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fix(historian): #689 size reverse-walk segments from the page, not the tie budget

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ReverseRawReader aimed every segment at half of MaxTieClusterOverfetch
(~32k rows by default) and carried that width page to page, so a paged
'latest N' read on a dense Gateway tag fetched ~32,770 rows per page
(2-5 s live) to return N. The walk now lives in a generic ReverseWalk
core: segments are aimed at ~N+8 rows (page budget 2N+16), a discovery
segment (first without a hint / after an empty one) may read 1,024, and
the tie budget is only the ceiling a single-timestamp cluster escalates
to. A truncated read never re-enters the stretch below its newest row
until the gap above it is bisected. Tests: per-page row counts on a
dense tag (node manager + walk), randomized exactness fuzz (walk and
node-manager paging), dense-tag-below-a-long-gap read bound.
This commit is contained in:
Joseph Doherty
2026-10-12 16:25:07 -04:00
parent 71d031fcd0
commit d33992034d
8 changed files with 663 additions and 183 deletions
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src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/ReverseRawReader.cs
+46 -167
View File
@@ -5,70 +5,29 @@ namespace ZB.MOM.WW.OtOpcUa.OpcUaServer;
/// <summary>One walked-back reverse raw page (<see cref="ReverseRawReader"/>).</summary>
/// <param name="Samples">At most the requested cap, newest first.</param>
/// <param name="WidthHintTicks">The segment width the walk ended on — the next page's starting width (0 = none).</param>
internal readonly record struct ReverseRawPage(IReadOnlyList<DataValueSnapshot> Samples, long WidthHintTicks);
internal readonly record struct ReverseRawPage(IReadOnlyList<DataValueSnapshot> Samples, long WidthHintTicks)
{
/// <summary>Everything the next page of the same read should resume with (its width is <see cref="WidthHintTicks"/>).</summary>
public ReverseWalkHint Hint { get; init; } = new(WidthHintTicks);
}
/// <summary>
/// Gitea #686: serves a Part 11 reverse raw read — <c>(lo, hi]</c> newest-first, capped — on a historian backend that can
/// only read forward (<see cref="IHistorianDataSource.SupportsReverseRaw"/> = <c>false</c>: the Gateway, whose
/// <c>ReadRaw</c> streams ascending and rejects <c>start &gt; end</c>).
/// <c>ReadRaw</c> streams ascending and rejects <c>start &gt; end</c>). The walk itself — segment sizing (#689), the
/// truncation rules, edges, ties and bounds — is <see cref="ReverseWalk"/>; this adapter supplies the raw read (a read is
/// cut short when it returned its full cap) and keeps the backend's tie order.
/// </summary>
/// <remarks>
/// <para>
/// <b>Uncapped</b> (<c>maxValues = 0</c>): ONE forward read of the whole window, filtered and reversed in memory — the
/// same rows, round trips and memory as an uncapped forward read (and as a native backend's uncapped reverse read).
/// </para>
/// <para>
/// <b>Capped: walk back in segments.</b> Starting at <c>hi</c>, read one segment <c>(segLo, segHi]</c> FORWARD, keep
/// it newest-first, and step to the next older segment until the cap is met or <c>lo</c> is reached. The first
/// segment is <see cref="InitialSegment"/> wide, or the width the previous page of the same read ended on (carried in
/// the continuation point). After each read the next width is re-aimed at about half the row budget from the density
/// just observed, growing at most ×4 per step — so a live tag's "latest N" is usually one read, a dense tag does not
/// throw away a full read per page, and sparse history is found in a few dozen tiny reads.
/// </para>
/// <para>
/// <b>Never answer from a truncated read.</b> The row budget is <c>M = max(1, MaxTieClusterOverfetch)</c>; a segment
/// read asks for <c>M + 2</c> rows (room for the row an AVEVA read fabricates at its query start, and one more to see
/// past the segment). Rows stream ascending, so a read is complete for its segment when it came back short OR its last
/// row is already past <c>segHi</c>. An incomplete read is discarded: when rows BEFORE the segment (a neighbouring
/// tie cluster inside the alignment slack, the fabricated row) crowded it out it is re-read once with room for them
/// (at most <c>2M + 4</c> rows); otherwise the segment is narrowed (to about half the budget at the density the
/// truncated read showed) and re-read. Only a single timestamp holding more than <c>M</c> samples — a segment already
/// one timestamp-resolution wide that still does not fit — is refused: <see cref="HistoryReadLimitExceededException"/>
/// (⇒ <c>BadTooManyOperations</c>, a request limit, never a backend fault).
/// </para>
/// <para>
/// <b>Edges.</b> A segment is requested as <c>[⌊segLo⌋, ⌈segHi + 1 tick⌉]</c>, both rounded outward to the millisecond
/// (the gateway's / AVEVA's resolution), and filtered to <c>(segLo, segHi]</c> on the samples' own timestamps. That is
/// exact whether the backend's forward end is exclusive (the interface contract) or inclusive (the gateway) and
/// whether it truncates its bounds (QuestDB-style); and because the request start is already on a millisecond, a row
/// an AVEVA read fabricates at its (possibly rounded-up) query start can never land inside the segment (#687).
/// Adjacent segments share no timestamp, so nothing is returned twice.
/// </para>
/// <para>
/// <b>Ties</b> come back in the backend's forward order reversed — the same order the node manager's tie-cluster
/// over-fetch uses for a reverse read — so a resumed page's boundary-tie skip lines up.
/// </para>
/// <para>
/// <b>Bounds.</b> The walk never asks for data after now + <see cref="LookAhead"/> (one day — the historian stores
/// nothing further ahead; a <c>StartTime = DateTime.MaxValue</c> read would otherwise send centuries-ahead query
/// starts), nor before <see cref="Floor"/> (the Unix epoch — AVEVA holds nothing earlier, and an end-only read's year-1
/// lower bound is outside the historian's time encoding). A walked-back read therefore returns only samples strictly
/// after the epoch and no later than now + one day.
/// </para>
/// </remarks>
internal static class ReverseRawReader
{
/// <summary>The first segment's width when no hint is carried.</summary>
internal static readonly TimeSpan InitialSegment = TimeSpan.FromMinutes(1);
internal static readonly TimeSpan InitialSegment = ReverseWalk.InitialSegment;
/// <summary>The walk's exclusive lower limit (see the remarks).</summary>
internal static readonly DateTime Floor = DateTime.UnixEpoch;
/// <summary>The walk's exclusive lower limit.</summary>
internal static readonly DateTime Floor = ReverseWalk.Floor;
/// <summary>How far past "now" the walk starts at most (the same horizon as the Gateway's raw-aggregate look-ahead).</summary>
internal static readonly TimeSpan LookAhead = TimeSpan.FromDays(1);
/// <summary>Request bounds are rounded outward to this (the gateway / AVEVA timestamp resolution).</summary>
private const long Alignment = TimeSpan.TicksPerMillisecond;
/// <summary>How far past "now" the walk starts at most.</summary>
internal static readonly TimeSpan LookAhead = ReverseWalk.LookAhead;
/// <summary>Reads <c>(lo, hi]</c> newest-first from forward reads.</summary>
/// <param name="source">A backend that reads forward.</param>
@@ -76,129 +35,49 @@ internal static class ReverseRawReader
/// <param name="hi">The later (inclusive) bound — the request's StartTime or a reverse resume point.</param>
/// <param name="lo">The earlier (exclusive) bound.</param>
/// <param name="maxValues">Cap; <c>0</c> = every sample in the window (one forward read).</param>
/// <param name="maxTieCluster">Row budget per segment read (<c>MaxTieClusterOverfetch</c>; values below 1 count as 1).</param>
/// <param name="maxTieCluster">The tie budget (<c>MaxTieClusterOverfetch</c>; values below 1 count as 1) — a ceiling only.</param>
/// <param name="nowUtc">Now, for the look-ahead clamp.</param>
/// <param name="widthHintTicks">Starting segment width (the previous page's <see cref="ReverseRawPage.WidthHintTicks"/>), 0 = none.</param>
/// <param name="ct">Cancellation.</param>
/// <returns>The page, newest first, and the width to start the next page with.</returns>
/// <exception cref="HistoryReadLimitExceededException">One timestamp holds more than the row budget.</exception>
/// <exception cref="HistoryReadLimitExceededException">One timestamp holds more than the tie budget.</exception>
public static Task<ReverseRawPage> ReadAsync(
IHistorianDataSource source, string tagname, DateTime hi, DateTime lo, uint maxValues, int maxTieCluster,
DateTime nowUtc, long widthHintTicks, CancellationToken ct) =>
ReadAsync(source, tagname, hi, lo, maxValues, maxTieCluster, nowUtc, new ReverseWalkHint(widthHintTicks), ct);
/// <summary>Reads <c>(lo, hi]</c> newest-first from forward reads, resuming with the previous page's full hint.</summary>
/// <param name="source">A backend that reads forward.</param>
/// <param name="tagname">Historian tag.</param>
/// <param name="hi">The later (inclusive) bound.</param>
/// <param name="lo">The earlier (exclusive) bound.</param>
/// <param name="maxValues">Cap; <c>0</c> = every sample in the window (one forward read).</param>
/// <param name="maxTieCluster">The tie budget — a ceiling only.</param>
/// <param name="nowUtc">Now, for the look-ahead clamp.</param>
/// <param name="hint">What the previous page carried (<see cref="ReverseRawPage.Hint"/>), default = none.</param>
/// <param name="ct">Cancellation.</param>
/// <returns>The page, newest first, and the hint for the next page.</returns>
/// <exception cref="HistoryReadLimitExceededException">One timestamp holds more than the tie budget.</exception>
public static async Task<ReverseRawPage> ReadAsync(
IHistorianDataSource source, string tagname, DateTime hi, DateTime lo, uint maxValues, int maxTieCluster,
DateTime nowUtc, long widthHintTicks, CancellationToken ct)
DateTime nowUtc, ReverseWalkHint hint, CancellationToken ct)
{
var resolution = Math.Max(1L, source.TimestampResolutionTicks);
var granularity = Math.Max(Alignment, resolution);
var budget = Math.Max(1, maxTieCluster);
var result = new List<DataValueSnapshot>();
var horizon = nowUtc.Ticks > DateTime.MaxValue.Ticks - LookAhead.Ticks ? DateTime.MaxValue.Ticks : nowUtc.Ticks + LookAhead.Ticks;
var hiTicks = Math.Min(hi.Ticks, horizon);
var stopTicks = Math.Max(lo.Ticks, Floor.Ticks);
if (hiTicks <= stopTicks) return new ReverseRawPage(result, widthHintTicks);
if (maxValues == 0)
{
// One forward read of the whole window, reversed — the uncapped forward read's own cost and bound.
var all = (await source.ReadRawAsync(tagname, Utc(AlignDown(stopTicks, granularity)),
Utc(AlignUp(hiTicks + 1, granularity)), 0, ct).ConfigureAwait(false)).Samples;
for (var i = all.Count - 1; i >= 0; i--)
{
var ts = Ticks(all[i]);
if (ts > stopTicks && ts <= hiTicks) result.Add(all[i]);
}
return new ReverseRawPage(result, widthHintTicks);
}
var maxWidth = DateTime.MaxValue.Ticks;
var width = widthHintTicks > 0 ? Math.Clamp(widthHintTicks, resolution, maxWidth) : Math.Max(resolution, InitialSegment.Ticks);
var baseCap = budget + 2;
var maxCap = 2 * budget + 4;
var segHi = hiTicks;
while (segHi > stopTicks && result.Count < maxValues)
{
var segLo = segHi - stopTicks <= width ? stopTicks : segHi - width;
var start = Utc(AlignDown(segLo, granularity));
var end = Utc(AlignUp(segHi + 1, granularity));
var cap = baseCap;
IReadOnlyList<DataValueSnapshot> rows;
var narrowed = false;
while (true)
{
ct.ThrowIfCancellationRequested();
rows = (await source.ReadRawAsync(tagname, start, end, (uint)cap, ct).ConfigureAwait(false)).Samples;
if (rows.Count < cap || Ticks(rows[^1]) > segHi) break; // complete for (segLo, segHi]
var leading = 0;
var inner = 0;
var firstInner = 0L;
foreach (var r in rows)
var (rows, next) = await ReverseWalk.WalkAsync(
async (start, end, cap, token) =>
{
var ts = Ticks(r);
if (ts <= segLo) leading++;
else if (ts <= segHi && inner++ == 0) firstInner = ts;
}
// Crowded out by rows before the segment, with the segment itself possibly within budget: re-read once
// with room for them.
var roomier = leading + budget + 2;
if (leading > 0 && inner <= budget && roomier > cap && roomier <= maxCap)
{
cap = roomier;
continue;
}
if (width <= resolution)
throw new HistoryReadLimitExceededException(budget,
$"A reverse raw read of '{tagname}' at {Utc(segHi):O} found more than {budget} samples at one " +
"timestamp; the backend reads forward only, so the latest of them cannot be selected. Increase " +
"ServerHistorian:MaxTieClusterOverfetch.");
// Narrow to about half the budget at the density the truncated read's own samples showed (their span, not
// the segment's — the segment may open on an empty stretch), at least halving.
var lastTs = Ticks(rows[^1]);
var aimed = inner > 1 && lastTs > firstInner
? (long)Math.Min((double)(lastTs - firstInner) * (budget / 2.0) / (inner - 1), width / 2.0)
: width / 4;
width = Math.Max(resolution, aimed);
narrowed = true;
break;
}
if (narrowed) continue; // re-read the same segHi, narrower
var count = 0;
for (var i = rows.Count - 1; i >= 0; i--)
{
var ts = Ticks(rows[i]);
if (ts <= segLo || ts > segHi) continue;
count++;
if (result.Count < maxValues) result.Add(rows[i]);
}
// Re-aim at about half the budget from the density just seen; grow at most x4.
var grown = width > maxWidth / 4 ? maxWidth : width * 4;
width = count == 0
? grown
: (long)Math.Clamp((double)width * Math.Max(1.0, budget / 2.0) / count, resolution, grown);
segHi = segLo;
}
return new ReverseRawPage(result, width);
var samples = (await source.ReadRawAsync(tagname, start, end, (uint)cap, token).ConfigureAwait(false)).Samples;
return new ForwardSegmentRead<DataValueSnapshot>(samples, cap > 0 && samples.Count >= cap);
},
Ticks, tieOrder: null, hi, lo, maxValues, budget, source.TimestampResolutionTicks, nowUtc, hint,
at => new HistoryReadLimitExceededException(budget,
$"A reverse raw read of '{tagname}' at {at:O} found more than {budget} samples at one timestamp; the " +
"backend reads forward only, so the latest of them cannot be selected. Increase " +
"ServerHistorian:MaxTieClusterOverfetch."),
ct)
.ConfigureAwait(false);
return new ReverseRawPage(rows, next.WidthTicks) { Hint = next };
}
private static long Ticks(DataValueSnapshot s) => (s.SourceTimestampUtc ?? DateTime.MinValue).Ticks;
private static DateTime Utc(long ticks) => new(ticks, DateTimeKind.Utc);
private static long AlignDown(long ticks, long step) => ticks - ticks % step;
private static long AlignUp(long ticks, long step)
{
var down = AlignDown(ticks, step);
if (down == ticks) return ticks;
return down > DateTime.MaxValue.Ticks - step ? DateTime.MaxValue.Ticks : down + step;
}
}