feat(server): add observability and operator controls (#16)

src/FinalFactory.Rendezvous.Server/Observability/AuditTrail.cs

@@ -0,0 +1,134 @@
+using System.Security.Cryptography;
+using System.Text;
+using Microsoft.Extensions.Options;
+
+namespace FinalFactory.Rendezvous.Server.Observability;
+
+internal sealed partial class AuditTrail
+{
+    private readonly object _gate = new();
+    private readonly LinkedList<AuditEntry> _entries = [];
+    private readonly AuditOptions _options;
+    private readonly TimeProvider _timeProvider;
+    private readonly ILogger<AuditTrail> _logger;
+    private readonly RendezvousTelemetry _telemetry;
+
+    public AuditTrail(
+        IOptions<AuditOptions> options,
+        ILogger<AuditTrail> logger,
+        RendezvousTelemetry telemetry,
+        TimeProvider? timeProvider = null)
+    {
+        _options = options.Value;
+        _logger = logger;
+        _telemetry = telemetry;
+        _timeProvider = timeProvider ?? TimeProvider.System;
+    }
+
+    public void Record(
+        string actorSubject,
+        string action,
+        string result,
+        string targetKind,
+        string targetIdentifier,
+        string correlationId)
+    {
+        DateTimeOffset now = _timeProvider.GetUtcNow();
+        AuditEntry entry = new(
+            now,
+            Fingerprint(actorSubject),
+            action,
+            result,
+            targetKind,
+            Fingerprint(targetIdentifier),
+            correlationId);
+        lock (_gate)
+        {
+            PurgeExpired(now);
+
+            while (_entries.Count >= _options.MaxEntries)
+            {
+                _entries.RemoveFirst();
+            }
+
+            _entries.AddLast(entry);
+        }
+
+        _telemetry.RecordAudit(action, result);
+        LogOperatorAction(
+            _logger,
+            entry.Timestamp,
+            entry.ActorFingerprint,
+            action,
+            result,
+            targetKind,
+            entry.TargetFingerprint,
+            correlationId);
+    }
+
+    public IReadOnlyDictionary<string, long> GetAggregateCounts()
+    {
+        lock (_gate)
+        {
+            PurgeExpired(_timeProvider.GetUtcNow());
+            return _entries
+                .GroupBy(static entry => $"{entry.Action}:{entry.Result}", StringComparer.Ordinal)
+                .ToDictionary(
+                    static group => group.Key,
+                    static group => (long)group.Count(),
+                    StringComparer.Ordinal);
+        }
+    }
+
+    internal IReadOnlyList<AuditEntry> GetEntriesForTests()
+    {
+        lock (_gate)
+        {
+            PurgeExpired(_timeProvider.GetUtcNow());
+            return _entries.ToArray();
+        }
+    }
+
+    private void PurgeExpired(DateTimeOffset now)
+    {
+        DateTimeOffset oldest = now.AddDays(-_options.RetentionDays);
+        while (_entries.First is { Value.Timestamp: var timestamp }
+               && timestamp < oldest)
+        {
+            _entries.RemoveFirst();
+        }
+    }
+
+    private static string Fingerprint(string value)
+    {
+        byte[] digest = SHA256.HashData(Encoding.UTF8.GetBytes(value));
+        return Convert.ToHexString(digest.AsSpan(0, 12));
+    }
+
+    [LoggerMessage(
+        EventId = 100,
+        Level = LogLevel.Information,
+        Message = "Operator audit at {Timestamp}: actor {ActorFingerprint} action {Action} completed with {Result} for {TargetKind} target {TargetFingerprint}; correlation {CorrelationId}")]
+    private static partial void LogOperatorAction(
+        ILogger logger,
+        DateTimeOffset timestamp,
+        string actorFingerprint,
+        string action,
+        string result,
+        string targetKind,
+        string targetFingerprint,
+        string correlationId);
+}
+
+internal sealed record AuditEntry(
+    DateTimeOffset Timestamp,
+    string ActorFingerprint,
+    string Action,
+    string Result,
+    string TargetKind,
+    string TargetFingerprint,
+    string CorrelationId)
+{
+    public override string ToString() =>
+        $"[AuditEntry {Action}/{Result}; actor and target fingerprinted]";
+}