
+            .WithMessageLimits().WithSiteCredentials(provider, "site-a"));
+
+    /// <summary>
+    /// Verifies that after a site rejects the cached (pre-rotation) key on a unary call, the next
+    /// call re-reads the key and is accepted — no restart needed.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task RejectedUnaryCall_DropsTheCachedKey_SoTheNextCallUsesTheRotatedOne()
+    {
+        var (provider, _) = RotatedProvider();
+        using var channel = Channel(provider);
+        var client = new SiteStreamService.SiteStreamServiceClient(channel);
+
+        var ex = await Assert.ThrowsAsync<RpcException>(
+            async () => await client.PullAuditEventsAsync(new PullAuditEventsRequest()));
+        Assert.Equal(StatusCode.PermissionDenied, ex.StatusCode);
+
+        var reply = await client.PullAuditEventsAsync(new PullAuditEventsRequest { BatchSize = 1 });
+        Assert.False(reply.MoreAvailable);
+    }
+
+    /// <summary>Verifies the same refresh for a rejected streaming call.</summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task RejectedStreamingCall_DropsTheCachedKey_SoTheNextCallUsesTheRotatedOne()
+    {
+        var (provider, _) = RotatedProvider();
+        using var channel = Channel(provider);
+        var client = new SiteStreamService.SiteStreamServiceClient(channel);
+
+        using (var rejected = client.SubscribeInstance(new InstanceStreamRequest { CorrelationId = "c1" }))
+        {
+            var ex = await Assert.ThrowsAsync<RpcException>(
+                async () => await rejected.ResponseStream.MoveNext(CancellationToken.None));
+            Assert.Equal(StatusCode.PermissionDenied, ex.StatusCode);
+        }
+
+        using var accepted = client.SubscribeInstance(new InstanceStreamRequest { CorrelationId = "c2" });
+        Assert.True(await accepted.ResponseStream.MoveNext(CancellationToken.None));
+        Assert.Equal("c2", accepted.ResponseStream.Current.CorrelationId);
+    }
+
+    /// <summary>
+    /// Negative control: accepted calls never drop the cache, so the store is read once no
+    /// matter how many calls succeed.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task AcceptedCalls_KeepTheCachedKey()
+    {
+        var resolver = Substitute.For<ISecretResolver>();
+        resolver.GetAsync(Arg.Any<SecretName>(), Arg.Any<CancellationToken>()).Returns(SiteKey);
+        var options = Substitute.For<IOptionsMonitor<CommunicationOptions>>();
+        options.CurrentValue.Returns(new CommunicationOptions());
+        var provider = new SitePskProvider(resolver, options, NullLogger<SitePskProvider>.Instance);
+        using var channel = Channel(provider);
+        var client = new SiteStreamService.SiteStreamServiceClient(channel);
+
+        await client.PullAuditEventsAsync(new PullAuditEventsRequest());
+        await client.PullAuditEventsAsync(new PullAuditEventsRequest());
+        await client.PullAuditEventsAsync(new PullAuditEventsRequest());
+
+        await resolver.Received(1).GetAsync(Arg.Any<SecretName>(), Arg.Any<CancellationToken>());
+    }
 }

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

@@ -204,4 +204,61 @@ public class SitePskProviderTests
         Assert.Equal("key-a", await provider.GetAsync("site-a", CancellationToken.None));
         Assert.Equal("key-b", await provider.GetAsync("site-b", CancellationToken.None));
     }
+    /// <summary>
+    /// Verifies that a reload of the configured <c>SitePsks</c> map drops cached keys, so a key
+    /// rotated in configuration is used without a restart (Gitea #96).
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task ReloadedSitePsks_DropsTheCache_SoAKeyRotatedInConfigIsUsed()
+    {
+        var resolver = Substitute.For<ISecretResolver>();
+        var options = new CommunicationOptions();
+        options.SitePsks["site-a"] = "old";
+        Action<CommunicationOptions, string?>? listener = null;
+        var monitor = Substitute.For<IOptionsMonitor<CommunicationOptions>>();
+        monitor.CurrentValue.Returns(_ => options);
+        monitor.OnChange(Arg.Do<Action<CommunicationOptions, string?>>(l => listener = l));
+        using var provider = new SitePskProvider(resolver, monitor, NullLogger<SitePskProvider>.Instance);
+
+        Assert.Equal("old", await provider.GetAsync("site-a", CancellationToken.None));
+
+        options = new CommunicationOptions();
+        options.SitePsks["site-a"] = "rotated";
+        Assert.NotNull(listener);
+        listener!(options, null);
+
+        Assert.Equal("rotated", await provider.GetAsync("site-a", CancellationToken.None));
+    }
+
+    /// <summary>Verifies the secret-name to site-id mapping the rotation hook relies on.</summary>
+    /// <param name="name">A secret name.</param>
+    /// <param name="expected">The expected site id, or null.</param>
+    [Theory]
+    [InlineData("SB-GRPC-PSK-site-a", "site-a")]
+    [InlineData("SB-GRPC-PSK-", null)]
+    [InlineData("OTHER-SECRET", null)]
+    [InlineData("sb-grpc-psk-site-a", "site-a")]
+    public void SiteIdFromSecretName_MapsOnlyPskNames(string name, string? expected)
+    {
+        Assert.Equal(expected, SitePskProvider.SiteIdFromSecretName(name));
+    }
+    /// <summary>
+    /// Verifies that invalidation ignores case: the rotation hook derives the site id from a
+    /// lowercased secret name, but the site may be cached under its mixed-case identifier.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task Invalidate_IgnoresCase_BecauseSecretNamesAreLowercased()
+    {
+        var resolver = Substitute.For<ISecretResolver>();
+        resolver.GetAsync(Arg.Any<SecretName>(), Arg.Any<CancellationToken>())
+            .Returns("old", "rotated");
+        var provider = Create(resolver);
+
+        Assert.Equal("old", await provider.GetAsync("Site-A", CancellationToken.None));
+        provider.Invalidate("site-a");
+
+        Assert.Equal("rotated", await provider.GetAsync("Site-A", CancellationToken.None));
+    }
 }

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

@@ -0,0 +1,89 @@
+using Akka.Actor;
+using Akka.Cluster.Tools.PublishSubscribe;
+using Akka.TestKit.Xunit2;
+using Microsoft.Extensions.Logging.Abstractions;
+using NSubstitute;
+using ZB.MOM.WW.ScadaBridge.Communication.Grpc;
+using ZB.MOM.WW.Secrets.Abstractions;
+
+namespace ZB.MOM.WW.ScadaBridge.Host.Tests;
+
+/// <summary>
+/// Gitea #96: an in-place rotation of <c>SB-GRPC-PSK-{siteId}</c> written through the secrets
+/// write path must drop central's cached key on this node and on the peer central node.
+/// </summary>
+public class SitePskRotationInvalidatorTests : TestKit
+{
+    private readonly ISecretCacheInvalidator _inner = Substitute.For<ISecretCacheInvalidator>();
+    private readonly ISitePskProvider _psk = Substitute.For<ISitePskProvider>();
+
+    private SitePskRotationInvalidator Create() =>
+        new(_inner, _psk, NullLogger<SitePskRotationInvalidator>.Instance);
+
+    /// <summary>
+    /// Verifies that writing a site's PSK secret evicts the resolver cache, drops the site's
+    /// cached key and publishes the invalidation to the peer.
+    /// </summary>
+    [Fact]
+    public void WritingAPskSecret_DropsTheCachedKey_AndTellsThePeer()
+    {
+        var mediator = CreateTestProbe();
+        var invalidator = Create();
+        invalidator.SetMediator(mediator.Ref);
+
+        // The secrets library lowercases names, so the site id arrives lowercased too.
+        invalidator.Invalidate(new SecretName("SB-GRPC-PSK-site-a"));
+
+        _inner.Received(1).Invalidate(new SecretName("SB-GRPC-PSK-site-a"));
+        _psk.Received(1).Invalidate("site-a");
+        var publish = mediator.ExpectMsg<Publish>();
+        Assert.Equal(SitePskRotationInvalidator.Topic, publish.Topic);
+        Assert.Equal(new SitePskInvalidated("site-a"), publish.Message);
+    }
+
+    /// <summary>Negative control: any other secret only evicts the resolver cache.</summary>
+    [Fact]
+    public void WritingAnyOtherSecret_OnlyEvictsTheResolverCache()
+    {
+        var mediator = CreateTestProbe();
+        var invalidator = Create();
+        invalidator.SetMediator(mediator.Ref);
+
+        invalidator.Invalidate(new SecretName("SMTP-PASSWORD"));
+
+        _inner.Received(1).Invalidate(new SecretName("SMTP-PASSWORD"));
+        _psk.DidNotReceiveWithAnyArgs().Invalidate(default!);
+        mediator.ExpectNoMsg(TimeSpan.FromMilliseconds(200));
+    }
+
+    /// <summary>Verifies that before the actor system exists an invalidation still applies locally.</summary>
+    [Fact]
+    public void WithoutAMediator_TheLocalInvalidationStillApplies()
+    {
+        Create().Invalidate(new SecretName("SB-GRPC-PSK-site-a"));
+
+        _psk.Received(1).Invalidate("site-a");
+    }
+
+    /// <summary>
+    /// Verifies that the subscriber applies a peer's invalidation to this node's resolver and PSK
+    /// caches, without re-publishing it.
+    /// </summary>
+    [Fact]
+    public void Subscriber_AppliesAPeerInvalidation_WithoutRepublishing()
+    {
+        var mediator = CreateTestProbe();
+        var invalidator = Create();
+        invalidator.SetMediator(mediator.Ref);
+        var subscriber = Sys.ActorOf(Props.Create(() => new SitePskInvalidationSubscriber(invalidator)));
+
+        subscriber.Tell(new SitePskInvalidated("site-b"));
+
+        AwaitAssert(() =>
+        {
+            _psk.Received(1).Invalidate("site-b");
+            _inner.Received(1).Invalidate(new SecretName("SB-GRPC-PSK-site-b"));
+        });
+        mediator.ExpectNoMsg(TimeSpan.FromMilliseconds(200));
+    }
+}