merge: transport hardening (register rows 51-53) — BundleLockedException curated to Commons/MapFault, fleet-wide MS SQL unlock-throttle window, import-session gauge + cap Warning

src/ZB.MOM.WW.ScadaBridge.Commons/Entities/Transport/BundleUnlockAttempt.cs

@@ -0,0 +1,42 @@
+namespace ZB.MOM.WW.ScadaBridge.Commons.Entities.Transport;
+
+/// <summary>
+/// One recorded bundle-unlock passphrase attempt, written to the central
+/// <c>BundleUnlockAttempt</c> table. Rows form the trailing-hour sliding window the
+/// Transport component's per-client unlock throttle
+/// (<c>TransportOptions.MaxUnlockAttemptsPerIpPerHour</c>) is evaluated against.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Register row 52: the throttle's counters used to be per-node and per-process, so the
+/// effective fleet cap was N x limit per client and a node restart cleared the window.
+/// Central MS SQL is the store both central nodes already share, so putting the window
+/// there makes the cap fleet-wide and restart-durable without new infrastructure.
+/// </para>
+/// <para>
+/// Operational history, NOT audit — the table is non-partitioned, lives on the standard
+/// <c>[PRIMARY]</c> filegroup and carries no DB-role restriction. Rows are pruned
+/// opportunistically once they age past the window; they are not a durable record of
+/// who tried to open which bundle (that is the <c>BundleImportUnlockFailed</c> audit row's
+/// job, and note this table deliberately records no bundle identity at all).
+/// </para>
+/// <para>
+/// Persistence-ignorant POCO; the EF Core mapping lives in the Configuration Database
+/// component. <see cref="AttemptedAtUtc"/> is UTC, like every timestamp in the system.
+/// </para>
+/// </remarks>
+public sealed class BundleUnlockAttempt
+{
+    /// <summary>Surrogate identity key assigned by the store.</summary>
+    public long Id { get; set; }
+
+    /// <summary>
+    /// The throttle bucket the attempt was counted against — the caller's remote IP, or
+    /// the shared <c>&lt;unattributed&gt;</c> bucket when no address could be attributed.
+    /// Sized for an IPv6 literal with a scope id.
+    /// </summary>
+    public required string ClientKey { get; set; }
+
+    /// <summary>The UTC instant at which the unlock attempt was registered.</summary>
+    public DateTime AttemptedAtUtc { get; set; }
+}

src/ZB.MOM.WW.ScadaBridge.Commons/Interfaces/Repositories/IBundleUnlockAttemptRepository.cs

@@ -0,0 +1,46 @@
+namespace ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Repositories;
+
+/// <summary>
+/// Register row 52: read/write access to the central <c>BundleUnlockAttempt</c> table,
+/// the fleet-wide sliding window behind the Transport component's per-client bundle-unlock
+/// throttle. Both central nodes share this store, so the documented
+/// <c>MaxUnlockAttemptsPerIpPerHour</c> cap is a fleet cap rather than a per-process one.
+/// </summary>
+/// <remarks>
+/// The throttle is a best-effort rate cap layered over an already-authenticated,
+/// role-gated surface. Callers are expected to treat a store failure as fail-open (the
+/// availability of legitimate imports outranks the cap), so no method here is required to
+/// be transactional with respect to the caller's own work.
+/// </remarks>
+public interface IBundleUnlockAttemptRepository
+{
+    /// <summary>
+    /// Counts the attempts recorded for <paramref name="clientKey"/> at or after
+    /// <paramref name="sinceUtc"/> — the trailing-window depth the cap is compared against.
+    /// </summary>
+    /// <param name="clientKey">Throttle bucket (client IP, or the shared unattributed bucket).</param>
+    /// <param name="sinceUtc">Inclusive UTC lower bound of the trailing window.</param>
+    /// <param name="ct">Cancellation token.</param>
+    /// <returns>The number of attempts inside the window.</returns>
+    Task<int> CountAttemptsSinceAsync(string clientKey, DateTime sinceUtc, CancellationToken ct = default);
+
+    /// <summary>
+    /// Appends one attempt row for <paramref name="clientKey"/>.
+    /// </summary>
+    /// <param name="clientKey">Throttle bucket (client IP, or the shared unattributed bucket).</param>
+    /// <param name="attemptedAtUtc">UTC instant of the attempt.</param>
+    /// <param name="ct">Cancellation token.</param>
+    /// <returns>A task that completes when the row is committed.</returns>
+    Task RecordAttemptAsync(string clientKey, DateTime attemptedAtUtc, CancellationToken ct = default);
+
+    /// <summary>
+    /// Deletes attempt rows older than <paramref name="beforeUtc"/>, up to
+    /// <paramref name="maxRows"/> in one bounded pass. Bounded so an opportunistic prune on
+    /// a request path can never turn into an unbounded delete.
+    /// </summary>
+    /// <param name="beforeUtc">Exclusive UTC upper bound — rows strictly older than this are eligible.</param>
+    /// <param name="maxRows">Maximum rows to delete in this pass; must be positive.</param>
+    /// <param name="ct">Cancellation token.</param>
+    /// <returns>The number of rows deleted.</returns>
+    Task<int> PruneAttemptsOlderThanAsync(DateTime beforeUtc, int maxRows, CancellationToken ct = default);
+}

src/ZB.MOM.WW.ScadaBridge.Commons/Interfaces/Transport/IBundleSessionStore.cs

@@ -18,6 +18,15 @@ public interface IBundleSessionStore
     /// <summary>Removes all sessions whose expiry has passed.</summary>
     void EvictExpired();
 
+    /// <summary>
+    /// Register row 53: the number of sessions currently held that have not yet expired.
+    /// Backs the <c>scadabridge.central.transport.import_sessions.open</c> gauge, so it
+    /// must be cheap and side-effect-free (it is read from the metrics collection thread)
+    /// and must NOT count expired-but-unswept entries — otherwise the gauge would read
+    /// as "at the cap" for up to a sweep interval after a burst drained.
+    /// </summary>
+    int OpenSessionCount { get; }
+
     /// <summary>
     /// T-003: returns the current unlock-failure count for a bundle keyed by its
     /// content hash. The counter is server-owned so a second tab / CLI caller

src/ZB.MOM.WW.ScadaBridge.Commons/Observability/ScadaBridgeTelemetry.cs

@@ -118,6 +118,14 @@ public static class ScadaBridgeTelemetry
     /// <summary>Provider that yields the live StoreAndForward queue depth; set by a later task.</summary>
     private static Func<long>? _queueDepthProvider;
 
+    /// <summary>
+    /// Register row 53: provider that yields the number of currently open (unexpired)
+    /// Transport bundle-import sessions, registered by the Transport component's
+    /// session-eviction hosted service. Null on a site node (Transport is central-only),
+    /// where the gauge simply reports 0.
+    /// </summary>
+    private static Func<long>? _importSessionCountProvider;
+
 #pragma warning disable IDE0052 // Held to keep the observable gauges alive for the meter's lifetime.
     /// <summary>Gauge reporting the number of currently open site connections.</summary>
     private static readonly ObservableGauge<long> _siteConnectionUp =
@@ -138,6 +146,20 @@ public static class ScadaBridgeTelemetry
             () => Interlocked.Read(ref _liveAlarmAggregatorsActive),
             unit: "1",
             description: "Number of per-site live-alarm aggregators currently running on the active central node.");
+
+    /// <summary>
+    /// Register row 53: gauge reporting the number of open Transport bundle-import
+    /// sessions on this central node. Each open session pins a fully decrypted bundle
+    /// in memory and consumes one of <c>TransportOptions.MaxConcurrentImportSessions</c>
+    /// (default 8), so a series that plateaus at the cap is the signature of leaked
+    /// sessions — the failure mode register row 43 could only be inferred from rotating
+    /// test failures because nothing counted them.
+    /// </summary>
+    private static readonly ObservableGauge<long> _transportImportSessionsOpen =
+        Meter.CreateObservableGauge<long>("scadabridge.central.transport.import_sessions.open",
+            () => Volatile.Read(ref _importSessionCountProvider) is { } p ? p() : 0L,
+            unit: "1",
+            description: "Number of currently open bundle-import sessions on this central node.");
 #pragma warning restore IDE0052
 
     // ---------------- Emit helpers ----------------
@@ -242,4 +264,42 @@ public static class ScadaBridgeTelemetry
         // delegate, so a stale stop cannot clobber a successor's provider.
         Interlocked.CompareExchange(ref _queueDepthProvider, null, provider);
     }
+
+    /// <summary>
+    /// Register row 53: registers the provider the open-import-sessions gauge reads on
+    /// each observation. Called by the Transport component's session-eviction hosted
+    /// service on start. A null provider is ignored so the gauge falls back to 0.
+    /// The callback is invoked synchronously on the metrics collection thread, so it
+    /// must stay cheap — the Transport implementation reads a
+    /// <see cref="System.Collections.Concurrent.ConcurrentDictionary{TKey,TValue}"/> it
+    /// already owns.
+    /// </summary>
+    /// <param name="provider">A callback returning the current open-session count.</param>
+    public static void SetImportSessionCountProvider(Func<long> provider)
+    {
+        if (provider is null)
+        {
+            return;
+        }
+
+        Volatile.Write(ref _importSessionCountProvider, provider);
+    }
+
+    /// <summary>
+    /// Clears the open-import-sessions provider, but only if the currently registered
+    /// provider is the exact <paramref name="provider"/> delegate passed in
+    /// (reference-equal compare-and-clear), so a late stop of a superseded instance
+    /// cannot stomp a newer registrant. Mirrors <see cref="ClearQueueDepthProvider"/>.
+    /// </summary>
+    /// <param name="provider">The provider delegate to remove; ignored unless it is the
+    /// one currently registered.</param>
+    public static void ClearImportSessionCountProvider(Func<long> provider)
+    {
+        if (provider is null)
+        {
+            return;
+        }
+
+        Interlocked.CompareExchange(ref _importSessionCountProvider, null, provider);
+    }
 }

src/ZB.MOM.WW.ScadaBridge.Commons/Types/Transport/BundleLockedException.cs

@@ -0,0 +1,44 @@
+namespace ZB.MOM.WW.ScadaBridge.Commons.Types.Transport;
+
+/// <summary>
+/// T-003: thrown by <c>IBundleImporter.LoadAsync</c> when an encrypted bundle has
+/// exceeded the configured failed-unlock attempt limit
+/// (<c>TransportOptions.MaxUnlockAttemptsPerSession</c>, default 3). The lockout is tracked
+/// server-side keyed by <see cref="BundleManifest.ContentHash"/>, so a second tab / CLI caller
+/// re-uploading the same bytes hits the same counter and cannot side-step the limit.
+/// <para>
+/// Distinct from <see cref="BundleUnlockThrottledException"/> on purpose: this one means
+/// "this BUNDLE has burned its strikes" and is keyed by the bundle's content hash; that one
+/// means "this CLIENT has attempted too many unlocks, across any bundles" and is keyed by
+/// the caller's IP.
+/// </para>
+/// <para>
+/// Register row 51: lives in Commons rather than beside the importer for the same reason
+/// <see cref="BundleUnlockThrottledException"/> does — it is part of the
+/// <c>IBundleImporter.LoadAsync</c> contract, which is declared here, and the management
+/// surface (which references Commons but not Transport) must be able to catch it to return
+/// a clear, curated error instead of a generic internal-failure message plus a correlation id.
+/// </para>
+/// </summary>
+public sealed class BundleLockedException : Exception
+{
+    /// <summary>Number of recorded unlock failures for this bundle.</summary>
+    public int FailedAttempts { get; }
+
+    /// <summary>SHA-256 (hex) of the bundle's content bytes, the lockout's tracking key.</summary>
+    public string BundleContentHash { get; }
+
+    /// <summary>
+    /// Initializes a new <see cref="BundleLockedException"/>.
+    /// </summary>
+    /// <param name="bundleContentHash">SHA-256 hex from <see cref="BundleManifest.ContentHash"/>.</param>
+    /// <param name="failedAttempts">Number of failures recorded against this bundle.</param>
+    public BundleLockedException(string bundleContentHash, int failedAttempts)
+        : base(
+            $"Bundle is locked after {failedAttempts} failed unlock attempts. "
+            + "Wait for the lockout window to expire or re-export the bundle to obtain a new content hash.")
+    {
+        BundleContentHash = bundleContentHash ?? throw new ArgumentNullException(nameof(bundleContentHash));
+        FailedAttempts = failedAttempts;
+    }
+}

src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Configurations/BundleUnlockAttemptEntityTypeConfiguration.cs

@@ -0,0 +1,46 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.Transport;
+
+namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Configurations;
+
+/// <summary>
+/// Maps the <see cref="BundleUnlockAttempt"/> POCO to the central
+/// <c>BundleUnlockAttempt</c> table — the fleet-wide sliding window behind the Transport
+/// component's per-client bundle-unlock throttle (register row 52). Operational history,
+/// NOT audit, so the table is non-partitioned, standard <c>[PRIMARY]</c> filegroup, no
+/// DB-role restriction.
+/// </summary>
+/// <remarks>
+/// One named index, <c>IX_BundleUnlockAttempt_Client_Attempted</c>, serves both access
+/// paths: the window count seeks on <c>ClientKey</c> and ranges on
+/// <c>AttemptedAtUtc</c>, and the opportunistic prune scans the same index by time. A
+/// second time-only index was deliberately not added — the table is kept small by the
+/// prune, so the leading-column seek is cheap enough for the prune to share.
+/// </remarks>
+public class BundleUnlockAttemptEntityTypeConfiguration : IEntityTypeConfiguration<BundleUnlockAttempt>
+{
+    /// <summary>
+    /// Configures the EF Core entity type mapping for <see cref="BundleUnlockAttempt"/>.
+    /// </summary>
+    /// <param name="builder">The entity type builder to configure.</param>
+    public void Configure(EntityTypeBuilder<BundleUnlockAttempt> builder)
+    {
+        builder.ToTable("BundleUnlockAttempt");
+
+        // Surrogate long identity assigned by the store.
+        builder.HasKey(a => a.Id);
+
+        // Bucket key: an IP literal or the "<unattributed>" sentinel. 64 chars covers an
+        // IPv6 address with a scope id; ASCII, so non-unicode (varchar) like every other
+        // catalog-bounded column in the schema.
+        builder.Property(a => a.ClientKey).HasMaxLength(64).IsUnicode(false).IsRequired();
+
+        builder.Property(a => a.AttemptedAtUtc).IsRequired();
+
+        // Window index — seeks the bucket, ranges over the trailing hour, and is
+        // reused by the prune's time scan. Name locked for migration discoverability.
+        builder.HasIndex(a => new { a.ClientKey, a.AttemptedAtUtc })
+            .HasDatabaseName("IX_BundleUnlockAttempt_Client_Attempted");
+    }
+}

src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Migrations/20260815154803_AddBundleUnlockAttemptTable.cs

@@ -0,0 +1,41 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddBundleUnlockAttemptTable : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "BundleUnlockAttempt",
+                columns: table => new
+                {
+                    Id = table.Column<long>(type: "bigint", nullable: false)
+                        .Annotation("SqlServer:Identity", "1, 1"),
+                    ClientKey = table.Column<string>(type: "varchar(64)", unicode: false, maxLength: 64, nullable: false),
+                    AttemptedAtUtc = table.Column<DateTime>(type: "datetime2", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_BundleUnlockAttempt", x => x.Id);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_BundleUnlockAttempt_Client_Attempted",
+                table: "BundleUnlockAttempt",
+                columns: new[] { "ClientKey", "AttemptedAtUtc" });
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "BundleUnlockAttempt");
+        }
+    }
+}

src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Migrations/ScadaBridgeDbContextModelSnapshot.cs

@@ -1716,6 +1716,31 @@ namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Migrations
                     b.ToTable("TemplateScripts");
                 });
 
+            modelBuilder.Entity("ZB.MOM.WW.ScadaBridge.Commons.Entities.Transport.BundleUnlockAttempt", b =>
+                {
+                    b.Property<long>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("bigint");
+
+                    SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<long>("Id"));
+
+                    b.Property<DateTime>("AttemptedAtUtc")
+                        .HasColumnType("datetime2");
+
+                    b.Property<string>("ClientKey")
+                        .IsRequired()
+                        .HasMaxLength(64)
+                        .IsUnicode(false)
+                        .HasColumnType("varchar(64)");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("ClientKey", "AttemptedAtUtc")
+                        .HasDatabaseName("IX_BundleUnlockAttempt_Client_Attempted");
+
+                    b.ToTable("BundleUnlockAttempt", (string)null);
+                });
+
             modelBuilder.Entity("ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Entities.AuditLogRow", b =>
                 {
                     b.Property<Guid>("EventId")

src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Repositories/BundleUnlockAttemptRepository.cs

@@ -0,0 +1,70 @@
+using Microsoft.EntityFrameworkCore;
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.Transport;
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Repositories;
+
+namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Repositories;
+
+/// <summary>
+/// EF Core implementation of <see cref="IBundleUnlockAttemptRepository"/> over the central
+/// <c>BundleUnlockAttempt</c> table (register row 52). Plain reads/writes against the
+/// shared <see cref="ScadaBridgeDbContext"/>; the prune uses <c>ExecuteDeleteAsync</c> so
+/// no rows are materialised into the change tracker.
+/// </summary>
+public class BundleUnlockAttemptRepository : IBundleUnlockAttemptRepository
+{
+    private readonly ScadaBridgeDbContext _context;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="BundleUnlockAttemptRepository"/> class.
+    /// </summary>
+    /// <param name="context">The EF Core database context.</param>
+    public BundleUnlockAttemptRepository(ScadaBridgeDbContext context)
+    {
+        _context = context ?? throw new ArgumentNullException(nameof(context));
+    }
+
+    /// <inheritdoc />
+    public async Task<int> CountAttemptsSinceAsync(
+        string clientKey, DateTime sinceUtc, CancellationToken ct = default)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(clientKey);
+
+        return await _context.BundleUnlockAttempts
+            .AsNoTracking()
+            .Where(a => a.ClientKey == clientKey && a.AttemptedAtUtc >= sinceUtc)
+            .CountAsync(ct);
+    }
+
+    /// <inheritdoc />
+    public async Task RecordAttemptAsync(
+        string clientKey, DateTime attemptedAtUtc, CancellationToken ct = default)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(clientKey);
+
+        _context.BundleUnlockAttempts.Add(new BundleUnlockAttempt
+        {
+            ClientKey = clientKey,
+            AttemptedAtUtc = attemptedAtUtc,
+        });
+        await _context.SaveChangesAsync(ct);
+    }
+
+    /// <inheritdoc />
+    public async Task<int> PruneAttemptsOlderThanAsync(
+        DateTime beforeUtc, int maxRows, CancellationToken ct = default)
+    {
+        if (maxRows < 1)
+        {
+            throw new ArgumentOutOfRangeException(nameof(maxRows), "Prune batch size must be at least 1.");
+        }
+
+        // Bounded by construction: OrderBy + Take caps the delete regardless of how much
+        // history accumulated while the prune was failing or the node was down, so an
+        // opportunistic sweep on a request path can never become an unbounded delete.
+        return await _context.BundleUnlockAttempts
+            .Where(a => a.AttemptedAtUtc < beforeUtc)
+            .OrderBy(a => a.AttemptedAtUtc)
+            .Take(maxRows)
+            .ExecuteDeleteAsync(ct);
+    }
+}

src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/ScadaBridgeDbContext.cs

@@ -13,6 +13,7 @@ using ZB.MOM.WW.ScadaBridge.Commons.Entities.Notifications;
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.Schemas;
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.Scripts;
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.SecuredWrites;
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.Transport;
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.Security;
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.Sites;
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.Templates;
@@ -152,6 +153,10 @@ public class ScadaBridgeDbContext : DbContext, IDataProtectionKeyContext
     /// <summary>Gets the set of hourly KPI rollups (pre-aggregated long-range trend backbone).</summary>
     public DbSet<KpiRollupHourly> KpiRollupHourly => Set<KpiRollupHourly>();
 
+    // Transport (bundle export/import)
+    /// <summary>Gets the set of recorded bundle-unlock attempts — the fleet-wide sliding window behind the per-client unlock throttle (register row 52).</summary>
+    public DbSet<BundleUnlockAttempt> BundleUnlockAttempts => Set<BundleUnlockAttempt>();
+
     // Data Protection Keys (for shared ASP.NET Data Protection across nodes)
     /// <summary>Gets the set of data protection keys.</summary>
     public DbSet<DataProtectionKey> DataProtectionKeys => Set<DataProtectionKey>();

src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/ServiceCollectionExtensions.cs

@@ -87,6 +87,10 @@ public static class ServiceCollectionExtensions
         services.AddScoped<ISecuredWriteRepository, SecuredWriteRepository>();
         services.AddScoped<ISharedSchemaRepository, SharedSchemaRepository>();
         services.AddScoped<IKpiHistoryRepository, KpiHistoryRepository>();
+        // Register row 52: the fleet-wide bundle-unlock throttle window. Central-only in
+        // practice (Transport is registered in the host's Central branch), but registered
+        // here with every other repository so the ConfigurationDatabase surface stays uniform.
+        services.AddScoped<IBundleUnlockAttemptRepository, BundleUnlockAttemptRepository>();
         // Auth re-arch: inbound API keys are no longer persisted in SQL Server —
         // the repository now exposes only API-method access, so a plain scoped
         // registration suffices (no peppered-hasher accessor to wire).

src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs

@@ -3567,6 +3567,17 @@ public class ManagementActor : ReceiveActor
             {
                 throw new ManagementCommandException(ex.Message);
             }
+            catch (BundleLockedException ex)
+            {
+                // Register row 51: the per-bundle 3-strike lockout. LoadAsync raises
+                // it from inside its own CryptographicException handler, so without
+                // this arm the operator saw MapFault's generic internal-error message
+                // plus a correlation id, never the reason. Listed ahead of the
+                // CryptographicException arm to keep the read order matching the
+                // importer's throw order (the two types are unrelated, so the
+                // ordering is documentation, not dispatch).
+                throw new ManagementCommandException(ex.Message);
+            }
             catch (System.Security.Cryptography.CryptographicException)
             {
                 throw new ManagementCommandException("Wrong passphrase or bundle was tampered.");
@@ -3632,6 +3643,17 @@ public class ManagementActor : ReceiveActor
             {
                 throw new ManagementCommandException(ex.Message);
             }
+            catch (BundleLockedException ex)
+            {
+                // Register row 51: the per-bundle 3-strike lockout. LoadAsync raises
+                // it from inside its own CryptographicException handler, so without
+                // this arm the operator saw MapFault's generic internal-error message
+                // plus a correlation id, never the reason. Listed ahead of the
+                // CryptographicException arm to keep the read order matching the
+                // importer's throw order (the two types are unrelated, so the
+                // ordering is documentation, not dispatch).
+                throw new ManagementCommandException(ex.Message);
+            }
             catch (System.Security.Cryptography.CryptographicException)
             {
                 throw new ManagementCommandException("Wrong passphrase or bundle was tampered.");

src/ZB.MOM.WW.ScadaBridge.Transport/Import/BundleImporter.cs

@@ -293,11 +293,18 @@ public sealed class BundleImporter : IBundleImporter
             //     operator is legitimately importing.
             // Registered on every unlock attempt (successful ones included) because the
             // documented budget counts attempts, not failures.
+            //
+            // Register row 52: the async overload consults the FLEET-WIDE window (central
+            // MS SQL) so the cap is not multiplied by the number of central nodes and does
+            // not reset on a node restart. It fails open on a store error — see
+            // BundleUnlockRateLimiter's remarks.
             var throttleKey = string.IsNullOrWhiteSpace(clientKey)
                 ? UnattributedClientKey
                 : clientKey;
             var perIpCap = _options.Value.MaxUnlockAttemptsPerIpPerHour;
-            if (!_unlockRateLimiter.TryRegisterAttempt(throttleKey, perIpCap))
+            if (!await _unlockRateLimiter
+                    .TryRegisterAttemptAsync(throttleKey, perIpCap, ct)
+                    .ConfigureAwait(false))
             {
                 _logger?.LogWarning(
                     "Bundle unlock throttled: client {ClientKey} exceeded {Cap} attempts in the trailing hour.",

src/ZB.MOM.WW.ScadaBridge.Transport/Import/BundleLockedException.cs

@@ -1,31 +0,0 @@
-namespace ZB.MOM.WW.ScadaBridge.Transport.Import;
-
-/// <summary>
-/// T-003: thrown by <see cref="BundleImporter.LoadAsync"/> when an encrypted bundle has
-/// exceeded the configured failed-unlock attempt limit
-/// (<see cref="TransportOptions.MaxUnlockAttemptsPerSession"/>). The lockout is tracked
-/// server-side keyed by <c>BundleManifest.ContentHash</c>, so a second tab / CLI caller
-/// re-uploading the same bytes hits the same counter and cannot side-step the limit.
-/// </summary>
-public sealed class BundleLockedException : Exception
-{
-    /// <summary>Number of recorded unlock failures for this bundle.</summary>
-    public int FailedAttempts { get; }
-
-    /// <summary>SHA-256 (hex) of the bundle's content bytes, the lockout's tracking key.</summary>
-    public string BundleContentHash { get; }
-
-    /// <summary>
-    /// Initializes a new <see cref="BundleLockedException"/>.
-    /// </summary>
-    /// <param name="bundleContentHash">SHA-256 hex from <c>BundleManifest.ContentHash</c>.</param>
-    /// <param name="failedAttempts">Number of failures recorded against this bundle.</param>
-    public BundleLockedException(string bundleContentHash, int failedAttempts)
-        : base(
-            $"Bundle is locked after {failedAttempts} failed unlock attempts. "
-            + "Wait for the lockout window to expire or re-export the bundle to obtain a new content hash.")
-    {
-        BundleContentHash = bundleContentHash ?? throw new ArgumentNullException(nameof(bundleContentHash));
-        FailedAttempts = failedAttempts;
-    }
-}

src/ZB.MOM.WW.ScadaBridge.Transport/Import/BundleSessionEvictionService.cs

@@ -1,6 +1,7 @@
 using Microsoft.Extensions.Hosting;
 using Microsoft.Extensions.Logging;
 using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Transport;
+using ZB.MOM.WW.ScadaBridge.Commons.Observability;
 
 namespace ZB.MOM.WW.ScadaBridge.Transport.Import;
 
@@ -21,6 +22,15 @@ internal sealed class BundleSessionEvictionService : BackgroundService
     private readonly IBundleSessionStore _sessionStore;
     private readonly ILogger<BundleSessionEvictionService> _logger;
 
+    /// <summary>
+    /// Register row 53: the open-session gauge callback, held in a field so
+    /// <see cref="StopAsync"/> can pass the SAME delegate reference to
+    /// <see cref="ScadaBridgeTelemetry.ClearImportSessionCountProvider"/> — the clear is
+    /// an identity-checked compare-and-clear, so a stale stop cannot unregister a newer
+    /// instance's provider. Mirrors StoreAndForwardService's queue-depth registration.
+    /// </summary>
+    private readonly Func<long> _openSessionCountProvider;
+
     /// <summary>
     /// Initializes a new instance of <see cref="BundleSessionEvictionService"/>.
     /// </summary>
@@ -32,6 +42,35 @@ internal sealed class BundleSessionEvictionService : BackgroundService
     {
         _sessionStore = sessionStore ?? throw new ArgumentNullException(nameof(sessionStore));
         _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+        _openSessionCountProvider = () => _sessionStore.OpenSessionCount;
+    }
+
+    /// <summary>
+    /// Register row 53: registers the open-import-sessions gauge provider before the
+    /// sweep loop starts. This service is the Transport component's only lifecycle seam
+    /// (the session store itself is a plain singleton), and it is registered by
+    /// <c>AddTransport</c>, which only the Central branch of the host calls — so the
+    /// gauge reports 0 on a site node, where there are no import sessions to count.
+    /// </summary>
+    /// <param name="cancellationToken">Host startup cancellation token.</param>
+    /// <returns>A task that completes when the base <see cref="BackgroundService"/> has started.</returns>
+    public override Task StartAsync(CancellationToken cancellationToken)
+    {
+        ScadaBridgeTelemetry.SetImportSessionCountProvider(_openSessionCountProvider);
+        return base.StartAsync(cancellationToken);
+    }
+
+    /// <summary>
+    /// Deregisters this instance's gauge provider on graceful stop so the gauge falls
+    /// back to 0 rather than reading through a dead store. The clear is identity-checked,
+    /// so a late stop cannot clobber a successor that already re-registered.
+    /// </summary>
+    /// <param name="cancellationToken">Host shutdown cancellation token.</param>
+    /// <returns>A task that completes when the base <see cref="BackgroundService"/> has stopped.</returns>
+    public override async Task StopAsync(CancellationToken cancellationToken)
+    {
+        ScadaBridgeTelemetry.ClearImportSessionCountProvider(_openSessionCountProvider);
+        await base.StopAsync(cancellationToken).ConfigureAwait(false);
     }
 
     /// <inheritdoc />

src/ZB.MOM.WW.ScadaBridge.Transport/Import/BundleSessionStore.cs

@@ -1,4 +1,6 @@
 using System.Collections.Concurrent;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
 using Microsoft.Extensions.Options;
 using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Transport;
 using ZB.MOM.WW.ScadaBridge.Commons.Types.Transport;
@@ -48,6 +50,7 @@ public sealed class BundleSessionStore : IBundleSessionStore
 
     private readonly TimeProvider _timeProvider;
     private readonly IOptions<TransportOptions> _options;
+    private readonly ILogger<BundleSessionStore> _logger;
 
     /// <summary>T-003: per-bundle unlock-failure entry with expiry.</summary>
     private sealed class UnlockFailureRecord
@@ -61,12 +64,42 @@ public sealed class BundleSessionStore : IBundleSessionStore
     /// </summary>
     /// <param name="options">Transport options. <see cref="TransportOptions.BundleSessionTtlMinutes"/> is also used as the TTL for the T-003 per-bundle unlock-failure tracker.</param>
     /// <param name="timeProvider">Time provider used to evaluate session expiry.</param>
-    public BundleSessionStore(IOptions<TransportOptions> options, TimeProvider timeProvider)
+    /// <param name="logger">Optional logger. Register row 53: a rejection at <see cref="TransportOptions.MaxConcurrentImportSessions"/> is logged at Warning, so a session leak is diagnosable from the node log instead of being inferred from rotating test failures.</param>
+    public BundleSessionStore(
+        IOptions<TransportOptions> options,
+        TimeProvider timeProvider,
+        ILogger<BundleSessionStore>? logger = null)
     {
         ArgumentNullException.ThrowIfNull(options);
         _options = options;
         _ = options.Value;
         _timeProvider = timeProvider ?? throw new ArgumentNullException(nameof(timeProvider));
+        _logger = logger ?? NullLogger<BundleSessionStore>.Instance;
+    }
+
+    /// <inheritdoc />
+    public int OpenSessionCount
+    {
+        get
+        {
+            // Register row 53: count only unexpired entries. _sessions.Count would
+            // include expired-but-unswept sessions (the eviction sweep runs once a
+            // minute), so the gauge would read "pinned at the cap" for up to a sweep
+            // interval after a burst had actually drained — exactly the false signal
+            // this instrument exists to rule out. Read-only: the gauge callback runs
+            // on the metrics collection thread and must not mutate the store.
+            var now = _timeProvider.GetUtcNow();
+            var count = 0;
+            foreach (var kv in _sessions)
+            {
+                if (kv.Value.ExpiresAt > now)
+                {
+                    count++;
+                }
+            }
+
+            return count;
+        }
     }
 
     /// <inheritdoc />
@@ -82,6 +115,17 @@ public sealed class BundleSessionStore : IBundleSessionStore
                 EvictExpired();
                 if (_sessions.Count >= cap)
                 {
+                    // Register row 53: this rejection used to be silent apart from the
+                    // exception the caller saw. A cap hit is either genuine concurrency
+                    // (many operators importing at once) or a session leak like register
+                    // row 43's — the count-vs-cap pair plus the recurrence rate in the
+                    // log is what distinguishes them.
+                    _logger.LogWarning(
+                        "Bundle import session rejected: {OpenSessions} of {Cap} concurrent import sessions are open "
+                        + "(ScadaBridge:Transport:MaxConcurrentImportSessions). Sessions release on apply, cancel or "
+                        + "the {TtlMinutes}-minute TTL; a count that stays pinned at the cap indicates leaked sessions.",
+                        _sessions.Count, cap, _options.Value.BundleSessionTtlMinutes);
+
                     throw new InvalidOperationException(
                         $"Too many concurrent import sessions ({cap}). Complete or cancel an in-progress import before starting another.");
                 }

src/ZB.MOM.WW.ScadaBridge.Transport/Import/BundleUnlockRateLimitedException.cs

@@ -1,35 +0,0 @@
-namespace ZB.MOM.WW.ScadaBridge.Transport.Import;
-
-/// <summary>
-/// Thrown by <see cref="BundleImporter.LoadAsync"/> when the caller
-/// has exceeded the configured per-IP-per-hour unlock attempt cap
-/// (<see cref="TransportOptions.MaxUnlockAttemptsPerIpPerHour"/>). The 429-equivalent
-/// signal: the caller must wait for the trailing-hour window to roll forward before
-/// another passphrase attempt is accepted.
-/// </summary>
-public sealed class BundleUnlockRateLimitedException : Exception
-{
-    /// <summary>
-    /// Rate-limit key the limiter rejected the attempt against — the caller IP when
-    /// supplied, or the bundle's content hash as the architectural fallback (the
-    /// importer has no <c>IHttpContext</c> dependency by design).
-    /// </summary>
-    public string ClientKey { get; }
-
-    /// <summary>Per-window cap that was reached.</summary>
-    public int Limit { get; }
-
-    /// <summary>
-    /// Initializes a new <see cref="BundleUnlockRateLimitedException"/>.
-    /// </summary>
-    /// <param name="clientKey">The rate-limit key that exceeded its budget.</param>
-    /// <param name="limit">The configured per-window cap.</param>
-    public BundleUnlockRateLimitedException(string clientKey, int limit)
-        : base(
-            $"Bundle unlock rate limit reached ({limit} attempts per hour). "
-            + "Wait for the trailing-hour window to expire before retrying.")
-    {
-        ClientKey = clientKey ?? throw new ArgumentNullException(nameof(clientKey));
-        Limit = limit;
-    }
-}

src/ZB.MOM.WW.ScadaBridge.Transport/Import/BundleUnlockRateLimiter.cs

@@ -1,26 +1,37 @@
 using System.Collections.Concurrent;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
 
 namespace ZB.MOM.WW.ScadaBridge.Transport.Import;
 
 /// <summary>
-/// In-memory sliding-window rate limiter for bundle-unlock passphrase
-/// attempts, keyed by client IP. The design doc (§11) declares a per-IP-per-hour cap
-/// (default 10) as a brute-force defence against a stolen bundle; this class is the
-/// minimal server-side implementation.
+/// Sliding-window rate limiter for bundle-unlock passphrase attempts, keyed by client IP.
+/// The design doc (§11) declares a per-IP-per-hour cap (default 10) as a brute-force
+/// defence against a stolen bundle; this class owns that policy.
 /// <para>
-/// Algorithm: each key (an IP string, or any opaque caller identifier) holds a queue
-/// of attempt timestamps. <see cref="TryRegisterAttempt"/> first prunes entries older
-/// than the configured window, then either appends the current timestamp and returns
-/// <c>true</c> if the count is still under the threshold, or refuses to append and
-/// returns <c>false</c> if appending would cross it. The trailing-hour count is the
-/// queue length post-prune.
+/// <b>The window is fleet-wide when an <see cref="IBundleUnlockAttemptWindow"/> is
+/// supplied (register row 52).</b> The counters used to be per-node AND per-process, so
+/// the effective cap was N x limit per client across the central pair and a node restart
+/// silently cleared the window. <see cref="TryRegisterAttemptAsync"/> now consults the
+/// shared central-MS-SQL window instead, which both central nodes already share. The
+/// production composition root always supplies one.
 /// </para>
 /// <para>
-/// Storage is a process-local <see cref="ConcurrentDictionary{TKey,TValue}"/>. The
-/// counters do not survive a host restart — that is by design: a restart resets the
-/// brute-force window in favour of legitimate operators after an outage. Persisting
-/// the counters would require a multi-node consensus story the simple in-memory
-/// design avoids.
+/// <b>Fail-open is deliberate.</b> A store failure in the throttle path logs a Warning and
+/// PERMITS the attempt. This is a best-effort rate cap layered over a surface that is
+/// already authenticated and role-gated (Administrator-only bundle import), so the
+/// availability of legitimate imports outranks the cap: a database blip must not make the
+/// import wizard unusable. Falling back to the process-local window instead was considered
+/// and rejected — it would still block a legitimate operator's 11th import during an
+/// outage, which is the outcome fail-open exists to prevent.
+/// </para>
+/// <para>
+/// Algorithm (the process-local fallback used when no shared window is registered): each
+/// key holds a queue of attempt timestamps. <see cref="TryRegisterAttempt"/> first prunes
+/// entries older than the configured window, then either appends the current timestamp and
+/// returns <c>true</c> if the count is still under the threshold, or refuses to append and
+/// returns <c>false</c> if appending would cross it. Either way a REJECTED attempt is never
+/// appended, so a denial burns no budget — the same contract the shared window implements.
 /// </para>
 /// <para>
 /// Thread-safety: the per-key queue is protected by a per-key lock taken inside the
@@ -39,10 +50,13 @@ public sealed class BundleUnlockRateLimiter
     private readonly ConcurrentDictionary<string, AttemptBucket> _buckets = new(StringComparer.OrdinalIgnoreCase);
     private readonly TimeProvider _timeProvider;
     private readonly TimeSpan _window;
+    private readonly IBundleUnlockAttemptWindow? _sharedWindow;
+    private readonly ILogger<BundleUnlockRateLimiter> _logger;
 
     /// <summary>
     /// Initializes a new <see cref="BundleUnlockRateLimiter"/> using the documented
-    /// 1-hour trailing window and the system clock. Suitable for production DI.
+    /// 1-hour trailing window and the system clock, with no shared window (process-local
+    /// counters only).
     /// </summary>
     public BundleUnlockRateLimiter() : this(TimeProvider.System, DefaultWindow)
     {
@@ -50,11 +64,28 @@ public sealed class BundleUnlockRateLimiter
 
     /// <summary>
     /// Initializes a new <see cref="BundleUnlockRateLimiter"/> with an injected clock
-    /// (for deterministic tests) and a custom trailing window.
+    /// (for deterministic tests) and a custom trailing window, with no shared window.
     /// </summary>
     /// <param name="timeProvider">Clock used for both timestamping new attempts and pruning expired ones.</param>
     /// <param name="window">Trailing window over which attempts are counted (typically 1 hour).</param>
     public BundleUnlockRateLimiter(TimeProvider timeProvider, TimeSpan window)
+        : this(timeProvider, window, sharedWindow: null, logger: null)
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new <see cref="BundleUnlockRateLimiter"/> over a fleet-wide shared
+    /// window (register row 52). This is the overload the production composition root uses.
+    /// </summary>
+    /// <param name="timeProvider">Clock used for the process-local fallback window.</param>
+    /// <param name="window">Trailing window over which attempts are counted (typically 1 hour).</param>
+    /// <param name="sharedWindow">Shared cross-node attempt window; when null the limiter falls back to process-local counters.</param>
+    /// <param name="logger">Optional logger. Used for the fail-open Warning when the shared window is unavailable.</param>
+    public BundleUnlockRateLimiter(
+        TimeProvider timeProvider,
+        TimeSpan window,
+        IBundleUnlockAttemptWindow? sharedWindow,
+        ILogger<BundleUnlockRateLimiter>? logger)
     {
         ArgumentNullException.ThrowIfNull(timeProvider);
         if (window <= TimeSpan.Zero)
@@ -64,13 +95,81 @@ public sealed class BundleUnlockRateLimiter
 
         _timeProvider = timeProvider;
         _window = window;
+        _sharedWindow = sharedWindow;
+        _logger = logger ?? NullLogger<BundleUnlockRateLimiter>.Instance;
     }
 
     /// <summary>
-    /// Attempts to register a new passphrase try against the configured per-key
-    /// limit. Returns <c>true</c> when the attempt is permitted (and recorded);
-    /// returns <c>false</c> when the key has exhausted its budget for the trailing
-    /// window — the caller should reject the unlock request with a 429-equivalent.
+    /// Gets a value indicating whether this limiter is backed by a fleet-wide shared
+    /// window. False means the cap degrades to per-node, per-process counters.
+    /// </summary>
+    public bool IsFleetWide => _sharedWindow is not null;
+
+    /// <summary>
+    /// Registers a passphrase try against the configured per-key limit, using the
+    /// fleet-wide shared window when one is registered and the process-local window
+    /// otherwise. This is the method the importer calls.
+    /// </summary>
+    /// <param name="clientKey">Opaque caller identifier — typically the remote IP.</param>
+    /// <param name="maxAttemptsPerWindow">The trailing-window cap (<c>MaxUnlockAttemptsPerIpPerHour</c>).</param>
+    /// <param name="ct">Cancellation token.</param>
+    /// <returns>
+    /// <c>true</c> if the attempt is permitted; <c>false</c> if the caller has exhausted
+    /// its budget. Also <c>true</c> when the shared window is unavailable — see the
+    /// class remarks on why the throttle fails open.
+    /// </returns>
+    public async Task<bool> TryRegisterAttemptAsync(
+        string clientKey, int maxAttemptsPerWindow, CancellationToken ct = default)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(clientKey);
+        if (maxAttemptsPerWindow < 1)
+        {
+            throw new ArgumentOutOfRangeException(
+                nameof(maxAttemptsPerWindow), "Limit must be at least 1.");
+        }
+
+        if (_sharedWindow is null)
+        {
+            return TryRegisterAttempt(clientKey, maxAttemptsPerWindow);
+        }
+
+        try
+        {
+            return await _sharedWindow
+                .TryRegisterAttemptAsync(clientKey, maxAttemptsPerWindow, _window, ct)
+                .ConfigureAwait(false);
+        }
+        catch (OperationCanceledException) when (ct.IsCancellationRequested)
+        {
+            // The caller's own cancellation, not a store failure — do not swallow it.
+            throw;
+        }
+        catch (Exception ex)
+        {
+            // Fail OPEN: a best-effort rate cap must never take the import surface down
+            // with the database. The Warning is the operator's signal that the fleet cap
+            // is currently not being enforced.
+            _logger.LogWarning(
+                ex,
+                "Fleet-wide bundle-unlock throttle unavailable; permitting the unlock attempt for client "
+                + "{ClientKey} without enforcing the {Cap}/hour cap.",
+                clientKey, maxAttemptsPerWindow);
+            return true;
+        }
+    }
+
+    /// <summary>
+    /// Attempts to register a new passphrase try against the configured per-key limit in
+    /// the PROCESS-LOCAL window. Returns <c>true</c> when the attempt is permitted (and
+    /// recorded); returns <c>false</c> when the key has exhausted its budget for the
+    /// trailing window — the caller should reject the unlock request with a 429-equivalent.
+    /// <para>
+    /// Register row 52: this bypasses the fleet-wide window. Callers on the unlock path
+    /// must use <see cref="TryRegisterAttemptAsync"/>, which routes to the shared window
+    /// when one is registered and falls back here otherwise. This overload remains public
+    /// for hosts that register Transport without a shared window, and for the tests that
+    /// pin the sliding-window arithmetic directly.
+    /// </para>
     /// </summary>
     /// <param name="clientKey">
     /// Opaque caller identifier — typically the remote IP, but any stable per-source

src/ZB.MOM.WW.ScadaBridge.Transport/Import/IBundleUnlockAttemptWindow.cs

@@ -0,0 +1,28 @@
+namespace ZB.MOM.WW.ScadaBridge.Transport.Import;
+
+/// <summary>
+/// Register row 52: a sliding window of bundle-unlock attempts that is shared across the
+/// central pair, so <c>TransportOptions.MaxUnlockAttemptsPerIpPerHour</c> is a FLEET cap
+/// rather than a per-node, per-process one.
+/// <para>
+/// Implementations are expected to THROW on store failure. The fail-open decision belongs
+/// to <see cref="BundleUnlockRateLimiter"/>, which owns the policy (log a Warning and
+/// permit the attempt), not to the storage layer.
+/// </para>
+/// </summary>
+public interface IBundleUnlockAttemptWindow
+{
+    /// <summary>
+    /// Registers one unlock attempt against the shared window and reports whether it was
+    /// within budget. Implementations must NOT record an attempt they are rejecting —
+    /// a denial burns no budget, matching the in-memory limiter's contract (register
+    /// row 44), so a client that keeps hammering cannot extend its own lockout.
+    /// </summary>
+    /// <param name="clientKey">Throttle bucket — the caller's remote IP, or the shared unattributed bucket.</param>
+    /// <param name="maxAttemptsPerWindow">The trailing-window cap (<c>MaxUnlockAttemptsPerIpPerHour</c>).</param>
+    /// <param name="window">Trailing window the attempts are counted over (one hour in production).</param>
+    /// <param name="ct">Cancellation token.</param>
+    /// <returns><c>true</c> when the attempt was recorded (within budget); <c>false</c> when the bucket is exhausted.</returns>
+    Task<bool> TryRegisterAttemptAsync(
+        string clientKey, int maxAttemptsPerWindow, TimeSpan window, CancellationToken ct = default);
+}

src/ZB.MOM.WW.ScadaBridge.Transport/Import/SqlBundleUnlockAttemptWindow.cs

@@ -0,0 +1,167 @@
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Repositories;
+
+namespace ZB.MOM.WW.ScadaBridge.Transport.Import;
+
+/// <summary>
+/// Register row 52: <see cref="IBundleUnlockAttemptWindow"/> backed by the central MS SQL
+/// <c>BundleUnlockAttempt</c> table — the HA store both central nodes already share, so
+/// the fleet-wide cap needs no new infrastructure, no new port and no consensus protocol.
+/// <para>
+/// Registered as a singleton (the limiter that consumes it is one) and therefore resolves
+/// a scoped <see cref="IBundleUnlockAttemptRepository"/> from its own
+/// <see cref="IServiceScopeFactory"/> scope per call, the same way the central purge
+/// singletons reach the database.
+/// </para>
+/// <para>
+/// <b>Accuracy is deliberately approximate.</b> The count-then-insert pair is not atomic
+/// across the two central nodes, so genuinely simultaneous attempts can overshoot the cap
+/// by up to the number in flight. A conditional single-statement insert would close that,
+/// at the cost of provider-specific SQL for a best-effort rate cap layered over an
+/// already-authenticated, role-gated surface. Overshoot is bounded and self-corrects
+/// within the window; it is not worth raw SQL.
+/// </para>
+/// </summary>
+public sealed class SqlBundleUnlockAttemptWindow : IBundleUnlockAttemptWindow
+{
+    /// <summary>
+    /// Minimum wall-clock gap between opportunistic prunes. The prune runs on the unlock
+    /// path (Transport owns no central scheduler, and the design constraint was "no new
+    /// background service"), so it is claimed at most this often per process — the same
+    /// rate-limited opportunistic-sweep shape <c>DeploymentService</c> uses for terminal
+    /// deployment records.
+    /// </summary>
+    internal static readonly TimeSpan PruneMinInterval = TimeSpan.FromMinutes(15);
+
+    /// <summary>
+    /// Upper bound on rows deleted by one opportunistic prune pass. Bounded so a sweep on
+    /// a request path can never become a long-running delete, even if the table grew while
+    /// prunes were failing.
+    /// </summary>
+    internal const int PruneBatchSize = 1000;
+
+    /// <summary>
+    /// Multiple of the trailing window that rows are retained for before becoming
+    /// prunable. Kept above 1 so a prune racing a count can never delete a row the count
+    /// still needs.
+    /// </summary>
+    internal const int PruneRetentionWindows = 2;
+
+    private readonly IServiceScopeFactory _scopeFactory;
+    private readonly TimeProvider _timeProvider;
+    private readonly ILogger<SqlBundleUnlockAttemptWindow> _logger;
+
+    /// <summary>Ticks of the last claimed prune, as a CAS-guarded process-local claim.</summary>
+    private long _lastPruneTicks;
+
+    /// <summary>
+    /// Initializes a new <see cref="SqlBundleUnlockAttemptWindow"/>.
+    /// </summary>
+    /// <param name="scopeFactory">Scope factory used to resolve the scoped attempt repository per call.</param>
+    /// <param name="timeProvider">Clock used for the window boundary and the prune cadence.</param>
+    /// <param name="logger">Optional logger for prune diagnostics.</param>
+    public SqlBundleUnlockAttemptWindow(
+        IServiceScopeFactory scopeFactory,
+        TimeProvider timeProvider,
+        ILogger<SqlBundleUnlockAttemptWindow>? logger = null)
+    {
+        _scopeFactory = scopeFactory ?? throw new ArgumentNullException(nameof(scopeFactory));
+        _timeProvider = timeProvider ?? throw new ArgumentNullException(nameof(timeProvider));
+        _logger = logger ?? NullLogger<SqlBundleUnlockAttemptWindow>.Instance;
+
+        // Seed the prune claim to "now" so a freshly started node does not prune on its
+        // very first unlock — the table is almost certainly already tidy, and a node that
+        // restarts under load would otherwise prune on every restart.
+        _lastPruneTicks = _timeProvider.GetUtcNow().UtcTicks;
+    }
+
+    /// <inheritdoc />
+    public async Task<bool> TryRegisterAttemptAsync(
+        string clientKey, int maxAttemptsPerWindow, TimeSpan window, CancellationToken ct = default)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(clientKey);
+        if (maxAttemptsPerWindow < 1)
+        {
+            throw new ArgumentOutOfRangeException(
+                nameof(maxAttemptsPerWindow), "Limit must be at least 1.");
+        }
+
+        if (window <= TimeSpan.Zero)
+        {
+            throw new ArgumentOutOfRangeException(nameof(window), "Window must be positive.");
+        }
+
+        var now = _timeProvider.GetUtcNow().UtcDateTime;
+        var key = clientKey.Trim();
+
+        using var scope = _scopeFactory.CreateScope();
+        var repo = scope.ServiceProvider.GetRequiredService<IBundleUnlockAttemptRepository>();
+
+        // Two round trips rather than one conditional insert — see the class remarks on
+        // why the resulting overshoot is accepted. Exceptions propagate: the fail-open
+        // decision lives in BundleUnlockRateLimiter.
+        var used = await repo.CountAttemptsSinceAsync(key, now - window, ct).ConfigureAwait(false);
+        if (used >= maxAttemptsPerWindow)
+        {
+            // Rejected attempts are NOT recorded — a denial burns no budget, so a client
+            // hammering the endpoint cannot roll its own window forward indefinitely.
+            return false;
+        }
+
+        await repo.RecordAttemptAsync(key, now, ct).ConfigureAwait(false);
+
+        await TryPruneAsync(repo, now, window, ct).ConfigureAwait(false);
+        return true;
+    }
+
+    /// <summary>
+    /// Opportunistic, rate-limited, bounded prune of aged-out attempt rows. Best-effort:
+    /// any failure is logged and swallowed, because retention maintenance must never fail
+    /// the operator's actual unlock.
+    /// </summary>
+    /// <param name="repo">Repository resolved for the current call's scope.</param>
+    /// <param name="nowUtc">Current UTC instant.</param>
+    /// <param name="window">The trailing window; rows older than a multiple of it are prunable.</param>
+    /// <param name="ct">Cancellation token.</param>
+    /// <returns>A task that completes when the prune attempt has finished (or been skipped).</returns>
+    private async Task TryPruneAsync(
+        IBundleUnlockAttemptRepository repo, DateTime nowUtc, TimeSpan window, CancellationToken ct)
+    {
+        var previous = Interlocked.Read(ref _lastPruneTicks);
+        if (nowUtc.Ticks - previous < PruneMinInterval.Ticks)
+        {
+            return;
+        }
+
+        // Claim the sweep so concurrent unlocks on this node don't all prune at once.
+        if (Interlocked.CompareExchange(ref _lastPruneTicks, nowUtc.Ticks, previous) != previous)
+        {
+            return;
+        }
+
+        try
+        {
+            var cutoff = nowUtc - (window * PruneRetentionWindows);
+            var deleted = await repo
+                .PruneAttemptsOlderThanAsync(cutoff, PruneBatchSize, ct)
+                .ConfigureAwait(false);
+
+            if (deleted > 0)
+            {
+                _logger.LogDebug(
+                    "Pruned {Deleted} bundle-unlock attempt row(s) older than {Cutoff:o}.", deleted, cutoff);
+            }
+        }
+        catch (OperationCanceledException) when (ct.IsCancellationRequested)
+        {
+            throw;
+        }
+        catch (Exception ex)
+        {
+            // Never fail the unlock for a retention sweep.
+            _logger.LogWarning(ex, "Opportunistic bundle-unlock attempt prune failed; will retry on a later attempt.");
+        }
+    }
+}

src/ZB.MOM.WW.ScadaBridge.Transport/ServiceCollectionExtensions.cs

@@ -1,5 +1,6 @@
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.DependencyInjection.Extensions;
+using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
 using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Transport;
 using ZB.MOM.WW.ScadaBridge.TemplateEngine.Validation;
@@ -43,10 +44,22 @@ public static class ServiceCollectionExtensions
         services.AddScoped<DependencyResolver>();
         services.AddScoped<IBundleExporter, BundleExporter>();
         services.AddSingleton<IBundleSessionStore, BundleSessionStore>();
-        // T-004: per-IP-per-hour unlock rate limiter — design doc §11. Singleton
-        // so the trailing-hour window is shared across every importer scope; the
-        // counters live in-memory and reset on host restart (by design).
-        services.AddSingleton<BundleUnlockRateLimiter>();
+        // T-004: per-IP-per-hour unlock rate limiter — design doc §11. Singleton so the
+        // trailing-hour window is shared across every importer scope.
+        //
+        // Register row 52: the window is FLEET-WIDE, backed by the central MS SQL
+        // BundleUnlockAttempt table that both central nodes already share — the counters
+        // used to be per-node and per-process, so the effective cap was N x limit per
+        // client and a restart cleared the window. The window resolves its scoped
+        // repository from its own scope per call, so it is safe as a singleton.
+        // Constructed explicitly rather than by convention so it is unambiguous which
+        // constructor overload production uses.
+        services.AddSingleton<IBundleUnlockAttemptWindow, SqlBundleUnlockAttemptWindow>();
+        services.AddSingleton(sp => new BundleUnlockRateLimiter(
+            sp.GetRequiredService<TimeProvider>(),
+            BundleUnlockRateLimiter.DefaultWindow,
+            sp.GetRequiredService<IBundleUnlockAttemptWindow>(),
+            sp.GetService<ILogger<BundleUnlockRateLimiter>>()));
         // T-007: periodic eviction sweep so abandoned sessions clear without
         // needing a fresh Get() to trigger lazy eviction.
         services.AddHostedService<BundleSessionEvictionService>();