fix(opcua): issue and resume continuation points for event HistoryRead (#669)

A capped events read now pages: the source's truncation flag triggers a session-bound cursor (time inclusive + tie skip), oversized tie clusters are paged within via a bounded half-open over-fetch, and release/unknown/ cross-kind points behave as for Raw.
2026-10-11 08:21:02 -04:00
parent db6a53036b
commit f9fd495292
5 changed files with 498 additions and 20 deletions
@@ -583,6 +583,23 @@ are disposed when the session closes). Resuming an unknown / evicted / released
 `BadContinuationPointInvalid`. `releaseContinuationPoints` drops the stored cursors without reading
 data.

+### Continuation-point paging (Events) (#669)
+
+A capped event read (`NumValuesPerNode > 0`) pages exactly like Raw, with one difference: the trigger is the **source's
+truncation flag** (`HistoricalEventsResult.ContinuationPoint != null`), never the page count, so a cap-sized page the source
+says is complete carries no continuation point. `NumValuesPerNode == 0` stays unbounded and never pages.
+
+- The cursor is `(source, next start, end, boundary skip, cap)` in the same session-bound store as Raw (flagged as an events
+  cursor — a Raw point presented to an events read, or the reverse, is `BadContinuationPointInvalid`). The next page reads from
+  the last event's `EventTimeUtc` **inclusive** and drops the events at that timestamp already emitted.
+- A tie cluster larger than the cap is paged within by over-fetching the half-open window `[T, T + resolution)` up to
+  `MaxTieClusterOverfetch`; beyond that bound the node answers `BadHistoryOperationUnsupported` (raise the bound).
+- **Tie exactness depends on the backend.** SqlServer and QuestDB order by `(EventTimeUtc, EventId)`, so the skip count is exact.
+  The Gateway backend has no stable intra-timestamp order (and only a truncation flag), so ties at a page boundary are
+  best-effort there.
+- Reverse event reads (`start > end`) are not paged server-side; no continuation point is issued for them.
+- No session on the request (in-process callers) degrades to single-shot, as for Raw.
+
 ### Total aggregate

 The OPC UA `Total` aggregate is **supported** over the HistorianGateway backend. The gateway exposes a
@@ -725,8 +742,8 @@ result never has both.

 `[start, end)` by `EventTimeUtc`; **a reversed window (`end < start`) is swapped and returned ascending**. A null `sourceName` means all
 sources, otherwise `SourceName = @source`. Ordered `(EventTimeUtc, EventId)` ascending. A bounded read fetches `maxEvents + 1` rows and
-returns a non-null continuation point when the extra row exists; the node manager does not currently issue or resume event
-continuation points, so a client-capped read returns only the first page.
+returns a non-null continuation point when the extra row exists; the node manager turns that flag into an OPC UA continuation
+point (see [Continuation-point paging (Events)](#continuation-point-paging-events-669)).

 ### Aggregates (Processed reads)

@@ -31,12 +31,19 @@ namespace ZB.MOM.WW.OtOpcUa.OpcUaServer;
 ///     prior pages and must be dropped from the head of the next page (tie de-dup).
 /// </param>
 /// <param name="NumValuesPerNode">The client's per-page cap; re-applied to every resumed page.</param>
+/// <param name="IsEvents">
+///     <c>true</c> for an EVENT HistoryRead cursor (Gitea #669): <paramref name="Tagname"/> then carries the
+///     registered historian event-source name and the boundary fields are over <c>EventTimeUtc</c>. A cursor
+///     is only ever resumed by the read kind that issued it, so a raw point presented to an events read
+///     (or vice versa) is a <c>BadContinuationPointInvalid</c> miss.
+/// </param>
 internal sealed record HistoryContinuationState(
    string Tagname,
    DateTime NextStartUtc,
    DateTime EndUtc,
    int BoundarySkipCount,
-    uint NumValuesPerNode);
+    uint NumValuesPerNode,
+    bool IsEvents = false);

 /// <summary>
 ///     Pure server-side continuation-point paging decisions for the count-capped variable-history arms
@@ -5086,6 +5086,7 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
    {
        // Snapshot the select clauses once — the same filter projects every node's events.
        var selectClauses = details.Filter?.SelectClauses ?? new SimpleAttributeOperandCollection();
+        var session = context.OperationContext?.Session;

        nodesToProcess = AuthorizeHistoryRead(context, nodesToProcess, results, errors);
        WithHistoryReadBatchLimiter(nodesToProcess, results, errors, () =>
@@ -5108,7 +5109,7 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
                }

                var sn = sourceName;
-                work.Add(() => ServeEventsAsync(handle, sn, details, selectClauses, results, errors, ct));
+                work.Add(() => ServeEventsAsync(handle, sn, details, selectClauses, nodesToRead, session, results, errors, ct));
            }

            RunBounded(work, HistoryReadBatchParallelism);
@@ -5134,32 +5135,120 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
        string? sourceName,
        ReadEventDetails details,
        SimpleAttributeOperandCollection selectClauses,
+        IList<HistoryReadValueId> nodesToRead,
+        ISession? session,
        IList<SdkHistoryReadResult> results,
        IList<ServiceResult> errors,
        CancellationToken ct)
    {
+        var inboundCp = nodesToRead[handle.Index].ContinuationPoint;
+        var resumed = inboundCp is { Length: > 0 };
        try
        {
-            // NOT under the node-manager Lock — awaiting the async source is safe here.
-            var sourceResult = await _historianDataSource.ReadEventsAsync(
-                sourceName,
-                details.StartTime,
-                details.EndTime,
-                // NumValuesPerNode==0 means "no limit — return ALL values" per OPC UA
-                // Part 4/11, so translate it to UNBOUNDED (int.MaxValue) here. Passing the int<=0
-                // backend-default-cap sentinel instead would silently truncate a "give me everything"
-                // events read at the backend default. A positive cap passes through (clamped).
-                EventMaxEvents(details.NumValuesPerNode),
-                ct).ConfigureAwait(false);
+            var startUtc = details.StartTime;
+            var endUtc = details.EndTime;
+            var numValuesPerNode = details.NumValuesPerNode;
+            var boundarySkip = 0;

-            var historyEvent = ProjectEvents(sourceResult.Events, selectClauses);
+            if (resumed)
+            {
+                // Single-use take. A miss (unknown / evicted / malformed / a RAW cursor) ⇒ BadContinuationPointInvalid
+                // for THIS node only.
+                var state = _historyContinuationStore.TryTake(session, inboundCp!);
+                if (state is null || !state.IsEvents)
+                {
+                    errors[handle.Index] = StatusCodes.BadContinuationPointInvalid;
+                    results[handle.Index] = new SdkHistoryReadResult { StatusCode = StatusCodes.BadContinuationPointInvalid };
+                    return;
+                }
+
+                sourceName = state.Tagname;
+                startUtc = state.NextStartUtc;
+                endUtc = state.EndUtc;
+                boundarySkip = state.BoundarySkipCount;
+                numValuesPerNode = state.NumValuesPerNode;
+            }
+
+            // Reverse event reads (start > end) are not paged: the cursor below is forward-only.
+            var forward = !(details.EndTime != DateTime.MinValue && details.StartTime > details.EndTime);
+
+            // NOT under the node-manager Lock — awaiting the async source is safe here.
+            // NumValuesPerNode==0 means "no limit — return ALL values" per OPC UA Part 4/11, so it is translated
+            // to UNBOUNDED (EventMaxEvents) rather than the backend's <=0 default-cap sentinel.
+            var sourceResult = await _historianDataSource.ReadEventsAsync(
+                sourceName, startUtc, endUtc, EventMaxEvents(numValuesPerNode), ct).ConfigureAwait(false);
+
+            // Truncation is the SOURCE's word (non-null continuation point) — never inferred from the count, so a
+            // page that happens to be exactly cap-sized but complete never emits a point.
+            var truncated = sourceResult.ContinuationPoint is not null && numValuesPerNode > 0 && forward;
+
+            IReadOnlyList<HistoricalEvent> events = sourceResult.Events;
+            if (resumed && boundarySkip > 0)
+            {
+                // Drop the events at the boundary timestamp already emitted on the prior page.
+                var i = 0;
+                while (i < events.Count && i < boundarySkip && events[i].EventTimeUtc == startUtc) i++;
+                if (i > 0) events = events.Skip(i).ToList();
+            }
+
+            byte[]? outboundCp = null;
+
+            if (truncated && resumed
+                && (events.Count == 0 || events[^1].EventTimeUtc == startUtc))
+            {
+                // Oversized tie cluster: more events share startUtc than the page cap, so (time, skip) alone
+                // cannot advance. Over-fetch the whole cluster over the half-open [T, T+resolution) window
+                // with an explicit bounded cap and page WITHIN it (same scheme as raw).
+                var resolution = Math.Max(1L, _historianDataSource.TimestampResolutionTicks);
+                var overfetchCap = (int)Math.Min(int.MaxValue - 1L, (long)Math.Max(1, MaxTieClusterOverfetch) + 1);
+                var cluster = (await _historianDataSource.ReadEventsAsync(
+                    sourceName, startUtc, startUtc.AddTicks(resolution), overfetchCap, ct)
+                    .ConfigureAwait(false)).Events;
+
+                if (cluster.Count > MaxTieClusterOverfetch)
+                {
+                    _logger.LogError(
+                        "OtOpcUaNodeManager: HistoryReadEvents tie cluster at {StartUtc:O} for source '{Source}' has {Count} events, " +
+                        "exceeding MaxTieClusterOverfetch={Max}; cannot page within it. Increase MaxTieClusterOverfetch.",
+                        startUtc, sourceName, cluster.Count, MaxTieClusterOverfetch);
+                    errors[handle.Index] = StatusCodes.BadHistoryOperationUnsupported;
+                    results[handle.Index] = new SdkHistoryReadResult { StatusCode = StatusCodes.BadHistoryOperationUnsupported };
+                    return;
+                }
+
+                HistoryPaging.SliceTieCluster(
+                    cluster.Count, boundarySkip, numValuesPerNode, startUtc, endUtc,
+                    out var sliceStart, out var sliceCount, out var nextStartUtc, out var nextSkip,
+                    reverse: false, resolutionTicks: resolution);
+
+                var slice = new List<HistoricalEvent>(sliceCount);
+                for (var i = sliceStart; i < sliceStart + sliceCount; i++) slice.Add(cluster[i]);
+                events = slice;
+
+                if (nextStartUtc is { } resumeAt)
+                {
+                    outboundCp = _historyContinuationStore.Save(session, new HistoryContinuationState(
+                        sourceName ?? string.Empty, resumeAt, endUtc, nextSkip, numValuesPerNode, IsEvents: true));
+                }
+            }
+            else if (truncated && events.Count > 0)
+            {
+                // Cursor from the last event: resume at its time INCLUSIVE, skipping the trailing ties emitted.
+                var boundary = events[^1].EventTimeUtc;
+                var skip = 0;
+                for (var i = events.Count - 1; i >= 0 && events[i].EventTimeUtc == boundary; i--) skip++;
+                outboundCp = _historyContinuationStore.Save(session, new HistoryContinuationState(
+                    sourceName ?? string.Empty, boundary, endUtc, skip, numValuesPerNode, IsEvents: true));
+            }
+
+            var historyEvent = ProjectEvents(events, selectClauses);
            results[handle.Index] = new SdkHistoryReadResult
            {
                // No events ⇒ GoodNoData (the notifier is historized, the window just held no events).
-                StatusCode = sourceResult.Events.Count == 0 ? StatusCodes.GoodNoData : StatusCodes.Good,
+                StatusCode = events.Count == 0 ? StatusCodes.GoodNoData : StatusCodes.Good,
                HistoryData = new ExtensionObject(historyEvent),
-                // We never issue continuation points — every read returns the full window in one shot.
-                ContinuationPoint = null,
+                // Null when the page was complete, or when no session is available to hold the cursor.
+                ContinuationPoint = outboundCp,
            };
            errors[handle.Index] = ServiceResult.Good;
        }
@@ -5843,7 +5932,7 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
                // Resume read: take the stored cursor. A miss (unknown / evicted / malformed point) ⇒
                // BadContinuationPointInvalid for THIS node.
                var state = _historyContinuationStore.TryTake(session, inboundCp);
-                if (state is null)
+                if (state is null || state.IsEvents)
                {
                    errors[handle.Index] = StatusCodes.BadContinuationPointInvalid;
                    results[handle.Index] = new SdkHistoryReadResult { StatusCode = StatusCodes.BadContinuationPointInvalid };
@@ -0,0 +1,107 @@
+using Opc.Ua;
+using Opc.Ua.Client;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Commons.OpcUa;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using CoreHistoryReadResult = ZB.MOM.WW.OtOpcUa.Core.Abstractions.HistoryReadResult;
+using static ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests.ServerEventRig;
+
+namespace ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests;
+
+/// <summary>
+///     Gitea #669 over a real SDK client session: an event HistoryRead capped below the available events pages
+///     through the PRODUCTION session-bound continuation store (<c>SessionHistoryContinuationStore</c>) — the
+///     path the session-less unit tests cannot reach — and the union of pages is every event exactly once.
+/// </summary>
+public sealed class EventHistoryPagingOverTheWireTests
+{
+    private static readonly DateTime Epoch = new(2026, 10, 1, 0, 0, 0, DateTimeKind.Utc);
+
+    [Fact]
+    public async Task A_capped_event_read_pages_through_the_session_store_exactly_once()
+    {
+        var ct = TestContext.Current.CancellationToken;
+        var rig = await StartAsync("EvtPagingWire", encrypted: false, ct);
+        try
+        {
+            // Source installed BEFORE the alarm materialises so the folder is promoted with the HistoryRead bit.
+            var events = Enumerable.Range(0, 25)
+                .Select(i => new HistoricalEvent($"e{i:00}", "src", Epoch.AddSeconds(i / 2), Epoch, "m", 500))
+                .ToList(); // pairs share a timestamp, so page boundaries cut through ties
+            rig.Nm.HistorianDataSource = new SqlLikeEventSource(events);
+            rig.MaterialiseNative();
+            rig.Enforce(false, [Key()]);
+
+            using var session = await rig.ConnectAsync(anonymous: true, ct);
+            var node = new NodeId(RawDeviceFolder, (ushort)session.NamespaceUris.GetIndex(V3NodeIds.RawNamespaceUri));
+
+            var collected = new List<string>();
+            byte[]? cp = null;
+            var pages = 0;
+            do
+            {
+                var (ids, next) = await ReadPageAsync(session, node, cp, ct);
+                collected.AddRange(ids);
+                cp = next;
+                (++pages).ShouldBeLessThan(30, "paging must terminate");
+            }
+            while (cp is not null);
+
+            pages.ShouldBeGreaterThan(2, "25 events at a cap of 10 must page over the wire");
+            collected.ShouldBe(events.Select(e => e.EventId));
+        }
+        finally { await rig.DisposeAsync(); }
+    }
+
+    private static async Task<(List<string> Ids, byte[]? Cp)> ReadPageAsync(
+        ISession session, NodeId node, byte[]? cp, CancellationToken ct)
+    {
+        var filter = new EventFilter();
+        filter.SelectClauses.Add(new SimpleAttributeOperand(ObjectTypeIds.BaseEventType, new QualifiedName("EventId"))
+        {
+            AttributeId = Attributes.Value,
+        });
+        var details = new ReadEventDetails
+        {
+            StartTime = Epoch.AddDays(-1),
+            EndTime = Epoch.AddDays(1),
+            NumValuesPerNode = 10,
+            Filter = filter,
+        };
+        var response = await session.HistoryReadAsync(
+            null, new ExtensionObject(details), TimestampsToReturn.Both, false,
+            new HistoryReadValueIdCollection { new HistoryReadValueId { NodeId = node, ContinuationPoint = cp } }, ct);
+        var result = response.Results.ShouldHaveSingleItem();
+        StatusCode.IsBad(result.StatusCode).ShouldBeFalse($"HistoryRead failed: {result.StatusCode}");
+        var he = (HistoryEvent)ExtensionObject.ToEncodeable(result.HistoryData);
+        var ids = he.Events.Select(e => System.Text.Encoding.UTF8.GetString((byte[])e.EventFields[0].Value)).ToList();
+        return (ids, result.ContinuationPoint is { Length: > 0 } c ? c : null);
+    }
+
+    /// <summary>(time, id) order, [start, end), cap + truncation flag — the SQL backends' contract.</summary>
+    private sealed class SqlLikeEventSource(List<HistoricalEvent> events) : IHistorianDataSource
+    {
+        private readonly List<HistoricalEvent> _all =
+            events.OrderBy(e => e.EventTimeUtc).ThenBy(e => e.EventId, StringComparer.Ordinal).ToList();
+
+        public async Task<HistoricalEventsResult> ReadEventsAsync(
+            string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents, CancellationToken ct)
+        {
+            await Task.Delay(1, ct).ConfigureAwait(false); // complete on a pool thread, like a real backend
+            var window = _all.Where(e => e.EventTimeUtc >= startUtc && e.EventTimeUtc < endUtc).ToList();
+            var cap = maxEvents <= 0 ? 1000 : maxEvents;
+            return new HistoricalEventsResult(window.Take(cap).ToList(), window.Count > cap ? new byte[] { 1 } : null);
+        }
+
+        public Task<CoreHistoryReadResult> ReadRawAsync(string r, DateTime s, DateTime e, uint m, CancellationToken ct) =>
+            NullHistorianDataSource.Instance.ReadRawAsync(r, s, e, m, ct);
+        public Task<CoreHistoryReadResult> ReadProcessedAsync(
+            string r, DateTime s, DateTime e, TimeSpan i, HistoryAggregateType a, CancellationToken ct) =>
+            NullHistorianDataSource.Instance.ReadProcessedAsync(r, s, e, i, a, ct);
+        public Task<CoreHistoryReadResult> ReadAtTimeAsync(string r, IReadOnlyList<DateTime> t, CancellationToken ct) =>
+            NullHistorianDataSource.Instance.ReadAtTimeAsync(r, t, ct);
+        public HistorianHealthSnapshot GetHealthSnapshot() => NullHistorianDataSource.Instance.GetHealthSnapshot();
+        public void Dispose() { }
+    }
+}
@@ -0,0 +1,258 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using Opc.Ua;
+using Opc.Ua.Server;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Commons.OpcUa;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using HistorianRead = ZB.MOM.WW.OtOpcUa.Core.Abstractions.HistoryReadResult;
+using SdkHistoryReadResult = Opc.Ua.HistoryReadResult;
+
+namespace ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests;
+
+/// <summary>
+///     Gitea #669 — server-side continuation-point paging for HistoryRead-Events. Same harness as
+///     <see cref="NodeManagerHistoryReadPagingTests"/>: a real SDK server, a session-less in-process
+///     HistoryRead and an <see cref="InMemoryHistoryContinuationStore"/>, over a fake source that behaves like
+///     the SQL backends (ordered by (time, id), half-open window, cap + truncation flag).
+/// </summary>
+public sealed class NodeManagerHistoryReadEventsPagingTests : IDisposable
+{
+    private static CancellationToken Ct => TestContext.Current.CancellationToken;
+    private static readonly DateTime Epoch = new(2026, 1, 1, 0, 0, 0, DateTimeKind.Utc);
+    private readonly string _pkiRoot = Path.Combine(Path.GetTempPath(), $"otopcua-eventpaging-{Guid.NewGuid():N}");
+
+    [Fact]
+    public async Task Events_page_across_continuation_points_exactly_once_in_order()
+    {
+        var (host, nm, node, src) = await SetupAsync(Distinct(250));
+        var ids = DrainAsync(nm, node, max: 100);
+        ids.ShouldBe(src.All.Select(e => e.EventId));
+        src.Calls.ShouldBeGreaterThan(2);
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task Events_tie_at_page_boundary_is_neither_duplicated_nor_skipped()
+    {
+        // Indices 2,3,4 share a timestamp; cap 4 cuts through the cluster.
+        var events = new List<HistoricalEvent>
+        {
+            Ev("e0", 0), Ev("e1", 1), Ev("e2", 2), Ev("e3", 2), Ev("e4", 2), Ev("e5", 3),
+        };
+        var (host, nm, node, _) = await SetupAsync(events);
+        DrainAsync(nm, node, max: 4).ShouldBe(["e0", "e1", "e2", "e3", "e4", "e5"]);
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task Events_tie_cluster_larger_than_the_cap_pages_within_the_cluster()
+    {
+        var events = new List<HistoricalEvent> { Ev("a0", 0) };
+        for (var i = 0; i < 7; i++) events.Add(Ev($"t{i}", 5));
+        events.Add(Ev("z0", 9));
+        var (host, nm, node, _) = await SetupAsync(events);
+        DrainAsync(nm, node, max: 3).ShouldBe(
+            ["a0", "t0", "t1", "t2", "t3", "t4", "t5", "t6", "z0"]);
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task Events_tie_cluster_beyond_the_overfetch_bound_fails_loudly()
+    {
+        var events = new List<HistoricalEvent>();
+        for (var i = 0; i < 10; i++) events.Add(Ev($"t{i:00}", 5));
+        var (host, nm, node, _) = await SetupAsync(events);
+        nm.MaxTieClusterOverfetch = 4;
+
+        var p1 = ReadEvents(nm, node, max: 3, inboundCp: null);
+        p1.Cp.ShouldNotBeNull();
+        var p2 = ReadEvents(nm, node, max: 3, inboundCp: p1.Cp);
+        p2.Error.StatusCode.Code.ShouldBe(StatusCodes.BadHistoryOperationUnsupported);
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task Events_unknown_continuation_point_is_BadContinuationPointInvalid()
+    {
+        var (host, nm, node, _) = await SetupAsync(Distinct(5));
+        var r = ReadEvents(nm, node, max: 2, inboundCp: Guid.NewGuid().ToByteArray());
+        r.Error.StatusCode.Code.ShouldBe(StatusCodes.BadContinuationPointInvalid);
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task Events_release_drops_the_cursor()
+    {
+        var (host, nm, node, _) = await SetupAsync(Distinct(10));
+        var p1 = ReadEvents(nm, node, max: 4, inboundCp: null);
+        p1.Cp.ShouldNotBeNull();
+        InvokeRelease(nm, node, p1.Cp!);
+        ReadEvents(nm, node, max: 4, inboundCp: p1.Cp).Error.StatusCode.Code
+            .ShouldBe(StatusCodes.BadContinuationPointInvalid);
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task Events_cap_sized_complete_page_emits_no_continuation_point()
+    {
+        var (host, nm, node, _) = await SetupAsync(Distinct(4));
+        var r = ReadEvents(nm, node, max: 4, inboundCp: null);
+        r.Ids.Count.ShouldBe(4);
+        r.Cp.ShouldBeNull("the source did not flag truncation, so a cap-sized page is not a paged one");
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task Events_unbounded_request_never_pages()
+    {
+        var (host, nm, node, _) = await SetupAsync(Distinct(300));
+        var r = ReadEvents(nm, node, max: 0, inboundCp: null);
+        r.Ids.Count.ShouldBe(300);
+        r.Cp.ShouldBeNull();
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task A_raw_cursor_presented_to_an_events_read_is_invalid()
+    {
+        var (host, nm, node, _) = await SetupAsync(Distinct(10));
+        var raw = nm.HistoryContinuationStore.Save(null, new HistoryContinuationState("t", Epoch, Epoch.AddDays(1), 0, 4));
+        ReadEvents(nm, node, max: 4, inboundCp: raw).Error.StatusCode.Code
+            .ShouldBe(StatusCodes.BadContinuationPointInvalid);
+        await host.DisposeAsync();
+    }
+
+    // --- helpers ---------------------------------------------------------------------------------
+
+    private static HistoricalEvent Ev(string id, int seconds) =>
+        new(id, "src", Epoch.AddSeconds(seconds), Epoch.AddSeconds(seconds), "m", 500);
+
+    private static List<HistoricalEvent> Distinct(int n) =>
+        Enumerable.Range(0, n).Select(i => Ev($"e{i:0000}", i)).ToList();
+
+    private async Task<(OpcUaApplicationHost Host, OtOpcUaNodeManager Nm, NodeId Node, SqlLikeEventSource Src)> SetupAsync(
+        List<HistoricalEvent> events)
+    {
+        var applicationUri = $"urn:OtOpcUa.EventPagingTest:{Guid.NewGuid():N}";
+        var started = await OpcUaHostStart.StartAsync(
+            port => new OpcUaApplicationHost(
+                new OpcUaApplicationHostOptions
+                {
+                    ApplicationName = "OtOpcUa.EventPagingTest",
+                    ApplicationUri = applicationUri,
+                    OpcUaPort = port,
+                    PublicHostname = "localhost",
+                    PkiStoreRoot = _pkiRoot,
+                },
+                NullLogger<OpcUaApplicationHost>.Instance),
+            () => new OtOpcUaSdkServer(), Ct);
+        var nm = started.Server.NodeManager!;
+        nm.HistoryContinuationStore = new InMemoryHistoryContinuationStore();
+        var src = new SqlLikeEventSource(events);
+        nm.HistorianDataSource = src;
+
+        const string equipmentId = "eq-evt";
+        nm.EnsureFolder(equipmentId, parentNodeId: null, displayName: "Equipment", realm: AddressSpaceRealm.Uns);
+        nm.MaterialiseAlarmCondition("alarm-1", equipmentId, "HighTemp", "OffNormalAlarm", severity: 700, realm: AddressSpaceRealm.Uns);
+        return (started.Host, nm, nm.TryGetFolder(equipmentId)!.NodeId, src);
+    }
+
+    private static List<string> DrainAsync(OtOpcUaNodeManager nm, NodeId node, uint max)
+    {
+        var all = new List<string>();
+        byte[]? cp = null;
+        var pages = 0;
+        do
+        {
+            var r = ReadEvents(nm, node, max, cp);
+            r.Error.StatusCode.Code.ShouldBe(StatusCodes.Good);
+            all.AddRange(r.Ids);
+            cp = r.Cp;
+            (++pages).ShouldBeLessThan(60, "paging must terminate");
+        }
+        while (cp is not null);
+        return all;
+    }
+
+    private static (List<string> Ids, ServiceResult Error, byte[]? Cp) ReadEvents(
+        OtOpcUaNodeManager nm, NodeId node, uint max, byte[]? inboundCp)
+    {
+        var context = new OperationContext(
+            new RequestHeader(), secureChannelContext: null, RequestType.HistoryRead, identity: null);
+        var filter = new EventFilter();
+        filter.SelectClauses.Add(new SimpleAttributeOperand(ObjectTypeIds.BaseEventType, new QualifiedName("EventId"))
+        {
+            AttributeId = Attributes.Value,
+        });
+        var details = new ReadEventDetails
+        {
+            StartTime = Epoch.AddDays(-1),
+            EndTime = Epoch.AddDays(1),
+            NumValuesPerNode = max,
+            Filter = filter,
+        };
+        var nodesToRead = new List<HistoryReadValueId> { new() { NodeId = node, ContinuationPoint = inboundCp } };
+        var results = new List<SdkHistoryReadResult> { null! };
+        var errors = new List<ServiceResult> { null! };
+        nm.HistoryRead(context, details, TimestampsToReturn.Both, false, nodesToRead, results, errors);
+
+        var ids = new List<string>();
+        if (results[0]?.HistoryData is { } ho && !ExtensionObject.IsNull(ho))
+        {
+            var he = (HistoryEvent)ExtensionObject.ToEncodeable(ho);
+            foreach (var e in he.Events)
+                ids.Add(System.Text.Encoding.UTF8.GetString((byte[])e.EventFields[0].Value));
+        }
+
+        return (ids, errors[0], results[0]?.ContinuationPoint);
+    }
+
+    private static void InvokeRelease(OtOpcUaNodeManager nm, NodeId node, byte[] cp)
+    {
+        var context = new OperationContext(
+            new RequestHeader(), secureChannelContext: null, RequestType.HistoryRead, identity: null);
+        var nodesToRead = new List<HistoryReadValueId> { new() { NodeId = node, ContinuationPoint = cp } };
+        var results = new List<SdkHistoryReadResult> { null! };
+        var errors = new List<ServiceResult> { null! };
+        nm.HistoryRead(context, new ReadEventDetails { StartTime = Epoch, EndTime = Epoch.AddDays(1), NumValuesPerNode = 4 },
+            TimestampsToReturn.Both, true, nodesToRead, results, errors);
+    }
+
+    /// <summary>Behaves like the SqlServer/QuestDB event reads: (time, id) order, [start, end), cap, truncation flag.</summary>
+    private sealed class SqlLikeEventSource(List<HistoricalEvent> events) : IHistorianDataSource
+    {
+        public IReadOnlyList<HistoricalEvent> All { get; } =
+            events.OrderBy(e => e.EventTimeUtc).ThenBy(e => e.EventId, StringComparer.Ordinal).ToList();
+        public int Calls { get; private set; }
+
+        public Task<HistoricalEventsResult> ReadEventsAsync(
+            string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents, CancellationToken ct)
+        {
+            Calls++;
+            var window = All.Where(e => e.EventTimeUtc >= startUtc && e.EventTimeUtc < endUtc).ToList();
+            var cap = maxEvents <= 0 ? 1000 : maxEvents;
+            var truncated = window.Count > cap;
+            return Task.FromResult(new HistoricalEventsResult(
+                window.Take(cap).ToList(), truncated ? new byte[] { 1 } : null));
+        }
+
+        public Task<HistorianRead> ReadRawAsync(string r, DateTime s, DateTime e, uint m, CancellationToken ct) =>
+            NullHistorianDataSource.Instance.ReadRawAsync(r, s, e, m, ct);
+        public Task<HistorianRead> ReadProcessedAsync(
+            string r, DateTime s, DateTime e, TimeSpan i, HistoryAggregateType a, CancellationToken ct) =>
+            NullHistorianDataSource.Instance.ReadProcessedAsync(r, s, e, i, a, ct);
+        public Task<HistorianRead> ReadAtTimeAsync(string r, IReadOnlyList<DateTime> t, CancellationToken ct) =>
+            NullHistorianDataSource.Instance.ReadAtTimeAsync(r, t, ct);
+        public HistorianHealthSnapshot GetHealthSnapshot() => NullHistorianDataSource.Instance.GetHealthSnapshot();
+        public void Dispose() { }
+    }
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_pkiRoot))
+        {
+            try { Directory.Delete(_pkiRoot, recursive: true); } catch { /* best effort */ }
+        }
+    }
+}