docs/implementation-plan-mxaccess-worker.md

@@ -277,6 +277,8 @@ Live tests:
 
 Labels: `area:worker`, `type:feature`, `priority:p0`
 
+Status: implemented.
+
 Deliverables:
 
 - handlers for `OnDataChange`,

docs/mxaccess-worker-instance-design.md

@@ -348,9 +348,28 @@ Event handling rules:
 - Enqueue to the outbound event queue.
 - Return quickly to preserve message pumping.
 
-If event conversion throws, catch it inside the event handler, enqueue a
+`MxAccessBaseEventSink` implements the COM connection-point handlers and keeps
-structured `WorkerFault` or diagnostic event, and keep the worker alive only if
+the handlers limited to event argument conversion plus enqueue. It uses
-the fault policy allows it.
+`MxAccessEventMapper` to create `MxEvent` DTOs for `OnDataChange`,
+`OnWriteComplete`, `OperationComplete`, and `OnBufferedDataChange`. The mapper
+converts scalar and array values through `VariantConverter`, converts
+`MXSTATUS_PROXY[]` through `MxStatusProxyConverter`, and maps installed
+`MxDataType` values to the public protobuf enum while preserving the raw data
+type on buffered events. `OperationComplete` is only emitted from the native
+`OperationComplete` handler; write completion does not synthesize it.
+
+`MxAccessEventQueue` is the bounded outbound event queue for one worker
+session. It assigns the monotonic `WorkerSequence` and `WorkerTimestamp` when an
+event is accepted, preserving the order in which MXAccess handlers enqueue
+events. The default capacity is `10000`. When the queue reaches capacity it
+records a `WorkerFaultCategory.QueueOverflow` fault and rejects further events.
+The event handler catches conversion and enqueue failures, records the first
+fault on the queue, and returns to the STA message pump instead of writing to
+the pipe.
+
+If event conversion throws, catch it inside the event handler, record a
+structured `WorkerFault`, and keep the worker alive only if the fault policy
+allows it.
 
 ## Command Queue
 

src/MxGateway.Worker.Tests/MxAccess/MxAccessCommandExecutorTests.cs

@@ -842,7 +842,9 @@ public sealed class MxAccessCommandExecutorTests
 
     private sealed class NoopEventSink : IMxAccessEventSink
     {
-        public void Attach(object mxAccessComObject)
+        public void Attach(
+            object mxAccessComObject,
+            string sessionId)
         {
         }
 

src/MxGateway.Worker.Tests/MxAccess/MxAccessEventMapperTests.cs

@@ -0,0 +1,117 @@
+using System;
+using MxGateway.Contracts.Proto;
+using MxGateway.Worker.MxAccess;
+
+namespace MxGateway.Worker.Tests.MxAccess;
+
+public sealed class MxAccessEventMapperTests
+{
+    private readonly MxAccessEventMapper mapper = new();
+
+    [Fact]
+    public void CreateOnDataChange_ConvertsValueTimestampQualityAndStatuses()
+    {
+        DateTime timestamp = new(2026, 4, 26, 12, 30, 0, DateTimeKind.Utc);
+        FakeStatus[] statuses =
+        {
+            new()
+            {
+                success = -1,
+                category = 0,
+                detectedBy = 5,
+                detail = 0,
+            },
+        };
+
+        MxEvent mxEvent = mapper.CreateOnDataChange(
+            "session-1",
+            serverHandle: 12,
+            itemHandle: 34,
+            value: 42,
+            quality: 192,
+            timestamp: timestamp,
+            statuses: statuses);
+
+        Assert.Equal(MxEventFamily.OnDataChange, mxEvent.Family);
+        Assert.Equal("session-1", mxEvent.SessionId);
+        Assert.Equal(12, mxEvent.ServerHandle);
+        Assert.Equal(34, mxEvent.ItemHandle);
+        Assert.Equal(42, mxEvent.Value.Int32Value);
+        Assert.Equal(192, mxEvent.Quality);
+        Assert.Equal(timestamp, mxEvent.SourceTimestamp.ToDateTime());
+        Assert.Equal(MxEvent.BodyOneofCase.OnDataChange, mxEvent.BodyCase);
+
+        MxStatusProxy status = Assert.Single(mxEvent.Statuses);
+        Assert.Equal(-1, status.Success);
+        Assert.Equal(MxStatusCategory.Ok, status.Category);
+        Assert.Equal(MxStatusSource.RespondingAutomationObject, status.DetectedBy);
+    }
+
+    [Fact]
+    public void CreateOnWriteCompleteAndOperationComplete_PreservesDistinctFamilies()
+    {
+        MxEvent writeComplete = mapper.CreateOnWriteComplete(
+            "session-1",
+            serverHandle: 1,
+            itemHandle: 2,
+            statuses: Array.Empty<FakeStatus>());
+        MxEvent operationComplete = mapper.CreateOperationComplete(
+            "session-1",
+            serverHandle: 1,
+            itemHandle: 2,
+            statuses: Array.Empty<FakeStatus>());
+
+        Assert.Equal(MxEventFamily.OnWriteComplete, writeComplete.Family);
+        Assert.Equal(MxEvent.BodyOneofCase.OnWriteComplete, writeComplete.BodyCase);
+        Assert.Equal(MxEventFamily.OperationComplete, operationComplete.Family);
+        Assert.Equal(MxEvent.BodyOneofCase.OperationComplete, operationComplete.BodyCase);
+    }
+
+    [Fact]
+    public void CreateOnBufferedDataChange_PreservesRawDataTypeAndArrayMetadata()
+    {
+        DateTime firstTimestamp = new(2026, 4, 26, 13, 0, 0, DateTimeKind.Utc);
+        DateTime secondTimestamp = new(2026, 4, 26, 13, 1, 0, DateTimeKind.Utc);
+
+        MxEvent mxEvent = mapper.CreateOnBufferedDataChange(
+            "session-1",
+            serverHandle: 10,
+            itemHandle: 20,
+            rawDataType: 2,
+            value: new[] { 7, 8 },
+            quality: new[] { 192, 0 },
+            timestamp: new[] { firstTimestamp, secondTimestamp },
+            statuses: null);
+
+        Assert.Equal(MxEventFamily.OnBufferedDataChange, mxEvent.Family);
+        Assert.Equal(MxDataType.Integer, mxEvent.OnBufferedDataChange.DataType);
+        Assert.Equal(2, mxEvent.OnBufferedDataChange.RawDataType);
+        Assert.Equal(MxDataType.Integer, mxEvent.Value.ArrayValue.ElementDataType);
+        Assert.Equal(new[] { 7, 8 }, mxEvent.Value.ArrayValue.Int32Values.Values);
+        Assert.Equal(new[] { 192, 0 }, mxEvent.OnBufferedDataChange.QualityValues.Int32Values.Values);
+        Assert.Equal(2, mxEvent.OnBufferedDataChange.TimestampValues.TimestampValues.Values.Count);
+    }
+
+    [Theory]
+    [InlineData(-1, MxDataType.Unknown)]
+    [InlineData(0, MxDataType.NoData)]
+    [InlineData(1, MxDataType.Boolean)]
+    [InlineData(2, MxDataType.Integer)]
+    [InlineData(6, MxDataType.Time)]
+    [InlineData(15, MxDataType.InternationalizedString)]
+    [InlineData(999, MxDataType.Unknown)]
+    public void MapMxDataType_MapsInstalledMxAccessValues(
+        int rawDataType,
+        MxDataType expectedDataType)
+    {
+        Assert.Equal(expectedDataType, MxAccessEventMapper.MapMxDataType(rawDataType));
+    }
+
+    private sealed class FakeStatus
+    {
+        public int success;
+        public int category;
+        public int detectedBy;
+        public int detail;
+    }
+}

src/MxGateway.Worker.Tests/MxAccess/MxAccessEventQueueTests.cs

@@ -0,0 +1,106 @@
+using System;
+using System.Collections.Generic;
+using MxGateway.Contracts.Proto;
+using MxGateway.Worker.MxAccess;
+
+namespace MxGateway.Worker.Tests.MxAccess;
+
+public sealed class MxAccessEventQueueTests
+{
+    [Fact]
+    public void Enqueue_AssignsMonotonicWorkerSequencesAndPreservesOrder()
+    {
+        MxAccessEventQueue queue = new(capacity: 4);
+
+        WorkerEvent first = queue.Enqueue(CreateEvent(MxEventFamily.OnDataChange, itemHandle: 10));
+        WorkerEvent second = queue.Enqueue(CreateEvent(MxEventFamily.OnWriteComplete, itemHandle: 11));
+
+        Assert.Equal(1UL, first.Event.WorkerSequence);
+        Assert.Equal(2UL, second.Event.WorkerSequence);
+        Assert.NotNull(first.Event.WorkerTimestamp);
+        Assert.Equal(2, queue.Count);
+        Assert.Equal(2UL, queue.LastEventSequence);
+
+        Assert.True(queue.TryDequeue(out WorkerEvent? dequeuedFirst));
+        Assert.True(queue.TryDequeue(out WorkerEvent? dequeuedSecond));
+        Assert.Equal(10, dequeuedFirst?.Event.ItemHandle);
+        Assert.Equal(11, dequeuedSecond?.Event.ItemHandle);
+        Assert.False(queue.TryDequeue(out _));
+    }
+
+    [Fact]
+    public void Drain_RemovesAtMostRequestedEvents()
+    {
+        MxAccessEventQueue queue = new(capacity: 4);
+        queue.Enqueue(CreateEvent(MxEventFamily.OnDataChange, itemHandle: 10));
+        queue.Enqueue(CreateEvent(MxEventFamily.OnDataChange, itemHandle: 11));
+        queue.Enqueue(CreateEvent(MxEventFamily.OnDataChange, itemHandle: 12));
+
+        IReadOnlyList<WorkerEvent> drained = queue.Drain(maxEvents: 2);
+
+        Assert.Equal(2, drained.Count);
+        Assert.Equal(10, drained[0].Event.ItemHandle);
+        Assert.Equal(11, drained[1].Event.ItemHandle);
+        Assert.Equal(1, queue.Count);
+    }
+
+    [Fact]
+    public void Enqueue_WhenCapacityIsExceeded_RecordsOverflowFaultAndRejectsNewEvents()
+    {
+        MxAccessEventQueue queue = new(capacity: 1);
+        queue.Enqueue(CreateEvent(MxEventFamily.OnDataChange, itemHandle: 10));
+
+        MxAccessEventQueueOverflowException overflow = Assert.Throws<MxAccessEventQueueOverflowException>(
+            () => queue.Enqueue(CreateEvent(MxEventFamily.OnDataChange, itemHandle: 11)));
+
+        Assert.Equal(1, overflow.Capacity);
+        Assert.True(queue.IsFaulted);
+        Assert.Equal(WorkerFaultCategory.QueueOverflow, queue.Fault?.Category);
+        Assert.Equal(ProtocolStatusCode.WorkerUnavailable, queue.Fault?.ProtocolStatus.Code);
+        Assert.Throws<InvalidOperationException>(
+            () => queue.Enqueue(CreateEvent(MxEventFamily.OnDataChange, itemHandle: 12)));
+    }
+
+    [Fact]
+    public void RecordFault_KeepsFirstFault()
+    {
+        MxAccessEventQueue queue = new(capacity: 1);
+        queue.RecordFault(new WorkerFault
+        {
+            Category = WorkerFaultCategory.MxaccessEventConversionFailed,
+        });
+        queue.RecordFault(new WorkerFault
+        {
+            Category = WorkerFaultCategory.QueueOverflow,
+        });
+
+        Assert.True(queue.IsFaulted);
+        Assert.Equal(WorkerFaultCategory.MxaccessEventConversionFailed, queue.Fault?.Category);
+    }
+
+    private static MxEvent CreateEvent(
+        MxEventFamily family,
+        int itemHandle)
+    {
+        MxEvent mxEvent = new()
+        {
+            Family = family,
+            SessionId = "session-1",
+            ServerHandle = 1,
+            ItemHandle = itemHandle,
+        };
+
+        switch (family)
+        {
+            case MxEventFamily.OnWriteComplete:
+                mxEvent.OnWriteComplete = new OnWriteCompleteEvent();
+                break;
+
+            default:
+                mxEvent.OnDataChange = new OnDataChangeEvent();
+                break;
+        }
+
+        return mxEvent;
+    }
+}

src/MxGateway.Worker.Tests/MxAccess/MxAccessStaSessionTests.cs

@@ -18,7 +18,7 @@ public sealed class MxAccessStaSessionTests
         using StaRuntime runtime = CreateRuntime();
         using MxAccessStaSession session = new(runtime, factory, eventSink);
 
-        WorkerReady ready = await session.StartAsync(workerProcessId: 1234);
+        WorkerReady ready = await session.StartAsync("session-1", workerProcessId: 1234);
 
         Assert.Equal(1234, ready.WorkerProcessId);
         Assert.Equal(MxAccessInteropInfo.ProgId, ready.MxaccessProgid);
@@ -28,6 +28,7 @@ public sealed class MxAccessStaSessionTests
         Assert.Equal(runtime.StaThreadId, eventSink.AttachThreadId);
         Assert.Equal(ApartmentState.STA, factory.CreateApartmentState);
         Assert.Same(factory.CreatedObject, eventSink.AttachedObject);
+        Assert.Equal("session-1", eventSink.SessionId);
     }
 
     [Fact]
@@ -107,10 +108,15 @@ public sealed class MxAccessStaSessionTests
 
         public int? DetachThreadId { get; private set; }
 
-        public void Attach(object mxAccessComObject)
+        public string? SessionId { get; private set; }
+
+        public void Attach(
+            object mxAccessComObject,
+            string sessionId)
         {
             AttachedObject = mxAccessComObject;
             AttachThreadId = Thread.CurrentThread.ManagedThreadId;
+            SessionId = sessionId;
         }
 
         public void Detach()

src/MxGateway.Worker/Ipc/WorkerPipeSession.cs

@@ -235,7 +235,7 @@ public sealed class WorkerPipeSession
         try
         {
             return await _mxAccessStaSession
-                .StartAsync(_processIdProvider(), cancellationToken)
+                .StartAsync(_options.SessionId, _processIdProvider(), cancellationToken)
                 .ConfigureAwait(false);
         }
         catch

src/MxGateway.Worker/MxAccess/IMxAccessEventSink.cs

@@ -2,7 +2,9 @@ namespace MxGateway.Worker.MxAccess;
 
 public interface IMxAccessEventSink
 {
-    void Attach(object mxAccessComObject);
+    void Attach(
+        object mxAccessComObject,
+        string sessionId);
 
     void Detach();
 }

src/MxGateway.Worker/MxAccess/MxAccessBaseEventSink.cs

@@ -1,13 +1,39 @@
+using System;
 using ArchestrA.MxAccess;
+using Proto = MxGateway.Contracts.Proto;
 
 namespace MxGateway.Worker.MxAccess;
 
 public sealed class MxAccessBaseEventSink : IMxAccessEventSink
 {
+    private readonly MxAccessEventMapper eventMapper;
+    private readonly MxAccessEventQueue eventQueue;
     private LMXProxyServerClass? server;
+    private string sessionId = string.Empty;
 
-    public void Attach(object mxAccessComObject)
+    public MxAccessBaseEventSink()
+        : this(new MxAccessEventQueue())
     {
+    }
+
+    public MxAccessBaseEventSink(MxAccessEventQueue eventQueue)
+        : this(eventQueue, new MxAccessEventMapper())
+    {
+    }
+
+    public MxAccessBaseEventSink(
+        MxAccessEventQueue eventQueue,
+        MxAccessEventMapper eventMapper)
+    {
+        this.eventQueue = eventQueue ?? throw new ArgumentNullException(nameof(eventQueue));
+        this.eventMapper = eventMapper ?? throw new ArgumentNullException(nameof(eventMapper));
+    }
+
+    public void Attach(
+        object mxAccessComObject,
+        string sessionId)
+    {
+        this.sessionId = sessionId ?? string.Empty;
         server = (LMXProxyServerClass)mxAccessComObject;
         server.OnDataChange += OnDataChange;
         server.OnWriteComplete += OnWriteComplete;
@@ -27,9 +53,10 @@ public sealed class MxAccessBaseEventSink : IMxAccessEventSink
         server.OperationComplete -= OperationComplete;
         server.OnBufferedDataChange -= OnBufferedDataChange;
         server = null;
+        sessionId = string.Empty;
     }
 
-    private static void OnDataChange(
+    private void OnDataChange(
         int hLMXServerHandle,
         int phItemHandle,
         object pvItemValue,
@@ -37,23 +64,44 @@ public sealed class MxAccessBaseEventSink : IMxAccessEventSink
         object pftItemTimeStamp,
         ref MXSTATUS_PROXY[] pVars)
     {
+        MXSTATUS_PROXY[] statuses = pVars;
+        EnqueueEvent(() => eventMapper.CreateOnDataChange(
+            sessionId,
+            hLMXServerHandle,
+            phItemHandle,
+            pvItemValue,
+            pwItemQuality,
+            pftItemTimeStamp,
+            statuses));
     }
 
-    private static void OnWriteComplete(
+    private void OnWriteComplete(
         int hLMXServerHandle,
         int phItemHandle,
         ref MXSTATUS_PROXY[] pVars)
     {
+        MXSTATUS_PROXY[] statuses = pVars;
+        EnqueueEvent(() => eventMapper.CreateOnWriteComplete(
+            sessionId,
+            hLMXServerHandle,
+            phItemHandle,
+            statuses));
     }
 
-    private static void OperationComplete(
+    private void OperationComplete(
         int hLMXServerHandle,
         int phItemHandle,
         ref MXSTATUS_PROXY[] pVars)
     {
+        MXSTATUS_PROXY[] statuses = pVars;
+        EnqueueEvent(() => eventMapper.CreateOperationComplete(
+            sessionId,
+            hLMXServerHandle,
+            phItemHandle,
+            statuses));
     }
 
-    private static void OnBufferedDataChange(
+    private void OnBufferedDataChange(
         int hLMXServerHandle,
         int phItemHandle,
         MxDataType dtDataType,
@@ -62,5 +110,42 @@ public sealed class MxAccessBaseEventSink : IMxAccessEventSink
         object pftItemTimeStamp,
         ref MXSTATUS_PROXY[] pVars)
     {
+        MXSTATUS_PROXY[] statuses = pVars;
+        EnqueueEvent(() => eventMapper.CreateOnBufferedDataChange(
+            sessionId,
+            hLMXServerHandle,
+            phItemHandle,
+            (int)dtDataType,
+            pvItemValue,
+            pwItemQuality,
+            pftItemTimeStamp,
+            statuses));
+    }
+
+    private void EnqueueEvent(Func<Proto.MxEvent> createEvent)
+    {
+        try
+        {
+            eventQueue.Enqueue(createEvent());
+        }
+        catch (Exception exception)
+        {
+            eventQueue.RecordFault(CreateEventConversionFault(exception));
+        }
+    }
+
+    private Proto.WorkerFault CreateEventConversionFault(Exception exception)
+    {
+        return new Proto.WorkerFault
+        {
+            Category = Proto.WorkerFaultCategory.MxaccessEventConversionFailed,
+            ExceptionType = exception.GetType().FullName ?? string.Empty,
+            DiagnosticMessage = $"{exception.GetType().FullName}: HRESULT 0x{unchecked((uint)exception.HResult):X8}",
+            ProtocolStatus = new Proto.ProtocolStatus
+            {
+                Code = Proto.ProtocolStatusCode.MxaccessFailure,
+                Message = "MXAccess event conversion failed.",
+            },
+        };
     }
 }

src/MxGateway.Worker/MxAccess/MxAccessEventMapper.cs

@@ -0,0 +1,221 @@
+using System;
+using MxGateway.Contracts.Proto;
+using MxGateway.Worker.Conversion;
+
+namespace MxGateway.Worker.MxAccess;
+
+public sealed class MxAccessEventMapper
+{
+    private readonly VariantConverter variantConverter;
+    private readonly MxStatusProxyConverter statusProxyConverter;
+
+    public MxAccessEventMapper()
+        : this(new VariantConverter(), new MxStatusProxyConverter())
+    {
+    }
+
+    public MxAccessEventMapper(
+        VariantConverter variantConverter,
+        MxStatusProxyConverter statusProxyConverter)
+    {
+        this.variantConverter = variantConverter ?? throw new ArgumentNullException(nameof(variantConverter));
+        this.statusProxyConverter = statusProxyConverter ?? throw new ArgumentNullException(nameof(statusProxyConverter));
+    }
+
+    public MxEvent CreateOnDataChange(
+        string sessionId,
+        int serverHandle,
+        int itemHandle,
+        object? value,
+        int quality,
+        object? timestamp,
+        Array? statuses)
+    {
+        MxEvent mxEvent = CreateBaseEvent(
+            MxEventFamily.OnDataChange,
+            sessionId,
+            serverHandle,
+            itemHandle,
+            statuses);
+        mxEvent.Value = variantConverter.Convert(value);
+        mxEvent.Quality = quality;
+        ApplySourceTimestamp(mxEvent, timestamp);
+        mxEvent.OnDataChange = new OnDataChangeEvent();
+
+        return mxEvent;
+    }
+
+    public MxEvent CreateOnWriteComplete(
+        string sessionId,
+        int serverHandle,
+        int itemHandle,
+        Array? statuses)
+    {
+        MxEvent mxEvent = CreateBaseEvent(
+            MxEventFamily.OnWriteComplete,
+            sessionId,
+            serverHandle,
+            itemHandle,
+            statuses);
+        mxEvent.OnWriteComplete = new OnWriteCompleteEvent();
+
+        return mxEvent;
+    }
+
+    public MxEvent CreateOperationComplete(
+        string sessionId,
+        int serverHandle,
+        int itemHandle,
+        Array? statuses)
+    {
+        MxEvent mxEvent = CreateBaseEvent(
+            MxEventFamily.OperationComplete,
+            sessionId,
+            serverHandle,
+            itemHandle,
+            statuses);
+        mxEvent.OperationComplete = new OperationCompleteEvent();
+
+        return mxEvent;
+    }
+
+    public MxEvent CreateOnBufferedDataChange(
+        string sessionId,
+        int serverHandle,
+        int itemHandle,
+        int rawDataType,
+        object? value,
+        object? quality,
+        object? timestamp,
+        Array? statuses)
+    {
+        MxDataType dataType = MapMxDataType(rawDataType);
+        MxEvent mxEvent = CreateBaseEvent(
+            MxEventFamily.OnBufferedDataChange,
+            sessionId,
+            serverHandle,
+            itemHandle,
+            statuses);
+        mxEvent.Value = variantConverter.Convert(value, dataType);
+        mxEvent.OnBufferedDataChange = new OnBufferedDataChangeEvent
+        {
+            DataType = dataType,
+            RawDataType = rawDataType,
+            QualityValues = ConvertBufferedArray(quality, MxDataType.Integer),
+            TimestampValues = ConvertBufferedArray(timestamp, MxDataType.Time),
+        };
+
+        return mxEvent;
+    }
+
+    public static MxDataType MapMxDataType(int rawDataType)
+    {
+        return rawDataType switch
+        {
+            -1 => MxDataType.Unknown,
+            0 => MxDataType.NoData,
+            1 => MxDataType.Boolean,
+            2 => MxDataType.Integer,
+            3 => MxDataType.Float,
+            4 => MxDataType.Double,
+            5 => MxDataType.String,
+            6 => MxDataType.Time,
+            7 => MxDataType.ElapsedTime,
+            8 => MxDataType.ReferenceType,
+            9 => MxDataType.StatusType,
+            10 => MxDataType.Enum,
+            11 => MxDataType.SecurityClassificationEnum,
+            12 => MxDataType.DataQualityType,
+            13 => MxDataType.QualifiedEnum,
+            14 => MxDataType.QualifiedStruct,
+            15 => MxDataType.InternationalizedString,
+            16 => MxDataType.BigString,
+            17 => MxDataType.End,
+            _ => MxDataType.Unknown,
+        };
+    }
+
+    private MxEvent CreateBaseEvent(
+        MxEventFamily family,
+        string sessionId,
+        int serverHandle,
+        int itemHandle,
+        Array? statuses)
+    {
+        MxEvent mxEvent = new()
+        {
+            Family = family,
+            SessionId = sessionId ?? string.Empty,
+            ServerHandle = serverHandle,
+            ItemHandle = itemHandle,
+        };
+        mxEvent.Statuses.Add(statusProxyConverter.ConvertMany(statuses));
+
+        return mxEvent;
+    }
+
+    private void ApplySourceTimestamp(
+        MxEvent mxEvent,
+        object? timestamp)
+    {
+        MxValue convertedTimestamp = variantConverter.Convert(timestamp, MxDataType.Time);
+        if (convertedTimestamp.KindCase == MxValue.KindOneofCase.TimestampValue)
+        {
+            mxEvent.SourceTimestamp = convertedTimestamp.TimestampValue;
+            return;
+        }
+
+        if (!string.IsNullOrWhiteSpace(convertedTimestamp.RawDiagnostic))
+        {
+            mxEvent.RawStatus = string.IsNullOrWhiteSpace(mxEvent.RawStatus)
+                ? convertedTimestamp.RawDiagnostic
+                : $"{mxEvent.RawStatus}; {convertedTimestamp.RawDiagnostic}";
+        }
+    }
+
+    private MxArray ConvertBufferedArray(
+        object? value,
+        MxDataType expectedElementDataType)
+    {
+        if (value is Array array)
+        {
+            return variantConverter.ConvertArray(array, expectedElementDataType);
+        }
+
+        MxValue converted = variantConverter.Convert(value, expectedElementDataType);
+        if (converted.KindCase == MxValue.KindOneofCase.ArrayValue)
+        {
+            return converted.ArrayValue;
+        }
+
+        MxArray mxArray = new()
+        {
+            ElementDataType = converted.DataType,
+            VariantType = converted.VariantType,
+            RawElementDataType = converted.RawDataType,
+            RawDiagnostic = string.IsNullOrWhiteSpace(converted.RawDiagnostic)
+                ? "Buffered MXAccess event argument was not a SAFEARRAY."
+                : converted.RawDiagnostic,
+        };
+
+        switch (converted.KindCase)
+        {
+            case MxValue.KindOneofCase.Int32Value:
+                mxArray.Int32Values = new Int32Array();
+                mxArray.Int32Values.Values.Add(converted.Int32Value);
+                break;
+
+            case MxValue.KindOneofCase.Int64Value:
+                mxArray.Int64Values = new Int64Array();
+                mxArray.Int64Values.Values.Add(converted.Int64Value);
+                break;
+
+            case MxValue.KindOneofCase.TimestampValue:
+                mxArray.TimestampValues = new TimestampArray();
+                mxArray.TimestampValues.Values.Add(converted.TimestampValue);
+                break;
+        }
+
+        return mxArray;
+    }
+}

src/MxGateway.Worker/MxAccess/MxAccessEventQueue.cs

@@ -0,0 +1,180 @@
+using System;
+using System.Collections.Generic;
+using Google.Protobuf.WellKnownTypes;
+using MxGateway.Contracts.Proto;
+
+namespace MxGateway.Worker.MxAccess;
+
+public sealed class MxAccessEventQueue
+{
+    public const int DefaultCapacity = 10000;
+
+    private readonly int capacity;
+    private readonly Queue<WorkerEvent> events;
+    private readonly object syncRoot = new();
+    private ulong lastEventSequence;
+    private WorkerFault? fault;
+
+    public MxAccessEventQueue()
+        : this(DefaultCapacity)
+    {
+    }
+
+    public MxAccessEventQueue(int capacity)
+    {
+        if (capacity <= 0)
+        {
+            throw new ArgumentOutOfRangeException(
+                nameof(capacity),
+                "MXAccess event queue capacity must be greater than zero.");
+        }
+
+        this.capacity = capacity;
+        events = new Queue<WorkerEvent>(capacity);
+    }
+
+    public int Capacity => capacity;
+
+    public int Count
+    {
+        get
+        {
+            lock (syncRoot)
+            {
+                return events.Count;
+            }
+        }
+    }
+
+    public ulong LastEventSequence
+    {
+        get
+        {
+            lock (syncRoot)
+            {
+                return lastEventSequence;
+            }
+        }
+    }
+
+    public bool IsFaulted
+    {
+        get
+        {
+            lock (syncRoot)
+            {
+                return fault is not null;
+            }
+        }
+    }
+
+    public WorkerFault? Fault
+    {
+        get
+        {
+            lock (syncRoot)
+            {
+                return fault?.Clone();
+            }
+        }
+    }
+
+    public WorkerEvent Enqueue(MxEvent mxEvent)
+    {
+        if (mxEvent is null)
+        {
+            throw new ArgumentNullException(nameof(mxEvent));
+        }
+
+        lock (syncRoot)
+        {
+            if (fault is not null)
+            {
+                throw new InvalidOperationException("MXAccess outbound event queue is faulted.");
+            }
+
+            if (events.Count >= capacity)
+            {
+                fault = CreateOverflowFault();
+                throw new MxAccessEventQueueOverflowException(capacity);
+            }
+
+            MxEvent queuedEvent = mxEvent.Clone();
+            queuedEvent.WorkerSequence = ++lastEventSequence;
+            queuedEvent.WorkerTimestamp = Timestamp.FromDateTime(DateTime.UtcNow);
+
+            WorkerEvent workerEvent = new()
+            {
+                Event = queuedEvent,
+            };
+            events.Enqueue(workerEvent);
+
+            return workerEvent.Clone();
+        }
+    }
+
+    public bool TryDequeue(out WorkerEvent? workerEvent)
+    {
+        lock (syncRoot)
+        {
+            if (events.Count == 0)
+            {
+                workerEvent = null;
+                return false;
+            }
+
+            workerEvent = events.Dequeue().Clone();
+            return true;
+        }
+    }
+
+    public IReadOnlyList<WorkerEvent> Drain(uint maxEvents)
+    {
+        lock (syncRoot)
+        {
+            int drainCount = maxEvents == 0
+                ? events.Count
+                : Math.Min(events.Count, checked((int)Math.Min(maxEvents, int.MaxValue)));
+            if (drainCount == 0)
+            {
+                return Array.Empty<WorkerEvent>();
+            }
+
+            List<WorkerEvent> drained = new(drainCount);
+            for (int index = 0; index < drainCount; index++)
+            {
+                drained.Add(events.Dequeue().Clone());
+            }
+
+            return drained;
+        }
+    }
+
+    public void RecordFault(WorkerFault workerFault)
+    {
+        if (workerFault is null)
+        {
+            throw new ArgumentNullException(nameof(workerFault));
+        }
+
+        lock (syncRoot)
+        {
+            fault ??= workerFault.Clone();
+        }
+    }
+
+    private WorkerFault CreateOverflowFault()
+    {
+        string message = $"MXAccess outbound event queue reached capacity {capacity}.";
+        return new WorkerFault
+        {
+            Category = WorkerFaultCategory.QueueOverflow,
+            DiagnosticMessage = message,
+            ProtocolStatus = new ProtocolStatus
+            {
+                Code = ProtocolStatusCode.WorkerUnavailable,
+                Message = message,
+            },
+        };
+    }
+}

src/MxGateway.Worker/MxAccess/MxAccessEventQueueOverflowException.cs

@@ -0,0 +1,14 @@
+using System;
+
+namespace MxGateway.Worker.MxAccess;
+
+public sealed class MxAccessEventQueueOverflowException : Exception
+{
+    public MxAccessEventQueueOverflowException(int capacity)
+        : base($"MXAccess outbound event queue reached its configured capacity of {capacity}.")
+    {
+        Capacity = capacity;
+    }
+
+    public int Capacity { get; }
+}

src/MxGateway.Worker/MxAccess/MxAccessSession.cs

@@ -44,7 +44,8 @@ public sealed class MxAccessSession : IDisposable
 
     public static MxAccessSession Create(
         IMxAccessComObjectFactory factory,
-        IMxAccessEventSink eventSink)
+        IMxAccessEventSink eventSink,
+        string sessionId)
     {
         if (factory is null)
         {
@@ -66,7 +67,7 @@ public sealed class MxAccessSession : IDisposable
                 throw new InvalidOperationException("MXAccess COM factory returned null.");
             }
 
-            eventSink.Attach(mxAccessComObject);
+            eventSink.Attach(mxAccessComObject, sessionId);
 
             return new MxAccessSession(
                 mxAccessComObject,

src/MxGateway.Worker/MxAccess/MxAccessStaSession.cs

@@ -11,6 +11,7 @@ public sealed class MxAccessStaSession : IDisposable
 {
     private readonly IMxAccessComObjectFactory factory;
     private readonly IMxAccessEventSink eventSink;
+    private readonly MxAccessEventQueue eventQueue;
     private readonly StaRuntime staRuntime;
     private StaCommandDispatcher? commandDispatcher;
     private MxAccessSession? session;
@@ -20,7 +21,7 @@ public sealed class MxAccessStaSession : IDisposable
         : this(
             new StaRuntime(),
             new MxAccessComObjectFactory(),
-            new MxAccessBaseEventSink())
+            new MxAccessEventQueue())
     {
     }
 
@@ -28,13 +29,41 @@ public sealed class MxAccessStaSession : IDisposable
         StaRuntime staRuntime,
         IMxAccessComObjectFactory factory,
         IMxAccessEventSink eventSink)
+        : this(staRuntime, factory, eventSink, new MxAccessEventQueue())
+    {
+    }
+
+    public MxAccessStaSession(
+        StaRuntime staRuntime,
+        IMxAccessComObjectFactory factory,
+        MxAccessEventQueue eventQueue)
+        : this(staRuntime, factory, new MxAccessBaseEventSink(eventQueue), eventQueue)
+    {
+    }
+
+    public MxAccessStaSession(
+        StaRuntime staRuntime,
+        IMxAccessComObjectFactory factory,
+        IMxAccessEventSink eventSink,
+        MxAccessEventQueue eventQueue)
     {
         this.staRuntime = staRuntime ?? throw new ArgumentNullException(nameof(staRuntime));
         this.factory = factory ?? throw new ArgumentNullException(nameof(factory));
         this.eventSink = eventSink ?? throw new ArgumentNullException(nameof(eventSink));
+        this.eventQueue = eventQueue ?? throw new ArgumentNullException(nameof(eventQueue));
+    }
+
+    public MxAccessEventQueue EventQueue => eventQueue;
+
+    public Task<WorkerReady> StartAsync(
+        int workerProcessId,
+        CancellationToken cancellationToken = default)
+    {
+        return StartAsync(string.Empty, workerProcessId, cancellationToken);
     }
 
     public Task<WorkerReady> StartAsync(
+        string sessionId,
         int workerProcessId,
         CancellationToken cancellationToken = default)
     {
@@ -48,7 +77,7 @@ public sealed class MxAccessStaSession : IDisposable
                     throw new InvalidOperationException("MXAccess COM session has already been created.");
                 }
 
-                session = MxAccessSession.Create(factory, eventSink);
+                session = MxAccessSession.Create(factory, eventSink, sessionId);
                 commandDispatcher = new StaCommandDispatcher(
                     staRuntime,
                     new MxAccessCommandExecutor(session));
@@ -68,6 +97,11 @@ public sealed class MxAccessStaSession : IDisposable
         return commandDispatcher.DispatchAsync(command);
     }
 
+    public IReadOnlyList<WorkerEvent> DrainEvents(uint maxEvents)
+    {
+        return eventQueue.Drain(maxEvents);
+    }
+
     public Task<IReadOnlyList<RegisteredServerHandle>> GetRegisteredServerHandlesAsync(
         CancellationToken cancellationToken = default)
     {