perf(dashboard): memoize Galaxy summary by cache sequence; document scope-provider identity invariant

src/ZB.MOM.WW.MxGateway.Server/Dashboard/DashboardSnapshotService.cs

@@ -30,6 +30,12 @@ public sealed class DashboardSnapshotService : IDashboardSnapshotService
     private readonly ILogger<DashboardSnapshotService> _logger;
     private readonly SemaphoreSlim _apiKeySummaryRefreshGate = new(1, 1);
     private IReadOnlyList<DashboardApiKeySummary> _apiKeySummaries = Array.Empty<DashboardApiKeySummary>();
+    // Memoizes the projected Galaxy summary against the immutable cache sequence. The shared
+    // library bumps Sequence only on a heavy refresh, so an unchanged sequence means the entry
+    // (and therefore its summary) is unchanged and the O(N) projection can be reused. This keeps
+    // the ~1s snapshot tick O(1) for Galaxy, restoring the pre-adoption behavior where the
+    // summary was computed once per refresh rather than once per tick.
+    private GalaxySummaryCache? _galaxySummaryCache;
 
     /// <summary>Initializes a new instance of the DashboardSnapshotService class.</summary>
     /// <param name="sessionRegistry">Registry of active gateway sessions.</param>
@@ -97,9 +103,30 @@ public sealed class DashboardSnapshotService : IDashboardSnapshotService
             Faults: CreateFaultSummaries(sessions, generatedAt),
             ApiKeys: Volatile.Read(ref _apiKeySummaries),
             Configuration: _configurationProvider.GetEffectiveConfiguration(),
-            Galaxy: DashboardGalaxyProjector.Project(_galaxyHierarchyCache.Current));
+            Galaxy: ResolveGalaxySummary());
     }
 
+    private DashboardGalaxySummary ResolveGalaxySummary()
+    {
+        GalaxyHierarchyCacheEntry entry = _galaxyHierarchyCache.Current;
+        long sequence = entry.Sequence;
+
+        // Lock-free reuse: a matching sequence means the entry is unchanged, so the previously
+        // projected summary is still correct. A racing recompute for a new sequence is harmless —
+        // the projection is pure, so any winner stores identical content for that sequence.
+        GalaxySummaryCache? cached = Volatile.Read(ref _galaxySummaryCache);
+        if (cached is not null && cached.Sequence == sequence)
+        {
+            return cached.Summary;
+        }
+
+        DashboardGalaxySummary summary = DashboardGalaxyProjector.Project(entry);
+        Volatile.Write(ref _galaxySummaryCache, new GalaxySummaryCache(sequence, summary));
+        return summary;
+    }
+
+    private sealed record GalaxySummaryCache(long Sequence, DashboardGalaxySummary Summary);
+
     /// <summary>
     /// Watches dashboard snapshots at regular intervals asynchronously.
     /// </summary>

src/ZB.MOM.WW.MxGateway.Server/Security/Authorization/GatewayBrowseScopeProvider.cs

@@ -11,6 +11,13 @@ public sealed class GatewayBrowseScopeProvider(IGatewayRequestIdentityAccessor i
     /// <inheritdoc />
     public IReadOnlyList<string>? ResolveBrowseSubtrees(ServerCallContext context)
     {
+        // Invariant: the caller identity is the ambient one pushed by
+        // GatewayGrpcAuthorizationInterceptor earlier in this same call. That interceptor is
+        // registered globally and therefore runs before the library's galaxy service, so
+        // Current is populated for any authenticated galaxy RPC. If it is somehow absent we
+        // return empty BrowseSubtrees, which the library treats as "no scoping" (full
+        // hierarchy) — safe only because the global interceptor has already authenticated and
+        // authorized the request first. Do not reorder the interceptor without revisiting this.
         ApiKeyConstraints constraints = identityAccessor.Current?.EffectiveConstraints ?? ApiKeyConstraints.Empty;
         return constraints.BrowseSubtrees;
     }