feat(historian): #675 reverse HistoryRead-Events paging + end-only backward reads

- IHistorianDataSource.ReadEventsAsync(..., bool reverse, ...) overload: reverse = Part 11 window (lo, hi] newest-first by (time, id) DESC. Default forwards forward reads and throws NotSupportedException for reverse (Null); Gateway refuses explicitly (ascending-only stream). Server answers BadHistoryOperationUnsupported instead of silently returning the earliest N ascending. - SqlServer / QuestDB serve reverse natively (ORDER BY time DESC, id DESC + TOP/LIMIT). - Node manager: reverse event reads page with continuation points; direction stored in the cursor (HistoryContinuationState.Reverse) so a resume continues in the cursor's direction; reverse tie clusters over-fetched forward and sliced from the newest end (SliceTieCluster(reverse:)); end-only reads with a cap page backward from EndTime (mirrors raw). - Resumed event pages over-fetch by the boundary skip, so every non-final page is a full cap page. - Tests: node-manager reverse paging (ties, clusters, coarse resolution, overfetch bound, resume direction both ways, end-only, unsupported backend); shared EventReadConformance on SqlServer + QuestDB; over-the-wire reverse paging; offline Gateway pin of the UTC DateTime.MaxValue bound through the published client's proto conversion (in-process gRPC capture) and FILETIME. - docs/Historian.md event paging section.
2026-10-11 11:19:54 -04:00
parent 825393299e
commit 8d154c6a81
20 changed files with 752 additions and 60 deletions
@@ -630,13 +630,31 @@ says is complete carries no continuation point. `NumValuesPerNode == 0` stays un

 - 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.
+  the last event's `EventTimeUtc` **inclusive** and drops the events at that timestamp already emitted — and asks the source for
+  that many more rows than the cap, so every page short of the last is a FULL `NumValuesPerNode` page (#675; resumed pages used
+  to come back short by the skip).
 - 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.
+- **Reverse event reads page newest-first (#675).** `StartTime > EndTime` reads the Part 11 reverse window `(EndTime, StartTime]`
+  with events in DESCENDING `(EventTimeUtc, EventId)` order, so a cap keeps the **latest** N and a continuation point is issued
+  when the source flags more. The direction is stored **in the cursor**: a reverse point resumes reverse (and a forward one
+  forward) whatever the resume request's details say. Reverse tie clusters are over-fetched forward and sliced from the newest
+  end, exactly as Raw does. Before #675 a capped reverse read silently returned the EARLIEST N, ascending, Good, with no
+  continuation point.
+- **End-only events reads read backward (#675).** `StartTime` unset (`DateTime.MinValue`) with an `EndTime` and
+  `NumValuesPerNode > 0` pages backward from `EndTime` over `(MinValue, EndTime]` — the same rule as Raw. Start-only reads still
+  page forward to an open upper bound (`DateTime.MaxValue`, UTC). Without a cap an end-only read keeps its forward
+  `[MinValue, EndTime)` meaning.
+- **Backends.** The server calls `IHistorianDataSource.ReadEventsAsync(…, bool reverse, …)`. SqlServer and QuestDB serve reverse
+  natively (`… > lo AND … <= hi ORDER BY time DESC, id DESC`). **Gateway and Null refuse a reverse read** (the interface default
+  throws `NotSupportedException`) and the node answers `BadHistoryOperationUnsupported` — the gateway streams ascending only and
+  rejects `end < start`, and draining the whole window to keep its tail would be exactly the unbounded read the cap prevents.
+- **Gateway open upper bound.** The `DateTime.MaxValue` (UTC) bound of a start-only forward read is pinned offline through the
+  published client's proto `Timestamp` conversion (captured by an in-process gRPC server) and the FILETIME encoding
+  (`GatewayEventBoundsTests`). What a live AVEVA Historian does with a year-9999 bound is still unverified (needs the VPN).
 - No session on the request (in-process callers) degrades to single-shot, as for Raw.

 ### Modified-value history (Part 11)
@@ -837,8 +855,10 @@ result never has both.

 ### Events

-`[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
+`[start, end)` by `EventTimeUtc`; through the five-argument read **a reversed window (`end < start`) is swapped and returned
+ascending**. The direction-aware overload with `reverse: true` (what the node manager calls for a reverse read, #675) reads
+`(lo, hi]` ordered `(EventTimeUtc, EventId)` **descending** — the clustered index scanned backwards. A null `sourceName` means all
+sources, otherwise `SourceName = @source`. Forward is 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 turns that flag into an OPC UA continuation
 point (see [Continuation-point paging (Events)](#continuation-point-paging-events-669)).

@@ -1179,7 +1199,8 @@ Every query filters `NOT deleted`; every value goes through `StoredSample`, so t
  from the last timestamp ± `TimestampResolutionTicks`.
 - **AtTime:** previous-or-equal and next neighbour of each time by `LIMIT 1` seeks, `UNION ALL`-ed, **50 times per statement** (QuestDB 10.0.1 refuses
  more than 126 branches). `AtTimeEvaluator` decides each value, as on `SqlServer`.
- **Events:** `[start, end)` by `ts`, ordered `(ts, event_id)`, `maxEvents + 1` fetched to produce the continuation point.
+- **Events:** `[start, end)` by `ts`, ordered `(ts, event_id)`, `maxEvents + 1` fetched to produce the continuation point; a
+  reverse read (#675) is `(lo, hi]` ordered `(ts, event_id)` descending.
 - **Processed:** one query per node; bucket index `(cast(ts AS LONG) * 10 - startUnixTicks) / intervalTicks` (never `SAMPLE BY`, whose alignment
  differs), so bucket edges are identical to `SqlServer` (start-inclusive, end-exclusive, partial last bucket, interval 0 ⇒ one bucket, 100,000-bucket cap).
  Min/max times use `row_number()` windows with the earliest-timestamp tie-break.
@@ -88,7 +88,7 @@ public sealed class HistoryReadLimitExceededException : InvalidOperationExceptio
 }

 /// <summary>
-///     One row returned by <see cref="IHistorianDataSource.ReadEventsAsync"/> — a historical
+///     One row returned by <see cref="IHistorianDataSource.ReadEventsAsync(string?, DateTime, DateTime, int, CancellationToken)"/> — a historical
 ///     alarm/event record, not the OPC UA live-event stream. Fields match the minimum set the
 ///     Server needs to populate a <c>HistoryEventFieldList</c> for HistoryReadEvents responses.
 /// </summary>
@@ -106,8 +106,8 @@ public sealed record HistoricalEvent(
    string? Message,
    ushort Severity);

-/// <summary>Result of a <see cref="IHistorianDataSource.ReadEventsAsync"/> call.</summary>
-/// <param name="Events">Events in chronological order by <c>EventTimeUtc</c>.</param>
+/// <summary>Result of a <see cref="IHistorianDataSource.ReadEventsAsync(string?, DateTime, DateTime, int, CancellationToken)"/> call.</summary>
+/// <param name="Events">Events ordered by <c>EventTimeUtc</c> in the read's direction — ascending, or descending for a reverse read (#675).</param>
 /// <param name="ContinuationPoint">Opaque token for the next call when more events are available; null when complete.</param>
 public sealed record HistoricalEventsResult(
    IReadOnlyList<HistoricalEvent> Events,
@@ -79,7 +79,7 @@ public interface IHistorianDataSource : IDisposable
    ///     <c>uint maxValuesPerNode</c> because their OPC UA HistoryRead surface has no
    ///     equivalent "use default" sentinel.
    ///
-    ///     This surface declares <see cref="ReadAtTimeAsync"/> and <see cref="ReadEventsAsync"/>
+    ///     This surface declares <see cref="ReadAtTimeAsync"/> and <see cref="ReadEventsAsync(string?, DateTime, DateTime, int, CancellationToken)"/>
    ///     as required members — a server-side historian owns the full read surface.
    /// </remarks>
    /// <param name="sourceName">The source name to filter events, or null to return events from all sources.</param>
@@ -101,6 +101,46 @@ public interface IHistorianDataSource : IDisposable
        int maxEvents,
        CancellationToken cancellationToken);

+    /// <summary>
+    ///     Read historical alarm / event records in a chosen direction — the overload the server's HistoryReadEvents
+    ///     paging calls (Gitea #675).
+    /// </summary>
+    /// <remarks>
+    ///     <para>
+    ///         The bounds may be given in either order; let <c>lo</c>/<c>hi</c> be the earlier/later of them. Forward
+    ///         (<paramref name="reverse"/> <c>false</c>) is exactly <see cref="ReadEventsAsync(string?, DateTime, DateTime, int, CancellationToken)"/>:
+    ///         <c>[lo, hi)</c> ascending by <c>(EventTimeUtc, EventId)</c>. Reverse is the Part 11 reverse window
+    ///         <c>(lo, hi]</c> in DESCENDING <c>(EventTimeUtc, EventId)</c> order, so a cap keeps the LATEST
+    ///         <paramref name="maxEvents"/> events — the mirror of forward, with the same tie-breaker reversed, so a
+    ///         resumed reverse page that re-reads its boundary timestamp sees the boundary ties in the same order.
+    ///         <paramref name="maxEvents"/> and the continuation-point contract are as for the forward read.
+    ///     </para>
+    ///     <para>
+    ///         The default forwards a forward read to the five-argument overload and throws
+    ///         <see cref="NotSupportedException"/> for a reverse one — a backend that cannot read newest-first (Gateway,
+    ///         Null) — which the server reports as <c>BadHistoryOperationUnsupported</c> rather than returning the
+    ///         earliest events in ascending order as if they were the answer.
+    ///     </para>
+    /// </remarks>
+    /// <param name="sourceName">The source name to filter events, or null to return events from all sources.</param>
+    /// <param name="startUtc">One bound of the window (UTC).</param>
+    /// <param name="endUtc">The other bound of the window (UTC).</param>
+    /// <param name="maxEvents">The cap; non-positive uses the backend default (see the five-argument overload).</param>
+    /// <param name="reverse"><c>true</c> to read newest-first over <c>(lo, hi]</c>.</param>
+    /// <param name="cancellationToken">Cancellation.</param>
+    /// <returns>The matching events in the read's direction, with a continuation point if truncated.</returns>
+    /// <exception cref="NotSupportedException">A reverse read on a backend that cannot read newest-first.</exception>
+    Task<HistoricalEventsResult> ReadEventsAsync(
+        string? sourceName,
+        DateTime startUtc,
+        DateTime endUtc,
+        int maxEvents,
+        bool reverse,
+        CancellationToken cancellationToken) =>
+        reverse
+            ? throw new NotSupportedException("This historian backend cannot read events newest-first (reverse).")
+            : ReadEventsAsync(sourceName, startUtc, endUtc, maxEvents, cancellationToken);
+
    /// <summary>
    ///     Point-in-time health snapshot for diagnostics and dashboards. Pure
    ///     observation; never blocks on backend I/O.
@@ -125,6 +125,21 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
        }
    }

+    /// <inheritdoc />
+    /// <remarks>
+    ///     A reverse (newest-first) read is refused with <see cref="NotSupportedException"/> — the server reports
+    ///     <c>BadHistoryOperationUnsupported</c> (Gitea #675). The gateway's <c>ReadEvents</c> streams ascending only and
+    ///     rejects <c>end &lt; start</c>, so the only way to serve "the latest N" would be to drain the whole window and
+    ///     keep its tail, an unbounded read the cap exists to prevent. Nothing is sent.
+    /// </remarks>
+    public Task<HistoricalEventsResult> ReadEventsAsync(
+        string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents, bool reverse,
+        CancellationToken cancellationToken) =>
+        reverse
+            ? throw new NotSupportedException(
+                "The HistorianGateway backend reads events oldest-first only; a reverse (newest-first) events read is not supported.")
+            : ReadEventsAsync(sourceName, startUtc, endUtc, maxEvents, cancellationToken);
+
    /// <inheritdoc />
    public async Task<HistoricalEventsResult> ReadEventsAsync(
        string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents,
@@ -361,8 +361,18 @@ public sealed class QuestDbHistorianDataSource : IHistorianDataSource
    ///     <c>maxEvents + 1</c> rows are fetched; when the extra row arrives it is dropped and the continuation point is
    ///     the UTF-8 of <see cref="EventContinuation.Encode"/> for that first not-returned event.
    /// </remarks>
+    public Task<HistoricalEventsResult> ReadEventsAsync(
+        string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents, CancellationToken cancellationToken) =>
+        ReadEventsAsync(sourceName, startUtc, endUtc, maxEvents, reverse: false, cancellationToken);
+
+    /// <inheritdoc />
+    /// <remarks>
+    ///     Forward is the five-argument read. Reverse (Gitea #675) is the (rounded) <c>(lo, hi]</c> ordered by
+    ///     <c>(ts, event_id)</c> DESCENDING, so <c>LIMIT</c> keeps the latest events; truncation and the continuation point
+    ///     work as forward.
+    /// </remarks>
    public async Task<HistoricalEventsResult> ReadEventsAsync(
-        string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents, CancellationToken cancellationToken)
+        string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents, bool reverse, CancellationToken cancellationToken)
    {
        // No stored event can carry a source longer than the limit, so an over-long filter matches nothing.
        if (sourceName is not null && sourceName.Length > HistorianTagNames.MaxLength)
@@ -376,9 +386,9 @@ public sealed class QuestDbHistorianDataSource : IHistorianDataSource
            var bounded = maxEvents > 0 && maxEvents < int.MaxValue;
            var sql =
                $"SELECT event_id, source, ts, received_utc, message, severity FROM {_conn.EventTable} " +
-                "WHERE ts >= $1 AND ts < $2" +
+                (reverse ? "WHERE ts > $1 AND ts <= $2" : "WHERE ts >= $1 AND ts < $2") +
                (sourceName is null ? "" : " AND source = $3") +
-                " ORDER BY ts ASC, event_id ASC" +
+                (reverse ? " ORDER BY ts DESC, event_id DESC" : " ORDER BY ts ASC, event_id ASC") +
                (bounded ? (sourceName is null ? " LIMIT $3" : " LIMIT $4") : "");

            await using var conn = await _conn.OpenAsync(cancellationToken).ConfigureAwait(false);
@@ -314,8 +314,18 @@ public sealed class SqlServerHistorianDataSource : IHistorianDataSource
    ///     of <c>"{EventTimeUtc.Ticks}|{EventId}"</c> of that first not-returned event (opaque to callers; only
    ///     its presence is contractual).
    /// </remarks>
+    public Task<HistoricalEventsResult> ReadEventsAsync(
+        string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents, CancellationToken cancellationToken) =>
+        ReadEventsAsync(sourceName, startUtc, endUtc, maxEvents, reverse: false, cancellationToken);
+
+    /// <inheritdoc />
+    /// <remarks>
+    ///     Forward is the five-argument read. Reverse (Gitea #675) is <c>(lo, hi]</c> ordered by
+    ///     <c>(EventTimeUtc, EventId)</c> DESCENDING — the clustered index scanned backwards — so <c>TOP</c> keeps the latest
+    ///     events; truncation and the continuation point work as forward.
+    /// </remarks>
    public async Task<HistoricalEventsResult> ReadEventsAsync(
-        string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents, CancellationToken cancellationToken)
+        string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents, bool reverse, CancellationToken cancellationToken)
    {
        // No stored event can carry a source longer than the column (the alarm writer dead-letters it rather than
        // truncating), so an over-long filter matches nothing — never a truncated prefix of someone else's source.
@@ -328,9 +338,10 @@ public sealed class SqlServerHistorianDataSource : IHistorianDataSource
            var bounded = maxEvents > 0 && maxEvents < int.MaxValue;
            var sql =
                $"SELECT {(bounded ? "TOP (@take) " : "")}EventId, SourceName, EventTimeUtc, ReceivedTimeUtc, Message, Severity " +
-                $"FROM {_conn.Q("Event")} WHERE EventTimeUtc >= @lo AND EventTimeUtc < @hi" +
+                $"FROM {_conn.Q("Event")} WHERE " +
+                (reverse ? "EventTimeUtc > @lo AND EventTimeUtc <= @hi" : "EventTimeUtc >= @lo AND EventTimeUtc < @hi") +
                (sourceName is null ? "" : " AND SourceName = @source") +
-                " ORDER BY EventTimeUtc ASC, EventId ASC";
+                (reverse ? " ORDER BY EventTimeUtc DESC, EventId DESC" : " ORDER BY EventTimeUtc ASC, EventId ASC");

            await using var conn = await _conn.OpenAsync(cancellationToken).ConfigureAwait(false);
            await using var cmd = _conn.CreateCommand(conn, sql);
@@ -46,6 +46,11 @@ namespace ZB.MOM.WW.OtOpcUa.OpcUaServer;
 ///     <see cref="HistoryCursorKind.Modified"/> only: the normalised window start, so a resumed page keeps the original
 ///     window and direction (the modified read resumes by key, not by moving the window start).
 /// </param>
+/// <param name="Reverse">
+///     <see cref="HistoryCursorKind.Events"/> only (Gitea #675): the read is newest-first over
+///     <c>(EndUtc, NextStartUtc]</c>. A resumed events read takes its direction from here, never from the resume
+///     request's details. Raw keeps deriving it from the window (<c>NextStartUtc &gt; EndUtc</c>).
+/// </param>
 internal sealed record HistoryContinuationState(
    string Tagname,
    DateTime NextStartUtc,
@@ -54,7 +59,8 @@ internal sealed record HistoryContinuationState(
    uint NumValuesPerNode,
    HistoryCursorKind Kind = HistoryCursorKind.Raw,
    long LastModifiedId = 0,
-    DateTime WindowStartUtc = default);
+    DateTime WindowStartUtc = default,
+    bool Reverse = false);

 /// <summary>The HistoryRead kind that issued a <see cref="HistoryContinuationState"/>.</summary>
 internal enum HistoryCursorKind
@@ -102,7 +102,7 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
    /// <see cref="_alarmConditions"/> / <see cref="_notifierFolders"/>.</summary>
    private readonly Dictionary<RealmKey, AlarmNotifierWiring> _alarmNotifierWiring = new();
    /// <summary>Phase C: event-notifier folder NodeId-identifier → the event-history source
-    /// name passed to <see cref="IHistorianDataSource.ReadEventsAsync"/>. The equipment-folder NodeId
+    /// name passed to <see cref="IHistorianDataSource.ReadEventsAsync(string?, DateTime, DateTime, int, CancellationToken)"/>. The equipment-folder NodeId
    /// identifier IS the equipment id, which IS the sourceName, so key and value are the same string;
    /// the map's presence (not its value) is what makes a folder an event-history source. Populated by
    /// <see cref="EnsureFolderIsEventNotifier"/> only when a real historian is wired at promotion time,
@@ -5153,6 +5153,7 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
            var endUtc = details.EndTime;
            var numValuesPerNode = details.NumValuesPerNode;
            var boundarySkip = 0;
+            bool reverse;

            if (resumed)
            {
@@ -5172,36 +5173,59 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
                endUtc = state.EndUtc;
                boundarySkip = state.BoundarySkipCount;
                numValuesPerNode = state.NumValuesPerNode;
+                // The direction is the CURSOR's (Gitea #675), never re-derived from the resume request's details: a
+                // reverse read resumes reverse, a forward one forward.
+                reverse = state.Reverse;
            }
+            else
+            {
+                // Part 11: with one bound unset, an unset EndTime (MinValue) with a StartTime means "page FORWARD from
+                // StartTime" — normalise it to an open upper bound, or the cursor would store MinValue and the read would
+                // look reversed. An unset StartTime with an EndTime and a count cap means "page BACKWARD from EndTime"
+                // (#675, mirroring raw): the reverse window (MinValue, EndTime]. Without a count cap an end-only read
+                // keeps its historical forward [MinValue, EndTime) meaning.
+                if (endUtc == DateTime.MinValue && startUtc != DateTime.MinValue)
+                    endUtc = new DateTime(DateTime.MaxValue.Ticks, DateTimeKind.Utc); // Utc kind: a proto Timestamp conversion rejects Unspecified
+                else if (numValuesPerNode > 0 && startUtc == DateTime.MinValue && endUtc != DateTime.MinValue)
+                    (startUtc, endUtc) = (endUtc, new DateTime(0, DateTimeKind.Utc));

-            // Part 11: an unset EndTime (MinValue) with a StartTime means "page forward from StartTime". Normalise it
-            // to an open upper bound, or the cursor would store MinValue and a backend would swap the reversed
-            // resume window. A resumed read is always forward (the cursor is only ever stored for a forward read),
-            // so its direction comes from the cursor, not from the resume request's details.
-            if (!resumed && endUtc == DateTime.MinValue && startUtc != DateTime.MinValue)
-                endUtc = new DateTime(DateTime.MaxValue.Ticks, DateTimeKind.Utc); // Utc kind: a proto Timestamp conversion rejects Unspecified
-            // Reverse event reads (start > end) are not paged: the cursor below is forward-only.
-            var forward = resumed || startUtc <= endUtc;
+                // Part 11: StartTime later than EndTime is a reverse read — events newest-first over (end, start].
+                reverse = startUtc > endUtc;
+            }

            // 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.
+            // to UNBOUNDED (EventMaxEvents) rather than the backend's <=0 default-cap sentinel. A resumed page re-reads its
+            // boundary timestamp inclusively and then drops the `boundarySkip` ties already emitted, so it asks for that
+            // many more rows than the cap — otherwise every resumed page would come back short by the skip.
+            var fetch = EventMaxEvents(numValuesPerNode);
+            if (numValuesPerNode > 0 && boundarySkip > 0)
+                fetch = (int)Math.Min(int.MaxValue - 1L, (long)fetch + boundarySkip);
            var sourceResult = await _historianDataSource.ReadEventsAsync(
-                sourceName, startUtc, endUtc, EventMaxEvents(numValuesPerNode), ct).ConfigureAwait(false);
+                sourceName, startUtc, endUtc, fetch, reverse, 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;
+            var truncated = sourceResult.ContinuationPoint is not null && numValuesPerNode > 0;

            IReadOnlyList<HistoricalEvent> events = sourceResult.Events;
            if (resumed && boundarySkip > 0)
            {
-                // Drop the events at the boundary timestamp already emitted on the prior page.
+                // Drop the events at the boundary timestamp already emitted on the prior page. The resumed read is in
+                // the cursor's direction, so in both directions those ties lead the page.
                var i = 0;
                while (i < events.Count && i < boundarySkip && events[i].EventTimeUtc == startUtc) i++;
                if (i > 0) events = events.Skip(i).ToList();
            }

+            // The skip over-fetch can leave more than the cap (fewer boundary ties came back than were skipped — pruned
+            // between pages): keep the cap, and the dropped tail means the read continues.
+            if (numValuesPerNode > 0 && events.Count > numValuesPerNode)
+            {
+                events = events.Take((int)Math.Min(int.MaxValue, numValuesPerNode)).ToList();
+                truncated = true;
+            }
+
            // The cursor to store, if any. Saved only AFTER the page is projected, so a projection throw can
            // never orphan a stored cursor.
            HistoryContinuationState? pendingCursor = null;
@@ -5214,10 +5238,14 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
                // 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)
+                IReadOnlyList<HistoricalEvent> cluster = (await _historianDataSource.ReadEventsAsync(
+                    sourceName, startUtc, startUtc.AddTicks(resolution), overfetchCap, reverse: false, ct)
                    .ConfigureAwait(false)).Events;

+                // The over-fetch reads the cluster FORWARD (ascending EventId), but a reverse read emitted its ties — and
+                // counted boundarySkip — in descending order. Slice in the read's own direction, as raw does.
+                if (reverse) cluster = cluster.Reverse().ToList();
+
                if (cluster.Count > MaxTieClusterOverfetch)
                {
                    _logger.LogError(
@@ -5232,7 +5260,7 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
                HistoryPaging.SliceTieCluster(
                    cluster.Count, boundarySkip, numValuesPerNode, startUtc, endUtc,
                    out var sliceStart, out var sliceCount, out var nextStartUtc, out var nextSkip,
-                    reverse: false, resolutionTicks: resolution);
+                    reverse: reverse, resolutionTicks: resolution);

                var slice = new List<HistoricalEvent>(sliceCount);
                for (var i = sliceStart; i < sliceStart + sliceCount; i++) slice.Add(cluster[i]);
@@ -5241,17 +5269,20 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
                if (nextStartUtc is { } resumeAt)
                {
                    pendingCursor = new HistoryContinuationState(
-                        sourceName ?? string.Empty, resumeAt, endUtc, nextSkip, numValuesPerNode, HistoryCursorKind.Events);
+                        sourceName ?? string.Empty, resumeAt, endUtc, nextSkip, numValuesPerNode, HistoryCursorKind.Events,
+                        Reverse: reverse);
                }
            }
            else if (truncated && events.Count > 0)
            {
-                // Cursor from the last event: resume at its time INCLUSIVE, skipping the trailing ties emitted.
+                // Cursor from the last event: resume at its time INCLUSIVE, skipping the trailing ties emitted. In a
+                // reverse read the last event is the OLDEST on the page, and the resumed (end, T] read starts there.
                var boundary = events[^1].EventTimeUtc;
                var skip = 0;
                for (var i = events.Count - 1; i >= 0 && events[i].EventTimeUtc == boundary; i--) skip++;
                pendingCursor = new HistoryContinuationState(
-                    sourceName ?? string.Empty, boundary, endUtc, skip, numValuesPerNode, HistoryCursorKind.Events);
+                    sourceName ?? string.Empty, boundary, endUtc, skip, numValuesPerNode, HistoryCursorKind.Events,
+                    Reverse: reverse);
            }

            var historyEvent = ProjectEvents(events, selectClauses);
@@ -5272,6 +5303,15 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
            errors[handle.Index] = StatusCodes.BadTimeout;
            results[handle.Index] = new SdkHistoryReadResult { StatusCode = StatusCodes.BadTimeout };
        }
+        catch (NotSupportedException ex)
+        {
+            // The backend cannot serve this read's shape — today a reverse (newest-first) read on Gateway / Null
+            // (#675). A client request the backend refuses, not a backend fault: Warning, not Error.
+            _logger.LogWarning("OtOpcUaNodeManager: HistoryReadEvents for node {NodeId} is not supported by the historian backend: {Reason}",
+                handle.NodeId, ex.Message);
+            errors[handle.Index] = StatusCodes.BadHistoryOperationUnsupported;
+            results[handle.Index] = new SdkHistoryReadResult { StatusCode = StatusCodes.BadHistoryOperationUnsupported };
+        }
        catch (Exception ex)
        {
            // One node's backend failure must not throw out of the batch — surface Bad for THIS node
@@ -5611,7 +5651,7 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
    private static int ClampToInt(uint value) => value > int.MaxValue ? int.MaxValue : (int)value;

    /// <summary>Map a HistoryRead-Events <c>NumValuesPerNode</c> request cap onto the
-    /// <see cref="IHistorianDataSource.ReadEventsAsync"/> <c>maxEvents</c> argument, honouring the OPC UA
+    /// <see cref="IHistorianDataSource.ReadEventsAsync(string?, DateTime, DateTime, int, CancellationToken)"/> <c>maxEvents</c> argument, honouring the OPC UA
    /// Part 4/11 semantics that <c>NumValuesPerNode == 0</c> means "no limit — return ALL values".
    /// We translate 0 to UNBOUNDED (<see cref="int.MaxValue"/>) — a very large positive cap — rather than
    /// the backend's <c>maxEvents &lt;= 0</c> "use the default cap" sentinel, so a client asking for the
@@ -45,9 +45,10 @@ public sealed class IHistorianDataSourceContractTests
    [InlineData("ReadEventsAsync", typeof(Task<HistoricalEventsResult>))]
    public void ReadMethods_ReturnExpectedTaskShape(string methodName, Type expectedReturnType)
    {
-        var method = typeof(IHistorianDataSource).GetMethod(methodName);
-        method.ShouldNotBeNull();
-        method!.ReturnType.ShouldBe(expectedReturnType);
+        // ReadEventsAsync is overloaded (the #675 direction-aware overload) — every overload must keep the shape.
+        var methods = Overloads(methodName);
+        methods.ShouldNotBeEmpty();
+        foreach (var method in methods) method.ReturnType.ShouldBe(expectedReturnType);
    }

    /// <summary>Verifies that GetHealthSnapshot is synchronous.</summary>
@@ -146,11 +147,14 @@ public sealed class IHistorianDataSourceContractTests
    [InlineData("ReadEventsAsync", "maxEvents", typeof(int))]
    public void HistoryRead_MaxParameter_TypePinned(string methodName, string parameterName, Type expectedType)
    {
-        var method = typeof(IHistorianDataSource).GetMethod(methodName);
-        method.ShouldNotBeNull();
-        var parameter = method!.GetParameters().FirstOrDefault(p => p.Name == parameterName);
-        parameter.ShouldNotBeNull($"Method {methodName} should expose a '{parameterName}' parameter.");
-        parameter!.ParameterType.ShouldBe(expectedType);
+        var methods = Overloads(methodName);
+        methods.ShouldNotBeEmpty();
+        foreach (var method in methods)
+        {
+            var parameter = method.GetParameters().FirstOrDefault(p => p.Name == parameterName);
+            parameter.ShouldNotBeNull($"Method {methodName} should expose a '{parameterName}' parameter.");
+            parameter!.ParameterType.ShouldBe(expectedType);
+        }
    }

    /// <summary>
@@ -163,13 +167,17 @@ public sealed class IHistorianDataSourceContractTests
    [Fact]
    public void ReadEventsAsync_sentinel_and_continuation_contract_types_pinned()
    {
-        var method = typeof(IHistorianDataSource).GetMethod("ReadEventsAsync");
-        method.ShouldNotBeNull();
+        var methods = Overloads("ReadEventsAsync");
+        methods.Count.ShouldBe(2, "the five-argument read plus the #675 direction-aware overload");
+        methods.ShouldContain(m => m.GetParameters().Any(p => p.Name == "reverse" && p.ParameterType == typeof(bool)));

-        var maxEventsParam = method!.GetParameters().FirstOrDefault(p => p.Name == "maxEvents");
-        maxEventsParam.ShouldNotBeNull("maxEvents parameter must exist for the sentinel contract");
-        maxEventsParam!.ParameterType.ShouldBe(typeof(int),
-            "maxEvents is int (not uint) so callers can pass <=0 as a 'use backend default cap' sentinel");
+        foreach (var method in methods)
+        {
+            var maxEventsParam = method.GetParameters().FirstOrDefault(p => p.Name == "maxEvents");
+            maxEventsParam.ShouldNotBeNull("maxEvents parameter must exist for the sentinel contract");
+            maxEventsParam!.ParameterType.ShouldBe(typeof(int),
+                "maxEvents is int (not uint) so callers can pass <=0 as a 'use backend default cap' sentinel");
+        }

        // HistoricalEventsResult must carry a nullable ContinuationPoint so implementations
        // can signal 'more results exist' (non-null) vs 'all results returned' (null).
@@ -179,4 +187,7 @@ public sealed class IHistorianDataSourceContractTests
        continuationProp!.PropertyType.ShouldBe(typeof(byte[]),
            "ContinuationPoint is byte[]? — null means no more pages, non-null means caller must page");
    }
+
+    private static List<System.Reflection.MethodInfo> Overloads(string name) =>
+        typeof(IHistorianDataSource).GetMethods().Where(m => m.Name == name).ToList();
 }
@@ -67,6 +67,20 @@ public sealed class NullHistorianDataSourceTests
        result.ContinuationPoint.ShouldBeNull();
    }

+    /// <summary>
+    ///     #675: the direction-aware overload's default — forward is the five-argument read, reverse is refused
+    ///     (the server answers <c>BadHistoryOperationUnsupported</c>).
+    /// </summary>
+    [Fact]
+    public async Task Direction_aware_ReadEventsAsync_forwards_forward_and_refuses_reverse()
+    {
+        IHistorianDataSource source = Source;
+        (await source.ReadEventsAsync(null, DateTime.UtcNow.AddHours(-1), DateTime.UtcNow, 0, reverse: false, Ct))
+            .Events.ShouldBeEmpty();
+        await Should.ThrowAsync<NotSupportedException>(() =>
+            source.ReadEventsAsync(null, DateTime.UtcNow, DateTime.UtcNow.AddHours(-1), 0, reverse: true, Ct));
+    }
+
    /// <summary>GetHealthSnapshot reports a fully-disabled (zeroed, disconnected, no-node) shape.</summary>
    [Fact]
    public void GetHealthSnapshot_is_disabled()
@@ -0,0 +1,106 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Conformance;
+
+/// <summary>
+///     The direction-aware event read (Gitea #675): forward is <c>[lo, hi)</c> ascending by <c>(EventTimeUtc, EventId)</c>;
+///     reverse is the Part 11 reverse window <c>(lo, hi]</c> DESCENDING by the same key, so a capped reverse read returns the
+///     LATEST events and flags truncation. Paging a reverse read the way the server does — resume at the last returned
+///     time inclusive and drop the boundary ties already emitted — must visit every event exactly once, newest-first,
+///     through a same-timestamp tie cluster.
+/// </summary>
+public abstract class EventReadConformance : ConformanceBase
+{
+    private static readonly CancellationToken Ct = CancellationToken.None;
+
+    /// <summary>e0@0, e1@1, then the ties t0..t4@2, then e5@3 and e6@4 (all source "s"), plus o0@2 from another source.</summary>
+    private async Task<IHistorianDataSource> SeedAsync()
+    {
+        await B.SeedEventsAsync(
+            (At(0), "e0", "s"), (At(1), "e1", "s"),
+            (At(2), "t0", "s"), (At(2), "t1", "s"), (At(2), "t2", "s"), (At(2), "t3", "s"), (At(2), "t4", "s"),
+            (At(3), "e5", "s"), (At(4), "e6", "s"),
+            (At(2), "o0", "other"));
+        return B.CreateDataSource();
+    }
+
+    private static readonly string[] NewestFirst = ["e6", "e5", "t4", "t3", "t2", "t1", "t0", "e1", "e0"];
+
+    [Fact]
+    public async Task Reverse_read_is_newest_first_by_time_then_EventId_descending()
+    {
+        using var src = await SeedAsync();
+        var r = await src.ReadEventsAsync("s", At(-1), At(10), 0, reverse: true, Ct);
+        r.Events.Select(e => e.EventId).ShouldBe(NewestFirst);
+        r.ContinuationPoint.ShouldBeNull();
+    }
+
+    [Fact]
+    public async Task Reverse_window_is_end_exclusive_start_inclusive_whichever_order_the_bounds_come_in()
+    {
+        using var src = await SeedAsync();
+        // (At(1), At(3)]: e1 at the lower bound is excluded, e5 at the upper bound included.
+        (await src.ReadEventsAsync("s", At(3), At(1), 0, reverse: true, Ct)).Events.Select(e => e.EventId)
+            .ShouldBe(["e5", "t4", "t3", "t2", "t1", "t0"]);
+        (await src.ReadEventsAsync("s", At(1), At(3), 0, reverse: true, Ct)).Events.Select(e => e.EventId)
+            .ShouldBe(["e5", "t4", "t3", "t2", "t1", "t0"]);
+        // Forward is unchanged: [At(1), At(3)) ascending.
+        (await src.ReadEventsAsync("s", At(1), At(3), 0, reverse: false, Ct)).Events.Select(e => e.EventId)
+            .ShouldBe(["e1", "t0", "t1", "t2", "t3", "t4"]);
+    }
+
+    [Fact]
+    public async Task A_capped_reverse_read_keeps_the_LATEST_events_and_flags_truncation()
+    {
+        using var src = await SeedAsync();
+        var r = await src.ReadEventsAsync("s", At(10), At(-1), 3, reverse: true, Ct);
+        r.Events.Select(e => e.EventId).ShouldBe(["e6", "e5", "t4"]);
+        r.ContinuationPoint.ShouldNotBeNull();
+        // An exact fit is complete.
+        (await src.ReadEventsAsync("s", At(10), At(-1), 9, reverse: true, Ct)).ContinuationPoint.ShouldBeNull();
+    }
+
+    [Fact]
+    public async Task Paging_a_reverse_read_through_a_tie_cluster_visits_every_event_once_newest_first()
+    {
+        using var src = await SeedAsync();
+        foreach (var cap in new[] { 1, 2, 3, 4 })
+        {
+            var all = new List<string>();
+            var lo = At(-1);
+            var from = At(10);
+            var skip = 0;
+            for (var page = 0; ; page++)
+            {
+                page.ShouldBeLessThan(40, "paging must terminate");
+                var r = await src.ReadEventsAsync("s", from, lo, cap + skip, reverse: true, Ct);
+                var events = r.Events.ToList();
+                var drop = 0;
+                while (drop < skip && drop < events.Count && events[drop].EventTimeUtc == from) drop++;
+                events = events.Skip(drop).ToList();
+                // The server asks for cap + skip rows (it re-reads the boundary ties it then drops), so a page is never
+                // emptied by the trim while more remains.
+                if (r.ContinuationPoint is not null) events.Count.ShouldBe(cap);
+
+                all.AddRange(events.Select(e => e.EventId));
+                if (r.ContinuationPoint is null) break;
+                var boundary = events[^1].EventTimeUtc;
+                var ties = events.AsEnumerable().Reverse().TakeWhile(e => e.EventTimeUtc == boundary).Count();
+                skip = boundary == from ? skip + ties : ties;
+                from = boundary;
+            }
+
+            all.SequenceEqual(NewestFirst).ShouldBeTrue($"cap {cap}: {string.Join(",", all)}");
+        }
+    }
+
+    [Fact]
+    public async Task Reverse_read_with_a_null_source_spans_every_source()
+    {
+        using var src = await SeedAsync();
+        var r = await src.ReadEventsAsync(null, At(2.5), At(1.5), 0, reverse: true, Ct);
+        r.Events.Select(e => e.EventId).ShouldBe(["t4", "t3", "t2", "t1", "t0", "o0"]);
+    }
+}
@@ -41,6 +41,13 @@ public interface IConformanceBackend : IAsyncDisposable
    /// <param name="samples">The rows to store.</param>
    Task SeedAsync(string tag, params (DateTime T, HistorianValue V, uint Status)[] samples);

+    /// <summary>
+    ///     Writes historical events directly in the backend's event table (never through the alarm writer): event id,
+    ///     source and time as given (rounded to the storage resolution), severity 500, received time = event time.
+    /// </summary>
+    /// <param name="events">The events to store.</param>
+    Task SeedEventsAsync(params (DateTime T, string Id, string Source)[] events);
+
    /// <summary>
    ///     Live (non-deleted) rows for a tag, ascending by time, read directly from storage. The status is returned
    ///     VERBATIM as stored — there is no read-side clamp (an Empty row stored non-Bad reads back non-Bad here).
@@ -0,0 +1,123 @@
+using System.Net;
+using Google.Protobuf.WellKnownTypes;
+using Grpc.Core;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Server.Kestrel.Core;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.HistorianGateway.Contracts.Grpc;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Runtime.Historian;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests;
+
+/// <summary>
+///     Gitea #675: the Gateway backend's event-read bounds and direction, offline.
+///     <list type="bullet">
+///       <item>A reverse (newest-first) read is refused before anything is sent — the gateway streams ascending only.</item>
+///       <item>The open upper bound a start-only forward read uses (<c>DateTime.MaxValue</c>, UTC kind) survives the
+///         PUBLISHED client package's proto conversion — captured on the wire by an in-process gRPC server — and the
+///         Windows FILETIME encoding the gateway converts it to. The live AVEVA leg (what the historian does with a
+///         year-9999 bound) stays open: it needs the VPN.</item>
+///     </list>
+/// </summary>
+public sealed class GatewayEventBoundsTests
+{
+    private static readonly DateTime UtcMax = new(DateTime.MaxValue.Ticks, DateTimeKind.Utc);
+    private static CancellationToken Ct => TestContext.Current.CancellationToken;
+
+    [Fact]
+    public async Task A_reverse_events_read_is_refused_and_sends_nothing()
+    {
+        var fake = new FakeHistorianGatewayClient();
+        IHistorianDataSource ds = new GatewayHistorianDataSource(fake, NullLogger<GatewayHistorianDataSource>.Instance);
+
+        await Should.ThrowAsync<NotSupportedException>(() =>
+            ds.ReadEventsAsync("Pump1", DateTime.UtcNow, DateTime.UtcNow.AddHours(-1), 10, reverse: true, Ct));
+        fake.ReadEventsCallCount.ShouldBe(0);
+
+        // Forward through the same overload is the five-argument read, unchanged.
+        await ds.ReadEventsAsync("Pump1", DateTime.UtcNow.AddHours(-1), DateTime.UtcNow, 10, reverse: false, Ct);
+        fake.ReadEventsCallCount.ShouldBe(1);
+    }
+
+    [Fact]
+    public void UtcMax_round_trips_through_the_proto_Timestamp_and_FILETIME_encodings()
+    {
+        var ts = Timestamp.FromDateTime(UtcMax);
+        ts.ToDateTime().ShouldBe(UtcMax);
+        ts.ToDateTime().Kind.ShouldBe(DateTimeKind.Utc);
+
+        // The gateway hands its bounds to the historian as FILETIME (100 ns since 1601) — in range up to year 30828.
+        var fileTime = ts.ToDateTime().ToFileTimeUtc();
+        DateTime.FromFileTimeUtc(fileTime).ShouldBe(UtcMax);
+    }
+
+    [Fact]
+    public async Task A_start_only_bound_of_UtcMax_reaches_the_wire_intact_through_the_published_client()
+    {
+        var capture = new CapturingReadService();
+        await using var server = await StartServerAsync(capture);
+        using var adapter = HistorianGatewayClientAdapter.Create(
+            new ServerHistorianOptions
+            {
+                Enabled = true,
+                Endpoint = $"http://127.0.0.1:{server.Port}",
+                ApiKey = "histgw_test_key",
+                UseTls = false,
+            },
+            NullLoggerFactory.Instance);
+        IHistorianDataSource ds = new GatewayHistorianDataSource(adapter, NullLogger<GatewayHistorianDataSource>.Instance);
+
+        var start = new DateTime(2026, 1, 1, 0, 0, 0, DateTimeKind.Utc);
+        var r = await ds.ReadEventsAsync("Pump1", start, UtcMax, 5, Ct);
+
+        r.Events.ShouldBeEmpty();
+        capture.Request.ShouldNotBeNull();
+        capture.Request!.StartTime.ToDateTime().ShouldBe(start);
+        capture.Request.EndTime.ToDateTime().ShouldBe(UtcMax);
+        DateTime.FromFileTimeUtc(capture.Request.EndTime.ToDateTime().ToFileTimeUtc()).ShouldBe(UtcMax);
+    }
+
+    private sealed class CapturingReadService : HistorianRead.HistorianReadBase
+    {
+        public ReadEventsRequest? Request { get; private set; }
+
+        public override Task ReadEvents(
+            ReadEventsRequest request, IServerStreamWriter<HistorianEvent> responseStream, ServerCallContext context)
+        {
+            Request = request;
+            return Task.CompletedTask;
+        }
+    }
+
+    private sealed class Server(WebApplication app, int port) : IAsyncDisposable
+    {
+        public int Port { get; } = port;
+
+        public async ValueTask DisposeAsync()
+        {
+            await app.StopAsync();
+            await app.DisposeAsync();
+        }
+    }
+
+    private static async Task<Server> StartServerAsync(CapturingReadService service)
+    {
+        var builder = WebApplication.CreateSlimBuilder();
+        builder.Logging.ClearProviders();
+        builder.WebHost.ConfigureKestrel(k =>
+            k.Listen(IPAddress.Loopback, 0, o => o.Protocols = HttpProtocols.Http2));
+        builder.Services.AddGrpc();
+        builder.Services.AddSingleton(service);
+        var app = builder.Build();
+        app.MapGrpcService<CapturingReadService>();
+        await app.StartAsync();
+        var port = new Uri(app.Urls.First()).Port;
+        return new Server(app, port);
+    }
+}
@@ -15,12 +15,18 @@
        <PackageReference Include="Microsoft.NET.Test.Sdk"/>
        <!-- ServiceCollection + BuildServiceProvider for the CreateDataSource factory test. -->
        <PackageReference Include="Microsoft.Extensions.DependencyInjection"/>
+        <!-- In-process gRPC server capturing the published client's ReadEvents request (#675 bounds pin). -->
+        <PackageReference Include="Grpc.AspNetCore"/>
        <PackageReference Include="xunit.runner.visualstudio">
            <PrivateAssets>all</PrivateAssets>
            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
        </PackageReference>
    </ItemGroup>

+    <ItemGroup>
+        <FrameworkReference Include="Microsoft.AspNetCore.App"/>
+    </ItemGroup>
+
    <ItemGroup>
        <ProjectReference Include="..\..\..\src\Drivers\ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway\ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.csproj"/>
    </ItemGroup>
@@ -60,3 +60,10 @@ public sealed class QuestDbModifiedValueConformance(QuestDbFixture fx) : Modifie
    /// <inheritdoc/>
    protected override Task<IConformanceBackend> CreateBackendAsync() => QuestDbConformanceBackend.CreateAsync(fx);
 }
+
+[Collection(QuestDbCollection.Name)]
+public sealed class QuestDbEventReadConformance(QuestDbFixture fx) : EventReadConformance
+{
+    /// <inheritdoc/>
+    protected override Task<IConformanceBackend> CreateBackendAsync() => QuestDbConformanceBackend.CreateAsync(fx);
+}
@@ -143,6 +143,27 @@ internal sealed class QuestDbConformanceBackend : IConformanceBackend
        await Wal.WaitAsync(Conn.SampleTable, CancellationToken.None);
    }

+    /// <inheritdoc/>
+    public async Task SeedEventsAsync(params (DateTime T, string Id, string Source)[] events)
+    {
+        await using (var c = await Conn.OpenAsync(CancellationToken.None))
+        {
+            foreach (var (t, id, source) in events)
+            {
+                await using var cmd = Conn.CreateCommand(c,
+                    $"INSERT INTO {Conn.EventTable} (ts, event_id, source, severity, message, received_utc) VALUES ($1, $2, $3, $4, $5, $6)");
+                QuestDbConnection.BindTimestamp(cmd, t);
+                QuestDbConnection.BindText(cmd, id);
+                QuestDbConnection.BindText(cmd, source);
+                cmd.Parameters.Add(new NpgsqlParameter { NpgsqlDbType = NpgsqlDbType.Integer, Value = 500 });
+                QuestDbConnection.BindText(cmd, null);
+                QuestDbConnection.BindTimestamp(cmd, t);
+                await cmd.ExecuteNonQueryAsync();
+            }
+        }
+        await Wal.WaitAsync(Conn.EventTable, CancellationToken.None);
+    }
+
    /// <summary>Runs a statement against this table set (test-only escape hatch for hand-made rows), then waits for WAL visibility of the sample table.</summary>
    /// <param name="sql">The statement; parameters are bound positionally by <paramref name="bind"/>.</param>
    /// <param name="bind">Binds the parameters.</param>
@@ -60,3 +60,10 @@ public sealed class SqlServerModifiedValueConformance(SqlHistorianFixture fx) :
    /// <inheritdoc/>
    protected override Task<IConformanceBackend> CreateBackendAsync() => SqlServerConformanceBackend.CreateAsync(fx);
 }
+
+[Collection(SqlHistorianCollection.Name)]
+public sealed class SqlServerEventReadConformance(SqlHistorianFixture fx) : EventReadConformance
+{
+    /// <inheritdoc/>
+    protected override Task<IConformanceBackend> CreateBackendAsync() => SqlServerConformanceBackend.CreateAsync(fx);
+}
@@ -40,6 +40,22 @@ internal sealed class SqlServerConformanceBackend : IConformanceBackend
    public async Task SeedAsync(string tag, params (DateTime T, HistorianValue V, uint Status)[] samples) =>
        await _db.SeedAsync(tag, samples);

+    /// <inheritdoc/>
+    public async Task SeedEventsAsync(params (DateTime T, string Id, string Source)[] events)
+    {
+        await using var c = await _db.Conn.OpenAsync(CancellationToken.None);
+        foreach (var (t, id, source) in events)
+        {
+            await using var cmd = _db.Conn.CreateCommand(c,
+                $"INSERT {_db.Conn.Q("Event")} (EventId, SourceName, AlarmId, AlarmName, AlarmTypeName, Severity, EventKind, Message, " +
+                "UserName, Comment, EventTimeUtc, ReceivedTimeUtc) VALUES (@id, @src, 'a', 'n', 'T', 500, 'Activated', NULL, NULL, NULL, @t, @t)");
+            cmd.Parameters.AddWithValue("@id", id);
+            cmd.Parameters.AddWithValue("@src", source);
+            SqlHistorianConnection.AddDateTime2(cmd, "@t", t);
+            await cmd.ExecuteNonQueryAsync();
+        }
+    }
+
    /// <inheritdoc/>
    public async Task<IReadOnlyList<(DateTime T, HistorianValue V, uint Status)>> ReadBackAsync(string tag)
    {
@@ -54,8 +54,48 @@ public sealed class EventHistoryPagingOverTheWireTests
        finally { await rig.DisposeAsync(); }
    }

+    [Fact]
+    public async Task A_capped_REVERSE_event_read_pages_newest_first_through_the_session_store_exactly_once()
+    {
+        // Gitea #675: StartTime later than EndTime. Before the fix the cap kept the EARLIEST 10, ascending, with no
+        // continuation point.
+        var ct = TestContext.Current.CancellationToken;
+        var rig = await StartAsync("EvtPagingWireRev", encrypted: false, ct);
+        try
+        {
+            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>();
+            var sizes = new List<int>();
+            byte[]? cp = null;
+            do
+            {
+                var (ids, next) = await ReadPageAsync(session, node, cp, ct, reverse: true);
+                collected.AddRange(ids);
+                sizes.Add(ids.Count);
+                cp = next;
+                sizes.Count.ShouldBeLessThan(30, "paging must terminate");
+            }
+            while (cp is not null);
+
+            sizes.ShouldBe([10, 10, 5]);
+            // Newest-first: time descending, ties by EventId descending.
+            collected.ShouldBe(events.OrderByDescending(e => e.EventTimeUtc)
+                .ThenByDescending(e => e.EventId, StringComparer.Ordinal).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)
+        ISession session, NodeId node, byte[]? cp, CancellationToken ct, bool reverse = false)
    {
        var filter = new EventFilter();
        filter.SelectClauses.Add(new SimpleAttributeOperand(ObjectTypeIds.BaseEventType, new QualifiedName("EventId"))
@@ -64,8 +104,8 @@ public sealed class EventHistoryPagingOverTheWireTests
        });
        var details = new ReadEventDetails
        {
-            StartTime = Epoch.AddDays(-1),
-            EndTime = Epoch.AddDays(1),
+            StartTime = reverse ? Epoch.AddDays(1) : Epoch.AddDays(-1),
+            EndTime = reverse ? Epoch.AddDays(-1) : Epoch.AddDays(1),
            NumValuesPerNode = 10,
            Filter = filter,
        };
@@ -94,6 +134,18 @@ public sealed class EventHistoryPagingOverTheWireTests
            return new HistoricalEventsResult(window.Take(cap).ToList(), window.Count > cap ? new byte[] { 1 } : null);
        }

+        /// <summary>The #675 overload: reverse is <c>(lo, hi]</c> by (time, id) descending.</summary>
+        public async Task<HistoricalEventsResult> ReadEventsAsync(
+            string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents, bool reverse, CancellationToken ct)
+        {
+            if (!reverse) return await ReadEventsAsync(sourceName, startUtc, endUtc, maxEvents, ct).ConfigureAwait(false);
+            await Task.Delay(1, ct).ConfigureAwait(false);
+            var (lo, hi) = startUtc <= endUtc ? (startUtc, endUtc) : (endUtc, startUtc);
+            var window = _all.Where(e => e.EventTimeUtc > lo && e.EventTimeUtc <= hi).Reverse().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(
@@ -199,11 +199,168 @@ public sealed class NodeManagerHistoryReadEventsPagingTests : IDisposable

        var mixed = ReadBatch(nm, 4, null, (node, good1[0].Cp), (other, Guid.NewGuid().ToByteArray()));
        mixed[0].Error.StatusCode.Code.ShouldBe(StatusCodes.Good);
-        mixed[0].Ids.ShouldBe(src.All.Skip(4).Take(3).Select(e => e.EventId));
+        // A resumed page is FULL (#675): it over-fetches by the boundary skip it then trims.
+        mixed[0].Ids.ShouldBe(src.All.Skip(4).Take(4).Select(e => e.EventId));
        mixed[1].Error.StatusCode.Code.ShouldBe(StatusCodes.BadContinuationPointInvalid);
        await host.DisposeAsync();
    }

+    // --- Gitea #675: reverse (newest-first) event paging ------------------------------------------------------
+
+    [Fact]
+    public async Task Reverse_events_page_newest_first_exactly_once_with_exact_page_counts()
+    {
+        var (host, nm, node, src) = await SetupAsync(Distinct(250));
+        var pages = new List<int>();
+        var ids = DrainAsync(nm, node, max: 100, start: Late, end: Early, pageSizes: pages);
+        ids.ShouldBe(src.All.Reverse().Select(e => e.EventId));
+        pages.ShouldBe([100, 100, 50]);
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task Reverse_first_page_is_the_LATEST_events_with_a_continuation_point()
+    {
+        // The pre-#675 shape: the cap kept the EARLIEST N, ascending, Good, and no continuation point.
+        var (host, nm, node, _) = await SetupAsync(Distinct(10));
+        var p1 = ReadEvents(nm, node, max: 3, inboundCp: null, end: Early, start: Late);
+        p1.Error.StatusCode.Code.ShouldBe(StatusCodes.Good);
+        p1.Ids.ShouldBe(["e0009", "e0008", "e0007"]);
+        p1.Cp.ShouldNotBeNull("a capped reverse read has more behind it and must page");
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task Reverse_tie_at_page_boundary_is_neither_duplicated_nor_skipped()
+    {
+        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);
+        // Newest-first, ties in descending EventId: cap 2 cuts the cluster at e4|e3.
+        DrainAsync(nm, node, max: 2, start: Late, end: Early).ShouldBe(["e5", "e4", "e3", "e2", "e1", "e0"]);
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task Reverse_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($"t{i}", 5));
+        events.Add(Ev("z0", 9));
+        var (host, nm, node, _) = await SetupAsync(events);
+        var pages = new List<int>();
+        DrainAsync(nm, node, max: 3, start: Late, end: Early, pageSizes: pages).ShouldBe(
+            ["z0", "t6", "t5", "t4", "t3", "t2", "t1", "t0", "a0"]);
+        pages.Sum().ShouldBe(9);
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task Reverse_tie_cluster_pages_on_a_coarse_resolution_backend()
+    {
+        const long res = 10;
+        var events = new List<HistoricalEvent>();
+        DateTime At(int slot) => Epoch.AddTicks(slot * res);
+        events.Add(new("a0", "s", At(0), Epoch, "m", 500));
+        for (var i = 0; i < 7; i++) events.Add(new($"t{i}", "s", At(3), Epoch, "m", 500));
+        events.Add(new("z0", "s", At(8), Epoch, "m", 500));
+        var (host, nm, node, _) = await SetupAsync(events, res);
+        DrainAsync(nm, node, max: 3, start: Late, end: Early).ShouldBe(
+            ["z0", "t6", "t5", "t4", "t3", "t2", "t1", "t0", "a0"]);
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task Reverse_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, end: Early, start: Late);
+        p1.Cp.ShouldNotBeNull();
+        ReadEvents(nm, node, max: 3, inboundCp: p1.Cp, end: Early, start: Late).Error.StatusCode.Code
+            .ShouldBe(StatusCodes.BadHistoryOperationUnsupported);
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task A_reverse_cursor_resumes_reverse_whatever_the_resume_request_says()
+    {
+        // The direction lives in the cursor: a resume carrying FORWARD details still continues newest-first.
+        var (host, nm, node, _) = await SetupAsync(Distinct(10));
+        var p1 = ReadEvents(nm, node, max: 4, inboundCp: null, end: Early, start: Late);
+        p1.Ids.ShouldBe(["e0009", "e0008", "e0007", "e0006"]);
+        var p2 = ReadEvents(nm, node, max: 4, inboundCp: p1.Cp); // default details: Early -> Late (forward)
+        p2.Error.StatusCode.Code.ShouldBe(StatusCodes.Good);
+        p2.Ids.ShouldBe(["e0005", "e0004", "e0003", "e0002"]);
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task A_forward_cursor_resumes_forward_whatever_the_resume_request_says()
+    {
+        var (host, nm, node, _) = await SetupAsync(Distinct(10));
+        var p1 = ReadEvents(nm, node, max: 4, inboundCp: null);
+        p1.Ids.ShouldBe(["e0000", "e0001", "e0002", "e0003"]);
+        var p2 = ReadEvents(nm, node, max: 4, inboundCp: p1.Cp, end: Early, start: Late);
+        p2.Ids.ShouldBe(["e0004", "e0005", "e0006", "e0007"]);
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task Reverse_unbounded_read_returns_everything_newest_first_without_paging()
+    {
+        var (host, nm, node, src) = await SetupAsync(Distinct(30));
+        var r = ReadEvents(nm, node, max: 0, inboundCp: null, end: Early, start: Late);
+        r.Ids.ShouldBe(src.All.Reverse().Select(e => e.EventId));
+        r.Cp.ShouldBeNull();
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task End_only_events_read_pages_backward_from_EndTime()
+    {
+        // StartTime unset + EndTime + NumValuesPerNode > 0 => backward from EndTime (Part 11; mirrors raw). The reverse
+        // window is (.., EndTime], so an event exactly AT EndTime is the first one returned.
+        var (host, nm, node, _) = await SetupAsync(Distinct(25));
+        var ids = DrainAsync(nm, node, max: 10, start: DateTime.MinValue, end: Epoch.AddSeconds(20));
+        ids.ShouldBe(Enumerable.Range(0, 21).Reverse().Select(i => $"e{i:0000}"));
+        await host.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task A_backend_without_reverse_support_answers_BadHistoryOperationUnsupported_not_the_earliest_events()
+    {
+        var (host, nm, node, src) = await SetupAsync(Distinct(10));
+        nm.HistorianDataSource = new ForwardOnlyEventSource(src);
+        ReadEvents(nm, node, max: 3, inboundCp: null, end: Early, start: Late).Error.StatusCode.Code
+            .ShouldBe(StatusCodes.BadHistoryOperationUnsupported);
+        // Forward reads on the same backend are unaffected.
+        DrainAsync(nm, node, max: 4).ShouldBe(src.All.Select(e => e.EventId));
+        await host.DisposeAsync();
+    }
+
+    private static readonly DateTime Early = Epoch.AddDays(-1);
+    private static readonly DateTime Late = Epoch.AddDays(1);
+
+    /// <summary>A backend that implements only the five-argument (forward) event read - Gateway / Null shaped.</summary>
+    private sealed class ForwardOnlyEventSource(IHistorianDataSource inner) : IHistorianDataSource
+    {
+        public Task<HistoricalEventsResult> ReadEventsAsync(string? n, DateTime s, DateTime e, int m, CancellationToken ct) =>
+            inner.ReadEventsAsync(n, s, e, m, ct);
+        public Task<HistorianRead> ReadRawAsync(string r, DateTime s, DateTime e, uint m, CancellationToken ct) =>
+            inner.ReadRawAsync(r, s, e, m, ct);
+        public Task<HistorianRead> ReadProcessedAsync(string r, DateTime s, DateTime e, TimeSpan i, HistoryAggregateType a, CancellationToken ct) =>
+            inner.ReadProcessedAsync(r, s, e, i, a, ct);
+        public Task<HistorianRead> ReadAtTimeAsync(string r, IReadOnlyList<DateTime> t, CancellationToken ct) =>
+            inner.ReadAtTimeAsync(r, t, ct);
+        public HistorianHealthSnapshot GetHealthSnapshot() => inner.GetHealthSnapshot();
+        public void Dispose() { }
+    }
+
    private sealed class RawSeriesSource : IHistorianDataSource
    {
        public Task<HistorianRead> ReadRawAsync(string r, DateTime s, DateTime e, uint m, CancellationToken ct)
@@ -265,16 +422,18 @@ public sealed class NodeManagerHistoryReadEventsPagingTests : IDisposable
        return (started.Host, nm, nm.TryGetFolder(equipmentId)!.NodeId, src);
    }

-    private static List<string> DrainAsync(OtOpcUaNodeManager nm, NodeId node, uint max, DateTime? end = null)
+    private static List<string> DrainAsync(
+        OtOpcUaNodeManager nm, NodeId node, uint max, DateTime? end = null, DateTime? start = null, List<int>? pageSizes = null)
    {
        var all = new List<string>();
        byte[]? cp = null;
        var pages = 0;
        do
        {
-            var r = ReadEvents(nm, node, max, cp, end);
+            var r = ReadEvents(nm, node, max, cp, end, start);
            r.Error.StatusCode.Code.ShouldBe(StatusCodes.Good);
            all.AddRange(r.Ids);
+            pageSizes?.Add(r.Ids.Count);
            cp = r.Cp;
            (++pages).ShouldBeLessThan(60, "paging must terminate");
        }
@@ -283,11 +442,15 @@ public sealed class NodeManagerHistoryReadEventsPagingTests : IDisposable
    }

    private static (List<string> Ids, ServiceResult Error, byte[]? Cp) ReadEvents(
-        OtOpcUaNodeManager nm, NodeId node, uint max, byte[]? inboundCp, DateTime? end = null) =>
-        ReadBatch(nm, max, end, (node, inboundCp))[0];
+        OtOpcUaNodeManager nm, NodeId node, uint max, byte[]? inboundCp, DateTime? end = null, DateTime? start = null) =>
+        ReadBatch(nm, max, end, start, (node, inboundCp))[0];

    private static List<(List<string> Ids, ServiceResult Error, byte[]? Cp)> ReadBatch(
-        OtOpcUaNodeManager nm, uint max, DateTime? end, params (NodeId Node, byte[]? Cp)[] reads)
+        OtOpcUaNodeManager nm, uint max, DateTime? end, params (NodeId Node, byte[]? Cp)[] reads) =>
+        ReadBatch(nm, max, end, null, reads);
+
+    private static List<(List<string> Ids, ServiceResult Error, byte[]? Cp)> ReadBatch(
+        OtOpcUaNodeManager nm, uint max, DateTime? end, DateTime? start, params (NodeId Node, byte[]? Cp)[] reads)
    {
        var context = new OperationContext(
            new RequestHeader(), secureChannelContext: null, RequestType.HistoryRead, identity: null);
@@ -298,7 +461,7 @@ public sealed class NodeManagerHistoryReadEventsPagingTests : IDisposable
        });
        var details = new ReadEventDetails
        {
-            StartTime = Epoch.AddDays(-1),
+            StartTime = start ?? Epoch.AddDays(-1),
            EndTime = end ?? Epoch.AddDays(1),
            NumValuesPerNode = max,
            Filter = filter,
@@ -361,6 +524,22 @@ public sealed class NodeManagerHistoryReadEventsPagingTests : IDisposable
                window.Take(cap).ToList(), truncated ? new byte[] { 1 } : null));
        }

+        /// <summary>Like the SQL backends' #675 overload: reverse => <c>(lo, hi]</c> newest-first by (time, id) descending.</summary>
+        public Task<HistoricalEventsResult> ReadEventsAsync(
+            string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents, bool reverse, CancellationToken ct)
+        {
+            if (!reverse) return ReadEventsAsync(sourceName, startUtc, endUtc, maxEvents, ct);
+            Calls++;
+            var (lo, hi) = startUtc <= endUtc ? (startUtc, endUtc) : (endUtc, startUtc);
+            lo = Trunc(lo);
+            hi = Trunc(hi);
+            var window = All.Where(e => e.EventTimeUtc > lo && e.EventTimeUtc <= hi).Reverse().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(