A tie cluster's truncated read left the width at one tick; once the gap
above it was under two milliseconds nothing floored it, so segHi
stepped down one tick per read: 30 rows at one ms 0.3 s below hi with a
carried 1 s width took 10,006 identical reads (S8). (a) The top-anchored
width is floored at the granularity, Min kept; (b) a segment no wider
than the granularity is the unit that escalates to
MaxTieClusterOverfetch and then refuses - per millisecond, exact for the
ms-resolution Gateway (documented). Both are needed: the floor alone
loops on crowded reads with a small tie budget. S8 is a repo test
(4 shapes, reads <= 30, 0 repeated requests); random-data tests now
measure the refusal unit as rows in any 1 ms window. Reviewer harness,
5,000 seeds x default/MSONLY/CROWD: worst page 70 reads (default/MSONLY),
63 (CROWD) at M=65536, p99.9 27-37; no page over 500 reads at any tie
budget.
Bisecting the gap above a truncated read halved below the 1 ms request
granularity (requests are ms-rounded, so it re-sent identical requests)
and bisected the WHOLE gap even when the burst ended right above the
row it saw: a 1,026-row burst ending 100 days ago took 46 reads. After
an empty top-anchored segment the walk now gallops up from the known
row ((known + d, top], d x4, following each truncated read), then
bisects the bracket, with halving floored at the granularity (the Min
kept; a crowded gallop falls back to top-anchored halving). Burst: 18
reads, 0 repeated requests; dense tag stopped 2 days ago: 26 (gallop
alone crawled: 94). Walk fuzzed at 4,000 seeds locally (400 in-tree).
Docs: measured counts; CLAUDE.md gateway prerequisites list
RuntimeDb:EventReadMaxRows.
A reverse or end-only capped events read on a backend without
SupportsReverseEvents (Gateway, Null) was refused
BadHistoryOperationUnsupported (explicit reverse) or answered with the
OLDEST N ascending (end-only, #675). It is now served newest-first by
the same ReverseWalk as raw reads: page-sized ms-aligned segments,
never answered from a read the source flagged truncated, ties sorted by
EventId (the gateway orders by event time only - the fake shuffles
them per call), one extra event walked to signal 'more', uncapped = one
forward read reversed, and the width + proven-empty stretch carried in
the events cursor. The oversized-cluster over-fetch keeps exactly the
events at T in the same order. A tie cluster over the budget is
BadTooManyOperations. Docs note the gateway's EventReadMaxRows must be
unlimited (it truncates silently).
The last pages of a deep reverse page-through on a forward-only backend
each walked down to 1970 one x4 growth step at a time (~14-16 reads,
~2.5 s live). After two empty segments the walk now asks once per page
for the whole remainder at the page budget (a truncated answer is
discarded and the walk carries on), and the stretch the page's last
complete read proved empty rides the continuation point
(HistoryContinuationState.EmptyFromTicks/EmptyToTicks, session-held, so
no versioning) so the next page steps over it. Fuzz now pages with the
walk's own hints and with true random empty stretches.
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.
Joseph Doherty