Resolve Concelier/Excititor merge conflicts

src/StellaOps.Scanner.Queue/AGENTS.md

@@ -0,0 +1,15 @@
+# StellaOps.Scanner.Queue — Agent Charter
+
+## Mission
+Deliver the scanner job queue backbone defined in `docs/ARCHITECTURE_SCANNER.md`, providing deterministic, offline-friendly leasing semantics for WebService producers and Worker consumers.
+
+## Responsibilities
+- Define queue abstractions with idempotent enqueue tokens, acknowledgement, lease renewal, and claim support.
+- Ship first-party adapters for Redis Streams and NATS JetStream, respecting offline deployments and allow-listed hosts.
+- Surface health probes, structured diagnostics, and metrics needed by Scanner WebService/Worker.
+- Document operational expectations and configuration binding hooks.
+
+## Interfaces & Dependencies
+- Consumes shared configuration primitives from `StellaOps.Configuration`.
+- Exposes dependency injection extensions for `StellaOps.DependencyInjection`.
+- Targets `net10.0` (preview) and aligns with scanner DTOs once `StellaOps.Scanner.Core` lands.

src/StellaOps.Scanner.Queue/IScanQueue.cs

@@ -0,0 +1,20 @@
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace StellaOps.Scanner.Queue;
+
+public interface IScanQueue
+{
+    ValueTask<QueueEnqueueResult> EnqueueAsync(
+        ScanQueueMessage message,
+        CancellationToken cancellationToken = default);
+
+    ValueTask<IReadOnlyList<IScanQueueLease>> LeaseAsync(
+        QueueLeaseRequest request,
+        CancellationToken cancellationToken = default);
+
+    ValueTask<IReadOnlyList<IScanQueueLease>> ClaimExpiredLeasesAsync(
+        QueueClaimOptions options,
+        CancellationToken cancellationToken = default);
+}

src/StellaOps.Scanner.Queue/IScanQueueLease.cs

@@ -0,0 +1,37 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace StellaOps.Scanner.Queue;
+
+public interface IScanQueueLease
+{
+    string MessageId { get; }
+
+    string JobId { get; }
+
+    ReadOnlyMemory<byte> Payload { get; }
+
+    int Attempt { get; }
+
+    DateTimeOffset EnqueuedAt { get; }
+
+    DateTimeOffset LeaseExpiresAt { get; }
+
+    string Consumer { get; }
+
+    string? IdempotencyKey { get; }
+
+    string? TraceId { get; }
+
+    IReadOnlyDictionary<string, string> Attributes { get; }
+
+    Task AcknowledgeAsync(CancellationToken cancellationToken = default);
+
+    Task RenewAsync(TimeSpan leaseDuration, CancellationToken cancellationToken = default);
+
+    Task ReleaseAsync(QueueReleaseDisposition disposition, CancellationToken cancellationToken = default);
+
+    Task DeadLetterAsync(string reason, CancellationToken cancellationToken = default);
+}

src/StellaOps.Scanner.Queue/Nats/NatsScanQueue.cs

@@ -0,0 +1,654 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.Logging;
+using NATS.Client.Core;
+using NATS.Client.JetStream;
+using NATS.Client.JetStream.Models;
+
+namespace StellaOps.Scanner.Queue.Nats;
+
+internal sealed class NatsScanQueue : IScanQueue, IAsyncDisposable
+{
+    private const string TransportName = "nats";
+
+    private static readonly INatsSerializer<byte[]> PayloadSerializer = NatsRawSerializer<byte[]>.Default;
+
+    private readonly ScannerQueueOptions _queueOptions;
+    private readonly NatsQueueOptions _options;
+    private readonly ILogger<NatsScanQueue> _logger;
+    private readonly TimeProvider _timeProvider;
+    private readonly SemaphoreSlim _connectionGate = new(1, 1);
+    private readonly Func<NatsOpts, CancellationToken, ValueTask<NatsConnection>> _connectionFactory;
+
+    private NatsConnection? _connection;
+    private NatsJSContext? _jsContext;
+    private INatsJSConsumer? _consumer;
+    private bool _disposed;
+
+    public NatsScanQueue(
+        ScannerQueueOptions queueOptions,
+        NatsQueueOptions options,
+        ILogger<NatsScanQueue> logger,
+        TimeProvider timeProvider,
+        Func<NatsOpts, CancellationToken, ValueTask<NatsConnection>>? connectionFactory = null)
+    {
+        _queueOptions = queueOptions ?? throw new ArgumentNullException(nameof(queueOptions));
+        _options = options ?? throw new ArgumentNullException(nameof(options));
+        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+        _timeProvider = timeProvider ?? TimeProvider.System;
+        _connectionFactory = connectionFactory ?? ((opts, cancellationToken) => new ValueTask<NatsConnection>(new NatsConnection(opts)));
+
+        if (string.IsNullOrWhiteSpace(_options.Url))
+        {
+            throw new InvalidOperationException("NATS connection URL must be configured for the scanner queue.");
+        }
+    }
+
+    public async ValueTask<QueueEnqueueResult> EnqueueAsync(
+        ScanQueueMessage message,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(message);
+
+        var js = await GetJetStreamAsync(cancellationToken).ConfigureAwait(false);
+        await EnsureStreamAndConsumerAsync(js, cancellationToken).ConfigureAwait(false);
+
+        var idempotencyKey = message.IdempotencyKey ?? message.JobId;
+        var headers = BuildHeaders(message, idempotencyKey);
+        var publishOpts = new NatsJSPubOpts
+        {
+            MsgId = idempotencyKey,
+            RetryAttempts = 0
+        };
+
+        var ack = await js.PublishAsync(
+                _options.Subject,
+                message.Payload.ToArray(),
+                PayloadSerializer,
+                publishOpts,
+                headers,
+                cancellationToken)
+            .ConfigureAwait(false);
+
+        if (ack.Duplicate)
+        {
+            _logger.LogDebug(
+                "Duplicate NATS enqueue detected for job {JobId} (token {Token}).",
+                message.JobId,
+                idempotencyKey);
+
+            QueueMetrics.RecordDeduplicated(TransportName);
+            return new QueueEnqueueResult(ack.Seq.ToString(), true);
+        }
+
+        QueueMetrics.RecordEnqueued(TransportName);
+        _logger.LogDebug(
+            "Enqueued job {JobId} into NATS stream {Stream} with sequence {Sequence}.",
+            message.JobId,
+            ack.Stream,
+            ack.Seq);
+
+        return new QueueEnqueueResult(ack.Seq.ToString(), false);
+    }
+
+    public async ValueTask<IReadOnlyList<IScanQueueLease>> LeaseAsync(
+        QueueLeaseRequest request,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(request);
+
+        var js = await GetJetStreamAsync(cancellationToken).ConfigureAwait(false);
+        var consumer = await EnsureStreamAndConsumerAsync(js, cancellationToken).ConfigureAwait(false);
+
+        var fetchOpts = new NatsJSFetchOpts
+        {
+            MaxMsgs = request.BatchSize,
+            Expires = request.LeaseDuration,
+            IdleHeartbeat = _options.IdleHeartbeat
+        };
+
+        var now = _timeProvider.GetUtcNow();
+        var leases = new List<IScanQueueLease>(capacity: request.BatchSize);
+
+        await foreach (var msg in consumer.FetchAsync(PayloadSerializer, fetchOpts, cancellationToken).ConfigureAwait(false))
+        {
+            var lease = CreateLease(msg, request.Consumer, now, request.LeaseDuration);
+            if (lease is not null)
+            {
+                leases.Add(lease);
+            }
+        }
+
+        return leases;
+    }
+
+    public async ValueTask<IReadOnlyList<IScanQueueLease>> ClaimExpiredLeasesAsync(
+        QueueClaimOptions options,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(options);
+
+        var js = await GetJetStreamAsync(cancellationToken).ConfigureAwait(false);
+        var consumer = await EnsureStreamAndConsumerAsync(js, cancellationToken).ConfigureAwait(false);
+
+        var fetchOpts = new NatsJSFetchOpts
+        {
+            MaxMsgs = options.BatchSize,
+            Expires = options.MinIdleTime,
+            IdleHeartbeat = _options.IdleHeartbeat
+        };
+
+        var now = _timeProvider.GetUtcNow();
+        var leases = new List<IScanQueueLease>(options.BatchSize);
+
+        await foreach (var msg in consumer.FetchAsync(PayloadSerializer, fetchOpts, cancellationToken).ConfigureAwait(false))
+        {
+            var deliveries = (int)(msg.Metadata?.NumDelivered ?? 1);
+            if (deliveries <= 1)
+            {
+                // Fresh message; surface back to queue and continue.
+                await msg.NakAsync(new AckOpts(), TimeSpan.Zero, cancellationToken).ConfigureAwait(false);
+                continue;
+            }
+
+            var lease = CreateLease(msg, options.ClaimantConsumer, now, _queueOptions.DefaultLeaseDuration);
+            if (lease is not null)
+            {
+                leases.Add(lease);
+            }
+        }
+
+        return leases;
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        if (_disposed)
+        {
+            return;
+        }
+
+        _disposed = true;
+
+        if (_connection is not null)
+        {
+            await _connection.DisposeAsync().ConfigureAwait(false);
+        }
+
+        _connectionGate.Dispose();
+        GC.SuppressFinalize(this);
+    }
+
+    internal async Task AcknowledgeAsync(
+        NatsScanQueueLease lease,
+        CancellationToken cancellationToken)
+    {
+        if (!lease.TryBeginCompletion())
+        {
+            return;
+        }
+
+        await lease.Message.AckAsync(new AckOpts(), cancellationToken).ConfigureAwait(false);
+
+        QueueMetrics.RecordAck(TransportName);
+        _logger.LogDebug(
+            "Acknowledged job {JobId} (seq {Seq}).",
+            lease.JobId,
+            lease.MessageId);
+    }
+
+    internal async Task RenewLeaseAsync(
+        NatsScanQueueLease lease,
+        TimeSpan leaseDuration,
+        CancellationToken cancellationToken)
+    {
+        await lease.Message.AckProgressAsync(new AckOpts(), cancellationToken).ConfigureAwait(false);
+        var expires = _timeProvider.GetUtcNow().Add(leaseDuration);
+        lease.RefreshLease(expires);
+
+        _logger.LogDebug(
+            "Renewed NATS lease for job {JobId} until {Expires:u}.",
+            lease.JobId,
+            expires);
+    }
+
+    internal async Task ReleaseAsync(
+        NatsScanQueueLease lease,
+        QueueReleaseDisposition disposition,
+        CancellationToken cancellationToken)
+    {
+        if (disposition == QueueReleaseDisposition.Retry
+            && lease.Attempt >= _queueOptions.MaxDeliveryAttempts)
+        {
+            _logger.LogWarning(
+                "Job {JobId} reached max delivery attempts ({Attempts}); shipping to dead-letter stream.",
+                lease.JobId,
+                lease.Attempt);
+
+            await DeadLetterAsync(
+                lease,
+                $"max-delivery-attempts:{lease.Attempt}",
+                cancellationToken).ConfigureAwait(false);
+            return;
+        }
+
+        if (!lease.TryBeginCompletion())
+        {
+            return;
+        }
+
+        if (disposition == QueueReleaseDisposition.Retry)
+        {
+            QueueMetrics.RecordRetry(TransportName);
+
+            var delay = CalculateBackoff(lease.Attempt);
+            await lease.Message.NakAsync(new AckOpts(), delay, cancellationToken).ConfigureAwait(false);
+
+            _logger.LogWarning(
+                "Rescheduled job {JobId} via NATS NAK with delay {Delay} (attempt {Attempt}).",
+                lease.JobId,
+                delay,
+                lease.Attempt);
+        }
+        else
+        {
+            await lease.Message.AckTerminateAsync(new AckOpts(), cancellationToken).ConfigureAwait(false);
+            QueueMetrics.RecordAck(TransportName);
+
+            _logger.LogInformation(
+                "Abandoned job {JobId} after {Attempt} attempt(s).",
+                lease.JobId,
+                lease.Attempt);
+        }
+    }
+
+    internal async Task DeadLetterAsync(
+        NatsScanQueueLease lease,
+        string reason,
+        CancellationToken cancellationToken)
+    {
+        if (!lease.TryBeginCompletion())
+        {
+            return;
+        }
+
+        await lease.Message.AckAsync(new AckOpts(), cancellationToken).ConfigureAwait(false);
+
+        var js = await GetJetStreamAsync(cancellationToken).ConfigureAwait(false);
+        await EnsureDeadLetterStreamAsync(js, cancellationToken).ConfigureAwait(false);
+
+        var headers = BuildDeadLetterHeaders(lease, reason);
+        await js.PublishAsync(
+                _options.DeadLetterSubject,
+                lease.Payload.ToArray(),
+                PayloadSerializer,
+                new NatsJSPubOpts(),
+                headers,
+                cancellationToken)
+            .ConfigureAwait(false);
+
+        QueueMetrics.RecordDeadLetter(TransportName);
+        _logger.LogError(
+            "Dead-lettered job {JobId} (attempt {Attempt}): {Reason}",
+            lease.JobId,
+            lease.Attempt,
+            reason);
+    }
+
+    private async Task<NatsJSContext> GetJetStreamAsync(CancellationToken cancellationToken)
+    {
+        if (_jsContext is not null)
+        {
+            return _jsContext;
+        }
+
+        var connection = await EnsureConnectionAsync(cancellationToken).ConfigureAwait(false);
+
+        await _connectionGate.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            _jsContext ??= new NatsJSContext(connection);
+            return _jsContext;
+        }
+        finally
+        {
+            _connectionGate.Release();
+        }
+    }
+
+    private async ValueTask<INatsJSConsumer> EnsureStreamAndConsumerAsync(
+        NatsJSContext js,
+        CancellationToken cancellationToken)
+    {
+        if (_consumer is not null)
+        {
+            return _consumer;
+        }
+
+        await _connectionGate.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            if (_consumer is not null)
+            {
+                return _consumer;
+            }
+
+            await EnsureStreamAsync(js, cancellationToken).ConfigureAwait(false);
+            await EnsureDeadLetterStreamAsync(js, cancellationToken).ConfigureAwait(false);
+
+            var consumerConfig = new ConsumerConfig
+            {
+                DurableName = _options.DurableConsumer,
+                AckPolicy = ConsumerConfigAckPolicy.Explicit,
+                ReplayPolicy = ConsumerConfigReplayPolicy.Instant,
+                DeliverPolicy = ConsumerConfigDeliverPolicy.All,
+                AckWait = ToNanoseconds(_options.AckWait),
+                MaxAckPending = _options.MaxInFlight,
+                MaxDeliver = Math.Max(1, _queueOptions.MaxDeliveryAttempts),
+                FilterSubjects = new[] { _options.Subject }
+            };
+
+            try
+            {
+                _consumer = await js.CreateConsumerAsync(
+                        _options.Stream,
+                        consumerConfig,
+                        cancellationToken)
+                    .ConfigureAwait(false);
+            }
+            catch (NatsJSApiException apiEx)
+            {
+                _logger.LogDebug(apiEx,
+                    "CreateConsumerAsync failed with code {Code}; attempting to fetch existing durable consumer {Durable}.",
+                    apiEx.Error?.Code,
+                    _options.DurableConsumer);
+
+                _consumer = await js.GetConsumerAsync(
+                        _options.Stream,
+                        _options.DurableConsumer,
+                        cancellationToken)
+                    .ConfigureAwait(false);
+            }
+
+            return _consumer;
+        }
+        finally
+        {
+            _connectionGate.Release();
+        }
+    }
+
+    private async Task<NatsConnection> EnsureConnectionAsync(CancellationToken cancellationToken)
+    {
+        if (_connection is not null)
+        {
+            return _connection;
+        }
+
+        await _connectionGate.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            if (_connection is not null)
+            {
+                return _connection;
+            }
+
+            var opts = new NatsOpts
+            {
+                Url = _options.Url!,
+                Name = "stellaops-scanner-queue",
+                CommandTimeout = TimeSpan.FromSeconds(10),
+                RequestTimeout = TimeSpan.FromSeconds(20),
+                PingInterval = TimeSpan.FromSeconds(30)
+            };
+
+            _connection = await _connectionFactory(opts, cancellationToken).ConfigureAwait(false);
+            await _connection.ConnectAsync().ConfigureAwait(false);
+            return _connection;
+        }
+        finally
+        {
+            _connectionGate.Release();
+        }
+    }
+
+    private async Task EnsureStreamAsync(NatsJSContext js, CancellationToken cancellationToken)
+    {
+        try
+        {
+            await js.GetStreamAsync(
+                    _options.Stream,
+                    new StreamInfoRequest(),
+                    cancellationToken)
+                .ConfigureAwait(false);
+        }
+        catch (NatsJSApiException)
+        {
+            var config = new StreamConfig(
+                name: _options.Stream,
+                subjects: new[] { _options.Subject })
+            {
+                Retention = StreamConfigRetention.Workqueue,
+                Storage = StreamConfigStorage.File,
+                MaxConsumers = -1,
+                MaxMsgs = -1,
+                MaxBytes = -1,
+                MaxAge = 0
+            };
+
+            await js.CreateStreamAsync(config, cancellationToken).ConfigureAwait(false);
+            _logger.LogInformation("Created NATS JetStream stream {Stream} ({Subject}).", _options.Stream, _options.Subject);
+        }
+    }
+
+    private async Task EnsureDeadLetterStreamAsync(NatsJSContext js, CancellationToken cancellationToken)
+    {
+        try
+        {
+            await js.GetStreamAsync(
+                    _options.DeadLetterStream,
+                    new StreamInfoRequest(),
+                    cancellationToken)
+                .ConfigureAwait(false);
+        }
+        catch (NatsJSApiException)
+        {
+            var config = new StreamConfig(
+                name: _options.DeadLetterStream,
+                subjects: new[] { _options.DeadLetterSubject })
+            {
+                Retention = StreamConfigRetention.Workqueue,
+                Storage = StreamConfigStorage.File,
+                MaxConsumers = -1,
+                MaxMsgs = -1,
+                MaxBytes = -1,
+                MaxAge = ToNanoseconds(_queueOptions.DeadLetter.Retention)
+            };
+
+            await js.CreateStreamAsync(config, cancellationToken).ConfigureAwait(false);
+            _logger.LogInformation("Created NATS dead-letter stream {Stream} ({Subject}).", _options.DeadLetterStream, _options.DeadLetterSubject);
+        }
+    }
+
+    internal async ValueTask PingAsync(CancellationToken cancellationToken)
+    {
+        var connection = await EnsureConnectionAsync(cancellationToken).ConfigureAwait(false);
+        await connection.PingAsync(cancellationToken).ConfigureAwait(false);
+    }
+
+    private NatsScanQueueLease? CreateLease(
+        NatsJSMsg<byte[]> message,
+        string consumer,
+        DateTimeOffset now,
+        TimeSpan leaseDuration)
+    {
+        var headers = message.Headers;
+        if (headers is null)
+        {
+            return null;
+        }
+
+        if (!headers.TryGetValue(QueueEnvelopeFields.JobId, out var jobIdValues) || jobIdValues.Count == 0)
+        {
+            return null;
+        }
+
+        var jobId = jobIdValues[0]!;
+        var idempotencyKey = headers.TryGetValue(QueueEnvelopeFields.IdempotencyKey, out var idemValues) && idemValues.Count > 0
+            ? idemValues[0]
+            : null;
+
+        var traceId = headers.TryGetValue(QueueEnvelopeFields.TraceId, out var traceValues) && traceValues.Count > 0
+            ? string.IsNullOrWhiteSpace(traceValues[0]) ? null : traceValues[0]
+            : null;
+
+        var enqueuedAt = headers.TryGetValue(QueueEnvelopeFields.EnqueuedAt, out var enqueuedValues) && enqueuedValues.Count > 0
+            && long.TryParse(enqueuedValues[0], out var unix)
+            ? DateTimeOffset.FromUnixTimeMilliseconds(unix)
+            : now;
+
+        var attempt = headers.TryGetValue(QueueEnvelopeFields.Attempt, out var attemptValues) && attemptValues.Count > 0
+            && int.TryParse(attemptValues[0], out var parsedAttempt)
+            ? parsedAttempt
+            : 1;
+
+        if (message.Metadata?.NumDelivered is ulong delivered && delivered > 0)
+        {
+            var deliveredInt = delivered > int.MaxValue ? int.MaxValue : (int)delivered;
+            if (deliveredInt > attempt)
+            {
+                attempt = deliveredInt;
+            }
+        }
+
+        var leaseExpires = now.Add(leaseDuration);
+        var attributes = ExtractAttributes(headers);
+
+        var messageId = message.Metadata?.Sequence.Stream.ToString() ?? Guid.NewGuid().ToString("n");
+        return new NatsScanQueueLease(
+            this,
+            message,
+            messageId,
+            jobId,
+            message.Data ?? Array.Empty<byte>(),
+            attempt,
+            enqueuedAt,
+            leaseExpires,
+            consumer,
+            idempotencyKey,
+            traceId,
+            attributes);
+    }
+
+    private static IReadOnlyDictionary<string, string> ExtractAttributes(NatsHeaders headers)
+    {
+        var attributes = new Dictionary<string, string>(StringComparer.Ordinal);
+
+        foreach (var key in headers.Keys)
+        {
+            if (!key.StartsWith(QueueEnvelopeFields.AttributePrefix, StringComparison.Ordinal))
+            {
+                continue;
+            }
+
+            if (headers.TryGetValue(key, out var values) && values.Count > 0)
+            {
+                attributes[key[QueueEnvelopeFields.AttributePrefix.Length..]] = values[0]!;
+            }
+        }
+
+        return attributes.Count == 0
+            ? EmptyReadOnlyDictionary<string, string>.Instance
+            : new ReadOnlyDictionary<string, string>(attributes);
+    }
+
+    private NatsHeaders BuildHeaders(ScanQueueMessage message, string idempotencyKey)
+    {
+        var headers = new NatsHeaders
+        {
+            { QueueEnvelopeFields.JobId, message.JobId },
+            { QueueEnvelopeFields.IdempotencyKey, idempotencyKey },
+            { QueueEnvelopeFields.Attempt, "1" },
+            { QueueEnvelopeFields.EnqueuedAt, _timeProvider.GetUtcNow().ToUnixTimeMilliseconds().ToString() }
+        };
+
+        if (!string.IsNullOrEmpty(message.TraceId))
+        {
+            headers.Add(QueueEnvelopeFields.TraceId, message.TraceId!);
+        }
+
+        if (message.Attributes is not null)
+        {
+            foreach (var kvp in message.Attributes)
+            {
+                headers.Add(QueueEnvelopeFields.AttributePrefix + kvp.Key, kvp.Value);
+            }
+        }
+
+        return headers;
+    }
+
+    private NatsHeaders BuildDeadLetterHeaders(NatsScanQueueLease lease, string reason)
+    {
+        var headers = new NatsHeaders
+        {
+            { QueueEnvelopeFields.JobId, lease.JobId },
+            { QueueEnvelopeFields.IdempotencyKey, lease.IdempotencyKey ?? lease.JobId },
+            { QueueEnvelopeFields.Attempt, lease.Attempt.ToString() },
+            { QueueEnvelopeFields.EnqueuedAt, lease.EnqueuedAt.ToUnixTimeMilliseconds().ToString() },
+            { "deadletter-reason", reason }
+        };
+
+        if (!string.IsNullOrWhiteSpace(lease.TraceId))
+        {
+            headers.Add(QueueEnvelopeFields.TraceId, lease.TraceId!);
+        }
+
+        foreach (var kvp in lease.Attributes)
+        {
+            headers.Add(QueueEnvelopeFields.AttributePrefix + kvp.Key, kvp.Value);
+        }
+
+        return headers;
+    }
+
+    private TimeSpan CalculateBackoff(int attempt)
+    {
+        var configuredInitial = _options.RetryDelay > TimeSpan.Zero
+            ? _options.RetryDelay
+            : _queueOptions.RetryInitialBackoff;
+
+        if (configuredInitial <= TimeSpan.Zero)
+        {
+            return TimeSpan.Zero;
+        }
+
+        if (attempt <= 1)
+        {
+            return configuredInitial;
+        }
+
+        var max = _queueOptions.RetryMaxBackoff > TimeSpan.Zero
+            ? _queueOptions.RetryMaxBackoff
+            : configuredInitial;
+
+        var exponent = attempt - 1;
+        var scaledTicks = configuredInitial.Ticks * Math.Pow(2, exponent - 1);
+        var cappedTicks = Math.Min(max.Ticks, scaledTicks);
+        var resultTicks = Math.Max(configuredInitial.Ticks, (long)cappedTicks);
+        return TimeSpan.FromTicks(resultTicks);
+    }
+
+    private static long ToNanoseconds(TimeSpan timeSpan)
+        => timeSpan <= TimeSpan.Zero ? 0 : timeSpan.Ticks * 100L;
+
+    private static class EmptyReadOnlyDictionary<TKey, TValue>
+        where TKey : notnull
+    {
+        public static readonly IReadOnlyDictionary<TKey, TValue> Instance =
+            new ReadOnlyDictionary<TKey, TValue>(new Dictionary<TKey, TValue>(0, EqualityComparer<TKey>.Default));
+    }
+}

src/StellaOps.Scanner.Queue/Nats/NatsScanQueueLease.cs

@@ -0,0 +1,82 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using NATS.Client.JetStream;
+
+namespace StellaOps.Scanner.Queue.Nats;
+
+internal sealed class NatsScanQueueLease : IScanQueueLease
+{
+    private readonly NatsScanQueue _queue;
+    private readonly NatsJSMsg<byte[]> _message;
+    private int _completed;
+
+    internal NatsScanQueueLease(
+        NatsScanQueue queue,
+        NatsJSMsg<byte[]> message,
+        string messageId,
+        string jobId,
+        byte[] payload,
+        int attempt,
+        DateTimeOffset enqueuedAt,
+        DateTimeOffset leaseExpiresAt,
+        string consumer,
+        string? idempotencyKey,
+        string? traceId,
+        IReadOnlyDictionary<string, string> attributes)
+    {
+        _queue = queue;
+        _message = message;
+        MessageId = messageId;
+        JobId = jobId;
+        Payload = payload;
+        Attempt = attempt;
+        EnqueuedAt = enqueuedAt;
+        LeaseExpiresAt = leaseExpiresAt;
+        Consumer = consumer;
+        IdempotencyKey = idempotencyKey;
+        TraceId = traceId;
+        Attributes = attributes;
+    }
+
+    public string MessageId { get; }
+
+    public string JobId { get; }
+
+    public ReadOnlyMemory<byte> Payload { get; }
+
+    public int Attempt { get; internal set; }
+
+    public DateTimeOffset EnqueuedAt { get; }
+
+    public DateTimeOffset LeaseExpiresAt { get; private set; }
+
+    public string Consumer { get; }
+
+    public string? IdempotencyKey { get; }
+
+    public string? TraceId { get; }
+
+    public IReadOnlyDictionary<string, string> Attributes { get; }
+
+    internal NatsJSMsg<byte[]> Message => _message;
+
+    public Task AcknowledgeAsync(CancellationToken cancellationToken = default)
+        => _queue.AcknowledgeAsync(this, cancellationToken);
+
+    public Task RenewAsync(TimeSpan leaseDuration, CancellationToken cancellationToken = default)
+        => _queue.RenewLeaseAsync(this, leaseDuration, cancellationToken);
+
+    public Task ReleaseAsync(QueueReleaseDisposition disposition, CancellationToken cancellationToken = default)
+        => _queue.ReleaseAsync(this, disposition, cancellationToken);
+
+    public Task DeadLetterAsync(string reason, CancellationToken cancellationToken = default)
+        => _queue.DeadLetterAsync(this, reason, cancellationToken);
+
+    internal bool TryBeginCompletion()
+        => Interlocked.CompareExchange(ref _completed, 1, 0) == 0;
+
+    internal void RefreshLease(DateTimeOffset expiresAt)
+        => LeaseExpiresAt = expiresAt;
+}

src/StellaOps.Scanner.Queue/QueueEnvelopeFields.cs

@@ -0,0 +1,12 @@
+namespace StellaOps.Scanner.Queue;
+
+internal static class QueueEnvelopeFields
+{
+    public const string Payload = "payload";
+    public const string JobId = "jobId";
+    public const string IdempotencyKey = "idempotency";
+    public const string Attempt = "attempt";
+    public const string EnqueuedAt = "enqueuedAt";
+    public const string TraceId = "traceId";
+    public const string AttributePrefix = "attr:";
+}

src/StellaOps.Scanner.Queue/QueueMetrics.cs

@@ -0,0 +1,28 @@
+using System.Diagnostics.Metrics;
+
+namespace StellaOps.Scanner.Queue;
+
+internal static class QueueMetrics
+{
+    private const string TransportTagName = "transport";
+
+    private static readonly Meter Meter = new("StellaOps.Scanner.Queue");
+    private static readonly Counter<long> EnqueuedCounter = Meter.CreateCounter<long>("scanner_queue_enqueued_total");
+    private static readonly Counter<long> DeduplicatedCounter = Meter.CreateCounter<long>("scanner_queue_deduplicated_total");
+    private static readonly Counter<long> AckCounter = Meter.CreateCounter<long>("scanner_queue_ack_total");
+    private static readonly Counter<long> RetryCounter = Meter.CreateCounter<long>("scanner_queue_retry_total");
+    private static readonly Counter<long> DeadLetterCounter = Meter.CreateCounter<long>("scanner_queue_deadletter_total");
+
+    public static void RecordEnqueued(string transport) => EnqueuedCounter.Add(1, BuildTags(transport));
+
+    public static void RecordDeduplicated(string transport) => DeduplicatedCounter.Add(1, BuildTags(transport));
+
+    public static void RecordAck(string transport) => AckCounter.Add(1, BuildTags(transport));
+
+    public static void RecordRetry(string transport) => RetryCounter.Add(1, BuildTags(transport));
+
+    public static void RecordDeadLetter(string transport) => DeadLetterCounter.Add(1, BuildTags(transport));
+
+    private static KeyValuePair<string, object?>[] BuildTags(string transport)
+        => new[] { new KeyValuePair<string, object?>(TransportTagName, transport) };
+}

src/StellaOps.Scanner.Queue/QueueTransportKind.cs

@@ -0,0 +1,7 @@
+namespace StellaOps.Scanner.Queue;
+
+public enum QueueTransportKind
+{
+    Redis,
+    Nats
+}

src/StellaOps.Scanner.Queue/Redis/RedisScanQueue.cs

@@ -0,0 +1,766 @@
+using System;
+using System.Buffers;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.Logging;
+using StackExchange.Redis;
+
+namespace StellaOps.Scanner.Queue.Redis;
+
+internal sealed class RedisScanQueue : IScanQueue, IAsyncDisposable
+{
+    private const string TransportName = "redis";
+
+    private readonly ScannerQueueOptions _queueOptions;
+    private readonly RedisQueueOptions _options;
+    private readonly ILogger<RedisScanQueue> _logger;
+    private readonly TimeProvider _timeProvider;
+    private readonly SemaphoreSlim _connectionLock = new(1, 1);
+    private readonly SemaphoreSlim _groupInitLock = new(1, 1);
+    private readonly Func<ConfigurationOptions, Task<IConnectionMultiplexer>> _connectionFactory;
+    private IConnectionMultiplexer? _connection;
+    private volatile bool _groupInitialized;
+    private bool _disposed;
+
+    private string BuildIdempotencyKey(string key)
+        => string.Concat(_options.IdempotencyKeyPrefix, key);
+
+    public RedisScanQueue(
+        ScannerQueueOptions queueOptions,
+        RedisQueueOptions options,
+        ILogger<RedisScanQueue> logger,
+        TimeProvider timeProvider,
+        Func<ConfigurationOptions, Task<IConnectionMultiplexer>>? connectionFactory = null)
+    {
+        _queueOptions = queueOptions ?? throw new ArgumentNullException(nameof(queueOptions));
+        _options = options ?? throw new ArgumentNullException(nameof(options));
+        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+        _timeProvider = timeProvider ?? TimeProvider.System;
+        _connectionFactory = connectionFactory ?? (config => Task.FromResult<IConnectionMultiplexer>(ConnectionMultiplexer.Connect(config)));
+
+        if (string.IsNullOrWhiteSpace(_options.ConnectionString))
+        {
+            throw new InvalidOperationException("Redis connection string must be configured for the scanner queue.");
+        }
+    }
+
+    public async ValueTask<QueueEnqueueResult> EnqueueAsync(
+        ScanQueueMessage message,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(message);
+        cancellationToken.ThrowIfCancellationRequested();
+
+        var db = await GetDatabaseAsync(cancellationToken).ConfigureAwait(false);
+        var now = _timeProvider.GetUtcNow();
+        await EnsureConsumerGroupAsync(db, cancellationToken).ConfigureAwait(false);
+
+        var attempt = 1;
+        var entries = BuildEntries(message, now, attempt);
+        var messageId = await AddToStreamAsync(
+                db,
+                _options.StreamName,
+                entries,
+                _options.ApproximateMaxLength,
+                _options.ApproximateMaxLength is not null)
+            .ConfigureAwait(false);
+
+        var idempotencyToken = message.IdempotencyKey ?? message.JobId;
+        var idempotencyKey = BuildIdempotencyKey(idempotencyToken);
+
+        var stored = await db.StringSetAsync(
+                key: idempotencyKey,
+                value: messageId,
+                when: When.NotExists,
+                expiry: _options.IdempotencyWindow)
+            .ConfigureAwait(false);
+
+        if (!stored)
+        {
+            // Duplicate enqueue – delete the freshly added entry and surface cached ID.
+            await db.StreamDeleteAsync(
+                    _options.StreamName,
+                    new RedisValue[] { messageId })
+                .ConfigureAwait(false);
+
+            var existing = await db.StringGetAsync(idempotencyKey).ConfigureAwait(false);
+            var duplicateId = existing.IsNullOrEmpty ? messageId : existing;
+
+            _logger.LogDebug(
+                "Duplicate queue enqueue detected for job {JobId} (token {Token}), returning existing stream id {StreamId}.",
+                message.JobId,
+                idempotencyToken,
+                duplicateId.ToString());
+
+            QueueMetrics.RecordDeduplicated(TransportName);
+            return new QueueEnqueueResult(duplicateId.ToString()!, true);
+        }
+
+        _logger.LogDebug(
+            "Enqueued job {JobId} into stream {Stream} with id {StreamId}.",
+            message.JobId,
+            _options.StreamName,
+            messageId.ToString());
+
+        QueueMetrics.RecordEnqueued(TransportName);
+        return new QueueEnqueueResult(messageId.ToString()!, false);
+    }
+
+    public async ValueTask<IReadOnlyList<IScanQueueLease>> LeaseAsync(
+        QueueLeaseRequest request,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(request);
+        cancellationToken.ThrowIfCancellationRequested();
+
+        var db = await GetDatabaseAsync(cancellationToken).ConfigureAwait(false);
+        await EnsureConsumerGroupAsync(db, cancellationToken).ConfigureAwait(false);
+
+        var entries = await db.StreamReadGroupAsync(
+                _options.StreamName,
+                _options.ConsumerGroup,
+                request.Consumer,
+                position: ">",
+                count: request.BatchSize,
+                flags: CommandFlags.None)
+            .ConfigureAwait(false);
+
+        if (entries is null || entries.Length == 0)
+        {
+            return Array.Empty<IScanQueueLease>();
+        }
+
+        var now = _timeProvider.GetUtcNow();
+        var leases = new List<IScanQueueLease>(entries.Length);
+
+        foreach (var entry in entries)
+        {
+            var lease = TryMapLease(
+                entry,
+                request.Consumer,
+                now,
+                request.LeaseDuration,
+                default);
+
+            if (lease is null)
+            {
+                _logger.LogWarning(
+                    "Stream entry {StreamId} is missing required metadata; acknowledging to avoid poison message.",
+                    entry.Id.ToString());
+                await db.StreamAcknowledgeAsync(
+                        _options.StreamName,
+                        _options.ConsumerGroup,
+                        new RedisValue[] { entry.Id })
+                    .ConfigureAwait(false);
+                await db.StreamDeleteAsync(
+                        _options.StreamName,
+                        new RedisValue[] { entry.Id })
+                    .ConfigureAwait(false);
+                continue;
+            }
+
+            leases.Add(lease);
+        }
+
+        return leases;
+    }
+
+    public async ValueTask<IReadOnlyList<IScanQueueLease>> ClaimExpiredLeasesAsync(
+        QueueClaimOptions options,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(options);
+        cancellationToken.ThrowIfCancellationRequested();
+
+        var db = await GetDatabaseAsync(cancellationToken).ConfigureAwait(false);
+        await EnsureConsumerGroupAsync(db, cancellationToken).ConfigureAwait(false);
+
+        var pending = await db.StreamPendingMessagesAsync(
+                _options.StreamName,
+                _options.ConsumerGroup,
+                options.BatchSize,
+                RedisValue.Null,
+                (long)options.MinIdleTime.TotalMilliseconds)
+            .ConfigureAwait(false);
+
+        if (pending is null || pending.Length == 0)
+        {
+            return Array.Empty<IScanQueueLease>();
+        }
+
+        var eligible = pending
+            .Where(p => p.IdleTimeInMilliseconds >= options.MinIdleTime.TotalMilliseconds)
+            .ToArray();
+
+        if (eligible.Length == 0)
+        {
+            return Array.Empty<IScanQueueLease>();
+        }
+
+        var messageIds = eligible
+            .Select(static p => (RedisValue)p.MessageId)
+            .ToArray();
+
+        var entries = await db.StreamClaimAsync(
+                _options.StreamName,
+                _options.ConsumerGroup,
+                options.ClaimantConsumer,
+                0,
+                messageIds,
+                CommandFlags.None)
+            .ConfigureAwait(false);
+
+        if (entries is null || entries.Length == 0)
+        {
+            return Array.Empty<IScanQueueLease>();
+        }
+
+        var now = _timeProvider.GetUtcNow();
+        var pendingById = Enumerable.ToDictionary<StreamPendingMessageInfo, string, StreamPendingMessageInfo>(
+            eligible,
+            static p => p.MessageId.IsNullOrEmpty ? string.Empty : p.MessageId.ToString(),
+            static p => p,
+            StringComparer.Ordinal);
+
+        var leases = new List<IScanQueueLease>(entries.Length);
+        foreach (var entry in entries)
+        {
+            var entryIdValue = entry.Id;
+            var entryId = entryIdValue.IsNullOrEmpty ? string.Empty : entryIdValue.ToString();
+            var hasPending = pendingById.TryGetValue(entryId, out var pendingInfo);
+            var attempt = hasPending
+                ? (int)Math.Max(1, pendingInfo.DeliveryCount)
+                : 1;
+
+            var lease = TryMapLease(
+                entry,
+                options.ClaimantConsumer,
+                now,
+                _queueOptions.DefaultLeaseDuration,
+                attempt);
+
+            if (lease is null)
+            {
+                _logger.LogWarning(
+                    "Unable to map claimed stream entry {StreamId}; acknowledging to unblock queue.",
+                    entry.Id.ToString());
+                await db.StreamAcknowledgeAsync(
+                        _options.StreamName,
+                        _options.ConsumerGroup,
+                        new RedisValue[] { entry.Id })
+                    .ConfigureAwait(false);
+                await db.StreamDeleteAsync(
+                        _options.StreamName,
+                        new RedisValue[] { entry.Id })
+                    .ConfigureAwait(false);
+                continue;
+            }
+
+            leases.Add(lease);
+        }
+
+        return leases;
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        if (_disposed)
+        {
+            return;
+        }
+
+        _disposed = true;
+        if (_connection is not null)
+        {
+            await _connection.CloseAsync();
+            _connection.Dispose();
+        }
+
+        _connectionLock.Dispose();
+        _groupInitLock.Dispose();
+        GC.SuppressFinalize(this);
+    }
+
+    internal async Task AcknowledgeAsync(
+        RedisScanQueueLease lease,
+        CancellationToken cancellationToken)
+    {
+        if (!lease.TryBeginCompletion())
+        {
+            return;
+        }
+
+        var db = await GetDatabaseAsync(cancellationToken).ConfigureAwait(false);
+
+        await db.StreamAcknowledgeAsync(
+                _options.StreamName,
+                _options.ConsumerGroup,
+                new RedisValue[] { lease.MessageId })
+            .ConfigureAwait(false);
+
+        await db.StreamDeleteAsync(
+                _options.StreamName,
+                new RedisValue[] { lease.MessageId })
+            .ConfigureAwait(false);
+
+        _logger.LogDebug(
+            "Acknowledged job {JobId} ({MessageId}) on consumer {Consumer}.",
+            lease.JobId,
+            lease.MessageId,
+            lease.Consumer);
+
+        QueueMetrics.RecordAck(TransportName);
+    }
+
+    internal async Task RenewLeaseAsync(
+        RedisScanQueueLease lease,
+        TimeSpan leaseDuration,
+        CancellationToken cancellationToken)
+    {
+        var db = await GetDatabaseAsync(cancellationToken).ConfigureAwait(false);
+
+        await db.StreamClaimAsync(
+                _options.StreamName,
+                _options.ConsumerGroup,
+                lease.Consumer,
+                0,
+                new RedisValue[] { lease.MessageId },
+                CommandFlags.None)
+            .ConfigureAwait(false);
+
+        var expires = _timeProvider.GetUtcNow().Add(leaseDuration);
+        lease.RefreshLease(expires);
+
+        _logger.LogDebug(
+            "Renewed lease for job {JobId} until {LeaseExpiry:u}.",
+            lease.JobId,
+            expires);
+    }
+
+    internal async Task ReleaseAsync(
+        RedisScanQueueLease lease,
+        QueueReleaseDisposition disposition,
+        CancellationToken cancellationToken)
+    {
+        if (disposition == QueueReleaseDisposition.Retry
+            && lease.Attempt >= _queueOptions.MaxDeliveryAttempts)
+        {
+            _logger.LogWarning(
+                "Job {JobId} reached max delivery attempts ({Attempts}); moving to dead-letter.",
+                lease.JobId,
+                lease.Attempt);
+
+            await DeadLetterAsync(
+                lease,
+                $"max-delivery-attempts:{lease.Attempt}",
+                cancellationToken).ConfigureAwait(false);
+            return;
+        }
+
+        if (!lease.TryBeginCompletion())
+        {
+            return;
+        }
+
+        var db = await GetDatabaseAsync(cancellationToken).ConfigureAwait(false);
+        await db.StreamAcknowledgeAsync(
+                _options.StreamName,
+                _options.ConsumerGroup,
+                new RedisValue[] { lease.MessageId })
+            .ConfigureAwait(false);
+        await db.StreamDeleteAsync(
+                _options.StreamName,
+                new RedisValue[] { lease.MessageId })
+            .ConfigureAwait(false);
+
+        QueueMetrics.RecordAck(TransportName);
+
+        if (disposition == QueueReleaseDisposition.Retry)
+        {
+            QueueMetrics.RecordRetry(TransportName);
+
+            var delay = CalculateBackoff(lease.Attempt);
+            if (delay > TimeSpan.Zero)
+            {
+                _logger.LogDebug(
+                    "Delaying retry for job {JobId} by {Delay} (attempt {Attempt}).",
+                    lease.JobId,
+                    delay,
+                    lease.Attempt);
+
+                try
+                {
+                    await Task.Delay(delay, cancellationToken).ConfigureAwait(false);
+                }
+                catch (TaskCanceledException)
+                {
+                    return;
+                }
+            }
+
+            var requeueMessage = new ScanQueueMessage(lease.JobId, lease.Payload)
+            {
+                IdempotencyKey = lease.IdempotencyKey,
+                Attributes = lease.Attributes,
+                TraceId = lease.TraceId
+            };
+
+            var now = _timeProvider.GetUtcNow();
+            var entries = BuildEntries(requeueMessage, now, lease.Attempt + 1);
+
+            await AddToStreamAsync(
+                    db,
+                    _options.StreamName,
+                    entries,
+                    _options.ApproximateMaxLength,
+                    _options.ApproximateMaxLength is not null)
+                .ConfigureAwait(false);
+
+            QueueMetrics.RecordEnqueued(TransportName);
+            _logger.LogWarning(
+                "Released job {JobId} for retry (attempt {Attempt}).",
+                lease.JobId,
+                lease.Attempt + 1);
+        }
+        else
+        {
+            _logger.LogInformation(
+                "Abandoned job {JobId} after {Attempt} attempt(s).",
+                lease.JobId,
+                lease.Attempt);
+        }
+    }
+
+    internal async Task DeadLetterAsync(
+        RedisScanQueueLease lease,
+        string reason,
+        CancellationToken cancellationToken)
+    {
+        if (!lease.TryBeginCompletion())
+        {
+            return;
+        }
+
+        var db = await GetDatabaseAsync(cancellationToken).ConfigureAwait(false);
+
+        await db.StreamAcknowledgeAsync(
+                _options.StreamName,
+                _options.ConsumerGroup,
+                new RedisValue[] { lease.MessageId })
+            .ConfigureAwait(false);
+
+        await db.StreamDeleteAsync(
+                _options.StreamName,
+                new RedisValue[] { lease.MessageId })
+            .ConfigureAwait(false);
+
+        var now = _timeProvider.GetUtcNow();
+        var entries = BuildEntries(
+            new ScanQueueMessage(lease.JobId, lease.Payload)
+            {
+                IdempotencyKey = lease.IdempotencyKey,
+                Attributes = lease.Attributes,
+                TraceId = lease.TraceId
+            },
+            now,
+            lease.Attempt);
+
+        await AddToStreamAsync(
+                db,
+                _queueOptions.DeadLetter.StreamName,
+                entries,
+                null,
+                false)
+            .ConfigureAwait(false);
+
+        _logger.LogError(
+            "Dead-lettered job {JobId} (attempt {Attempt}): {Reason}",
+            lease.JobId,
+            lease.Attempt,
+            reason);
+
+        QueueMetrics.RecordDeadLetter(TransportName);
+    }
+
+    private async ValueTask<IDatabase> GetDatabaseAsync(CancellationToken cancellationToken)
+    {
+        if (_connection is not null)
+        {
+            return _connection.GetDatabase(_options.Database ?? -1);
+        }
+
+        await _connectionLock.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            if (_connection is null)
+            {
+                var config = ConfigurationOptions.Parse(_options.ConnectionString!);
+                config.AbortOnConnectFail = false;
+                config.ConnectTimeout = (int)_options.InitializationTimeout.TotalMilliseconds;
+                config.ConnectRetry = 3;
+                if (_options.Database is not null)
+                {
+                    config.DefaultDatabase = _options.Database;
+                }
+
+                _connection = await _connectionFactory(config).ConfigureAwait(false);
+            }
+
+            return _connection.GetDatabase(_options.Database ?? -1);
+        }
+        finally
+        {
+            _connectionLock.Release();
+        }
+    }
+
+    private async Task EnsureConsumerGroupAsync(
+        IDatabase database,
+        CancellationToken cancellationToken)
+    {
+        if (_groupInitialized)
+        {
+            return;
+        }
+
+        await _groupInitLock.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            if (_groupInitialized)
+            {
+                return;
+            }
+
+            try
+            {
+                await database.StreamCreateConsumerGroupAsync(
+                        _options.StreamName,
+                        _options.ConsumerGroup,
+                        StreamPosition.Beginning,
+                        createStream: true)
+                    .ConfigureAwait(false);
+            }
+            catch (RedisServerException ex) when (ex.Message.Contains("BUSYGROUP", StringComparison.OrdinalIgnoreCase))
+            {
+                // Already exists.
+            }
+
+            _groupInitialized = true;
+        }
+        finally
+        {
+            _groupInitLock.Release();
+        }
+    }
+
+    private NameValueEntry[] BuildEntries(
+        ScanQueueMessage message,
+        DateTimeOffset enqueuedAt,
+        int attempt)
+    {
+        var attributeCount = message.Attributes?.Count ?? 0;
+        var entries = ArrayPool<NameValueEntry>.Shared.Rent(6 + attributeCount);
+        var index = 0;
+
+        entries[index++] = new NameValueEntry(QueueEnvelopeFields.JobId, message.JobId);
+        entries[index++] = new NameValueEntry(QueueEnvelopeFields.Attempt, attempt);
+        entries[index++] = new NameValueEntry(QueueEnvelopeFields.EnqueuedAt, enqueuedAt.ToUnixTimeMilliseconds());
+        entries[index++] = new NameValueEntry(
+            QueueEnvelopeFields.IdempotencyKey,
+            message.IdempotencyKey ?? message.JobId);
+        entries[index++] = new NameValueEntry(
+            QueueEnvelopeFields.Payload,
+            message.Payload.ToArray());
+        entries[index++] = new NameValueEntry(
+            QueueEnvelopeFields.TraceId,
+            message.TraceId ?? string.Empty);
+
+        if (attributeCount > 0)
+        {
+            foreach (var kvp in message.Attributes!)
+            {
+                entries[index++] = new NameValueEntry(
+                    QueueEnvelopeFields.AttributePrefix + kvp.Key,
+                    kvp.Value);
+            }
+        }
+
+        var result = entries.AsSpan(0, index).ToArray();
+        ArrayPool<NameValueEntry>.Shared.Return(entries, clearArray: true);
+        return result;
+    }
+
+    private RedisScanQueueLease? TryMapLease(
+        StreamEntry entry,
+        string consumer,
+        DateTimeOffset now,
+        TimeSpan leaseDuration,
+        int? attemptOverride)
+    {
+        if (entry.Values is null || entry.Values.Length == 0)
+        {
+            return null;
+        }
+
+        string? jobId = null;
+        string? idempotency = null;
+        long? enqueuedAtUnix = null;
+        byte[]? payload = null;
+        string? traceId = null;
+        var attributes = new Dictionary<string, string>(StringComparer.Ordinal);
+        var attempt = attemptOverride ?? 1;
+
+        foreach (var field in entry.Values)
+        {
+            var name = field.Name.ToString();
+            if (name.Equals(QueueEnvelopeFields.JobId, StringComparison.Ordinal))
+            {
+                jobId = field.Value.ToString();
+            }
+            else if (name.Equals(QueueEnvelopeFields.IdempotencyKey, StringComparison.Ordinal))
+            {
+                idempotency = field.Value.ToString();
+            }
+            else if (name.Equals(QueueEnvelopeFields.EnqueuedAt, StringComparison.Ordinal))
+            {
+                if (long.TryParse(field.Value.ToString(), out var unix))
+                {
+                    enqueuedAtUnix = unix;
+                }
+            }
+            else if (name.Equals(QueueEnvelopeFields.Payload, StringComparison.Ordinal))
+            {
+                payload = (byte[]?)field.Value ?? Array.Empty<byte>();
+            }
+            else if (name.Equals(QueueEnvelopeFields.Attempt, StringComparison.Ordinal))
+            {
+                if (int.TryParse(field.Value.ToString(), out var parsedAttempt))
+                {
+                    attempt = Math.Max(parsedAttempt, attempt);
+                }
+            }
+            else if (name.Equals(QueueEnvelopeFields.TraceId, StringComparison.Ordinal))
+            {
+                var value = field.Value.ToString();
+                traceId = string.IsNullOrWhiteSpace(value) ? null : value;
+            }
+            else if (name.StartsWith(QueueEnvelopeFields.AttributePrefix, StringComparison.Ordinal))
+            {
+                attributes[name[QueueEnvelopeFields.AttributePrefix.Length..]] = field.Value.ToString();
+            }
+        }
+
+        if (jobId is null || payload is null || enqueuedAtUnix is null)
+        {
+            return null;
+        }
+
+        var enqueuedAt = DateTimeOffset.FromUnixTimeMilliseconds(enqueuedAtUnix.Value);
+        var leaseExpires = now.Add(leaseDuration);
+
+        var attributeView = attributes.Count == 0
+            ? EmptyReadOnlyDictionary<string, string>.Instance
+            : new ReadOnlyDictionary<string, string>(attributes);
+
+        return new RedisScanQueueLease(
+            this,
+            entry.Id.ToString(),
+            jobId,
+            payload,
+            attempt,
+            enqueuedAt,
+            leaseExpires,
+            consumer,
+            idempotency,
+            traceId,
+            attributeView);
+    }
+
+    private TimeSpan CalculateBackoff(int attempt)
+    {
+        var configuredInitial = _options.RetryInitialBackoff > TimeSpan.Zero
+            ? _options.RetryInitialBackoff
+            : _queueOptions.RetryInitialBackoff;
+
+        var initial = configuredInitial > TimeSpan.Zero
+            ? configuredInitial
+            : TimeSpan.Zero;
+
+        if (initial <= TimeSpan.Zero)
+        {
+            return TimeSpan.Zero;
+        }
+
+        if (attempt <= 1)
+        {
+            return initial;
+        }
+
+        var configuredMax = _queueOptions.RetryMaxBackoff > TimeSpan.Zero
+            ? _queueOptions.RetryMaxBackoff
+            : initial;
+
+        var max = configuredMax <= TimeSpan.Zero
+            ? initial
+            : configuredMax;
+
+        var exponent = attempt - 1;
+        var scale = Math.Pow(2, exponent - 1);
+        var scaledTicks = initial.Ticks * scale;
+        var cappedTicks = Math.Min(max.Ticks, scaledTicks);
+
+        var resultTicks = Math.Max(initial.Ticks, (long)cappedTicks);
+        return TimeSpan.FromTicks(resultTicks);
+    }
+
+    private async Task<RedisValue> AddToStreamAsync(
+        IDatabase database,
+        RedisKey stream,
+        NameValueEntry[] entries,
+        int? maxLength,
+        bool useApproximateLength)
+    {
+        var capacity = 4 + (entries.Length * 2);
+        var args = new List<object>(capacity)
+        {
+            stream
+        };
+
+        if (maxLength.HasValue)
+        {
+            args.Add("MAXLEN");
+            if (useApproximateLength)
+            {
+                args.Add("~");
+            }
+
+            args.Add(maxLength.Value);
+        }
+
+        args.Add("*");
+        for (var i = 0; i < entries.Length; i++)
+        {
+            args.Add(entries[i].Name);
+            args.Add(entries[i].Value);
+        }
+
+        var result = await database.ExecuteAsync("XADD", args.ToArray()).ConfigureAwait(false);
+        return (RedisValue)result!;
+    }
+
+    private static class EmptyReadOnlyDictionary<TKey, TValue>
+        where TKey : notnull
+    {
+        public static readonly IReadOnlyDictionary<TKey, TValue> Instance =
+            new ReadOnlyDictionary<TKey, TValue>(new Dictionary<TKey, TValue>(0, EqualityComparer<TKey>.Default));
+    }
+
+    internal async ValueTask PingAsync(CancellationToken cancellationToken)
+    {
+        var db = await GetDatabaseAsync(cancellationToken).ConfigureAwait(false);
+        await db.ExecuteAsync("PING").ConfigureAwait(false);
+    }
+}

src/StellaOps.Scanner.Queue/Redis/RedisScanQueueLease.cs

@@ -0,0 +1,76 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace StellaOps.Scanner.Queue.Redis;
+
+internal sealed class RedisScanQueueLease : IScanQueueLease
+{
+    private readonly RedisScanQueue _queue;
+    private int _completed;
+
+    internal RedisScanQueueLease(
+        RedisScanQueue queue,
+        string messageId,
+        string jobId,
+        byte[] payload,
+        int attempt,
+        DateTimeOffset enqueuedAt,
+        DateTimeOffset leaseExpiresAt,
+        string consumer,
+        string? idempotencyKey,
+        string? traceId,
+        IReadOnlyDictionary<string, string> attributes)
+    {
+        _queue = queue;
+        MessageId = messageId;
+        JobId = jobId;
+        Payload = payload;
+        Attempt = attempt;
+        EnqueuedAt = enqueuedAt;
+        LeaseExpiresAt = leaseExpiresAt;
+        Consumer = consumer;
+        IdempotencyKey = idempotencyKey;
+        TraceId = traceId;
+        Attributes = attributes;
+    }
+
+    public string MessageId { get; }
+
+    public string JobId { get; }
+
+    public ReadOnlyMemory<byte> Payload { get; }
+
+    public int Attempt { get; }
+
+    public DateTimeOffset EnqueuedAt { get; }
+
+    public DateTimeOffset LeaseExpiresAt { get; private set; }
+
+    public string Consumer { get; }
+
+    public string? IdempotencyKey { get; }
+
+    public string? TraceId { get; }
+
+    public IReadOnlyDictionary<string, string> Attributes { get; }
+
+    public Task AcknowledgeAsync(CancellationToken cancellationToken = default)
+        => _queue.AcknowledgeAsync(this, cancellationToken);
+
+    public Task RenewAsync(TimeSpan leaseDuration, CancellationToken cancellationToken = default)
+        => _queue.RenewLeaseAsync(this, leaseDuration, cancellationToken);
+
+    public Task ReleaseAsync(QueueReleaseDisposition disposition, CancellationToken cancellationToken = default)
+        => _queue.ReleaseAsync(this, disposition, cancellationToken);
+
+    public Task DeadLetterAsync(string reason, CancellationToken cancellationToken = default)
+        => _queue.DeadLetterAsync(this, reason, cancellationToken);
+
+    internal bool TryBeginCompletion()
+        => Interlocked.CompareExchange(ref _completed, 1, 0) == 0;
+
+    internal void RefreshLease(DateTimeOffset expiresAt)
+        => LeaseExpiresAt = expiresAt;
+}

src/StellaOps.Scanner.Queue/ScanQueueContracts.cs

@@ -0,0 +1,115 @@
+using System;
+using System.Collections.Generic;
+
+namespace StellaOps.Scanner.Queue;
+
+public sealed class ScanQueueMessage
+{
+    private readonly byte[] _payload;
+
+    public ScanQueueMessage(string jobId, ReadOnlyMemory<byte> payload)
+    {
+        if (string.IsNullOrWhiteSpace(jobId))
+        {
+            throw new ArgumentException("Job identifier must be provided.", nameof(jobId));
+        }
+
+        JobId = jobId;
+        _payload = CopyPayload(payload);
+    }
+
+    public string JobId { get; }
+
+    public string? IdempotencyKey { get; init; }
+
+    public string? TraceId { get; init; }
+
+    public IReadOnlyDictionary<string, string>? Attributes { get; init; }
+
+    public ReadOnlyMemory<byte> Payload => _payload;
+
+    private static byte[] CopyPayload(ReadOnlyMemory<byte> payload)
+    {
+        if (payload.Length == 0)
+        {
+            return Array.Empty<byte>();
+        }
+
+        var copy = new byte[payload.Length];
+        payload.Span.CopyTo(copy);
+        return copy;
+    }
+}
+
+public readonly record struct QueueEnqueueResult(string MessageId, bool Deduplicated);
+
+public sealed class QueueLeaseRequest
+{
+    public QueueLeaseRequest(string consumer, int batchSize, TimeSpan leaseDuration)
+    {
+        if (string.IsNullOrWhiteSpace(consumer))
+        {
+            throw new ArgumentException("Consumer name must be provided.", nameof(consumer));
+        }
+
+        if (batchSize <= 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(batchSize), batchSize, "Batch size must be positive.");
+        }
+
+        if (leaseDuration <= TimeSpan.Zero)
+        {
+            throw new ArgumentOutOfRangeException(nameof(leaseDuration), leaseDuration, "Lease duration must be positive.");
+        }
+
+        Consumer = consumer;
+        BatchSize = batchSize;
+        LeaseDuration = leaseDuration;
+    }
+
+    public string Consumer { get; }
+
+    public int BatchSize { get; }
+
+    public TimeSpan LeaseDuration { get; }
+}
+
+public sealed class QueueClaimOptions
+{
+    public QueueClaimOptions(
+        string claimantConsumer,
+        int batchSize,
+        TimeSpan minIdleTime)
+    {
+        if (string.IsNullOrWhiteSpace(claimantConsumer))
+        {
+            throw new ArgumentException("Consumer must be provided.", nameof(claimantConsumer));
+        }
+
+        if (batchSize <= 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(batchSize), batchSize, "Batch size must be positive.");
+        }
+
+        if (minIdleTime < TimeSpan.Zero)
+        {
+            throw new ArgumentOutOfRangeException(nameof(minIdleTime), minIdleTime, "Idle time cannot be negative.");
+        }
+
+        ClaimantConsumer = claimantConsumer;
+        BatchSize = batchSize;
+        MinIdleTime = minIdleTime;
+    }
+
+    public string ClaimantConsumer { get; }
+
+    public int BatchSize { get; }
+
+    public TimeSpan MinIdleTime { get; }
+}
+
+public enum QueueReleaseDisposition
+{
+    Retry,
+    Abandon
+}

src/StellaOps.Scanner.Queue/ScannerQueueHealthCheck.cs

@@ -0,0 +1,55 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.Diagnostics.HealthChecks;
+using Microsoft.Extensions.Logging;
+using StellaOps.Scanner.Queue.Nats;
+using StellaOps.Scanner.Queue.Redis;
+
+namespace StellaOps.Scanner.Queue;
+
+public sealed class ScannerQueueHealthCheck : IHealthCheck
+{
+    private readonly IScanQueue _queue;
+    private readonly ILogger<ScannerQueueHealthCheck> _logger;
+
+    public ScannerQueueHealthCheck(
+        IScanQueue queue,
+        ILogger<ScannerQueueHealthCheck> logger)
+    {
+        _queue = queue ?? throw new ArgumentNullException(nameof(queue));
+        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+    }
+
+    public async Task<HealthCheckResult> CheckHealthAsync(
+        HealthCheckContext context,
+        CancellationToken cancellationToken = default)
+    {
+        cancellationToken.ThrowIfCancellationRequested();
+
+        try
+        {
+            switch (_queue)
+            {
+                case RedisScanQueue redisQueue:
+                    await redisQueue.PingAsync(cancellationToken).ConfigureAwait(false);
+                    return HealthCheckResult.Healthy("Redis queue reachable.");
+
+                case NatsScanQueue natsQueue:
+                    await natsQueue.PingAsync(cancellationToken).ConfigureAwait(false);
+                    return HealthCheckResult.Healthy("NATS queue reachable.");
+
+                default:
+                    return HealthCheckResult.Healthy("Queue transport without dedicated ping returned healthy.");
+            }
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Scanner queue health check failed.");
+            return new HealthCheckResult(
+                context.Registration.FailureStatus,
+                "Queue transport unreachable.",
+                ex);
+        }
+    }
+}

src/StellaOps.Scanner.Queue/ScannerQueueOptions.cs

@@ -0,0 +1,92 @@
+using System;
+
+namespace StellaOps.Scanner.Queue;
+
+public sealed class ScannerQueueOptions
+{
+    public QueueTransportKind Kind { get; set; } = QueueTransportKind.Redis;
+
+    public RedisQueueOptions Redis { get; set; } = new();
+
+    public NatsQueueOptions Nats { get; set; } = new();
+
+    /// <summary>
+    /// Default lease duration applied when callers do not override the visibility timeout.
+    /// </summary>
+    public TimeSpan DefaultLeaseDuration { get; set; } = TimeSpan.FromMinutes(5);
+
+    /// <summary>
+    /// Maximum number of times a message may be delivered before it is shunted to the dead-letter queue.
+    /// </summary>
+    public int MaxDeliveryAttempts { get; set; } = 5;
+
+    /// <summary>
+    /// Options controlling retry/backoff/dead-letter handling.
+    /// </summary>
+    public DeadLetterQueueOptions DeadLetter { get; set; } = new();
+
+    /// <summary>
+    /// Initial backoff applied when a job is retried after failure.
+    /// </summary>
+    public TimeSpan RetryInitialBackoff { get; set; } = TimeSpan.FromSeconds(5);
+
+    /// <summary>
+    /// Maximum backoff window applied for exponential retry.
+    /// </summary>
+    public TimeSpan RetryMaxBackoff { get; set; } = TimeSpan.FromMinutes(2);
+}
+
+public sealed class RedisQueueOptions
+{
+    public string? ConnectionString { get; set; }
+
+    public int? Database { get; set; }
+
+    public string StreamName { get; set; } = "scanner:jobs";
+
+    public string ConsumerGroup { get; set; } = "scanner-workers";
+
+    public string IdempotencyKeyPrefix { get; set; } = "scanner:jobs:idemp:";
+
+    public TimeSpan IdempotencyWindow { get; set; } = TimeSpan.FromHours(12);
+
+    public int? ApproximateMaxLength { get; set; }
+
+    public TimeSpan InitializationTimeout { get; set; } = TimeSpan.FromSeconds(30);
+
+    public TimeSpan ClaimIdleThreshold { get; set; } = TimeSpan.FromMinutes(10);
+
+    public TimeSpan PendingScanWindow { get; set; } = TimeSpan.FromMinutes(30);
+
+    public TimeSpan RetryInitialBackoff { get; set; } = TimeSpan.FromSeconds(5);
+}
+
+public sealed class NatsQueueOptions
+{
+    public string? Url { get; set; }
+
+    public string Stream { get; set; } = "SCANNER_JOBS";
+
+    public string Subject { get; set; } = "scanner.jobs";
+
+    public string DurableConsumer { get; set; } = "scanner-workers";
+
+    public int MaxInFlight { get; set; } = 64;
+
+    public TimeSpan AckWait { get; set; } = TimeSpan.FromMinutes(5);
+
+    public string DeadLetterStream { get; set; } = "SCANNER_JOBS_DEAD";
+
+    public string DeadLetterSubject { get; set; } = "scanner.jobs.dead";
+
+    public TimeSpan RetryDelay { get; set; } = TimeSpan.FromSeconds(10);
+
+    public TimeSpan IdleHeartbeat { get; set; } = TimeSpan.FromSeconds(30);
+}
+
+public sealed class DeadLetterQueueOptions
+{
+    public string StreamName { get; set; } = "scanner:jobs:dead";
+
+    public TimeSpan Retention { get; set; } = TimeSpan.FromDays(7);
+}

src/StellaOps.Scanner.Queue/ScannerQueueServiceCollectionExtensions.cs

@@ -0,0 +1,67 @@
+using System;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
+using Microsoft.Extensions.Diagnostics.HealthChecks;
+using Microsoft.Extensions.Logging;
+using StellaOps.Scanner.Queue.Nats;
+using StellaOps.Scanner.Queue.Redis;
+
+namespace StellaOps.Scanner.Queue;
+
+public static class ScannerQueueServiceCollectionExtensions
+{
+    public static IServiceCollection AddScannerQueue(
+        this IServiceCollection services,
+        IConfiguration configuration,
+        string sectionName = "scanner:queue")
+    {
+        ArgumentNullException.ThrowIfNull(services);
+        ArgumentNullException.ThrowIfNull(configuration);
+
+        var options = new ScannerQueueOptions();
+        configuration.GetSection(sectionName).Bind(options);
+
+        services.TryAddSingleton(TimeProvider.System);
+        services.AddSingleton(options);
+
+        services.AddSingleton<IScanQueue>(sp =>
+        {
+            var loggerFactory = sp.GetRequiredService<ILoggerFactory>();
+            var timeProvider = sp.GetService<TimeProvider>() ?? TimeProvider.System;
+
+            return options.Kind switch
+            {
+                QueueTransportKind.Redis => new RedisScanQueue(
+                    options,
+                    options.Redis,
+                    loggerFactory.CreateLogger<RedisScanQueue>(),
+                    timeProvider),
+                QueueTransportKind.Nats => new NatsScanQueue(
+                    options,
+                    options.Nats,
+                    loggerFactory.CreateLogger<NatsScanQueue>(),
+                    timeProvider),
+                _ => throw new InvalidOperationException($"Unsupported queue transport kind '{options.Kind}'.")
+            };
+        });
+
+        services.AddSingleton<ScannerQueueHealthCheck>();
+
+        return services;
+    }
+
+    public static IHealthChecksBuilder AddScannerQueueHealthCheck(
+        this IHealthChecksBuilder builder)
+    {
+        ArgumentNullException.ThrowIfNull(builder);
+
+        builder.Services.TryAddSingleton<ScannerQueueHealthCheck>();
+        builder.AddCheck<ScannerQueueHealthCheck>(
+            name: "scanner-queue",
+            failureStatus: HealthStatus.Unhealthy,
+            tags: new[] { "scanner", "queue" });
+
+        return builder;
+    }
+}

src/StellaOps.Scanner.Queue/StellaOps.Scanner.Queue.csproj

@@ -0,0 +1,21 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Extensions.Configuration.Abstractions" Version="8.0.0" />
+    <PackageReference Include="Microsoft.Extensions.Configuration.Binder" Version="8.0.1" />
+    <PackageReference Include="Microsoft.Extensions.DependencyInjection.Abstractions" Version="8.0.0" />
+    <PackageReference Include="Microsoft.Extensions.Diagnostics.HealthChecks" Version="8.0.0" />
+    <PackageReference Include="Microsoft.Extensions.Diagnostics.HealthChecks.Abstractions" Version="8.0.0" />
+    <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" Version="8.0.0" />
+    <PackageReference Include="Microsoft.Extensions.Options" Version="8.0.1" />
+    <PackageReference Include="StackExchange.Redis" Version="2.7.33" />
+    <PackageReference Include="NATS.Client.Core" Version="2.0.0" />
+    <PackageReference Include="NATS.Client.JetStream" Version="2.0.0" />
+  </ItemGroup>
+</Project>

src/StellaOps.Scanner.Queue/TASKS.md

@@ -0,0 +1,7 @@
+# Scanner Queue Task Board (Sprint 9)
+
+| ID | Status | Owner(s) | Depends on | Description | Exit Criteria |
+|----|--------|----------|------------|-------------|---------------|
+| SCANNER-QUEUE-09-401 | DONE (2025-10-19) | Scanner Queue Guild | — | Implement queue abstraction + Redis Streams adapter with ack/lease semantics, idempotency tokens, and deterministic job IDs. | Interfaces finalized; Redis adapter passes enqueue/dequeue/ack/claim lease tests; structured logs exercised. |
+| SCANNER-QUEUE-09-402 | DONE (2025-10-19) | Scanner Queue Guild | SCANNER-QUEUE-09-401 | Add pluggable backend support (Redis, NATS) with configuration binding, health probes, failover documentation. | NATS adapter + DI bindings delivered; health checks documented; configuration tests green. |
+| SCANNER-QUEUE-09-403 | DONE (2025-10-19) | Scanner Queue Guild | SCANNER-QUEUE-09-401 | Implement retry and dead-letter flow with structured metrics/logs for offline deployments. | Retry policy configurable; dead-letter queue persisted; metrics counters validated in integration tests. |