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fix(store-client): advance the PostgreSQL change stream cursor in poll order - #2265
Fixes#2264. I found it while looking into #2171, and it's the polling side of what came up in the review of #2250.
The PostgreSQL change stream polls the changelog in (changed_at, id) order, but its cursor only moved to a row with a higher id. A row that gets a high id and an early changed_at, which the in line GIN pending list flush produces under load, left the cursor behind rows it had already delivered. Every later poll returned them again, and with 100 or more of them the consumer never got past that batch.
The cursor now only moves forward in (changed_at, id) order. A late row fetched from its NOTIFY is still delivered without moving the cursor back, which is the guarantee #1469 added. MarkProcessed gets the same check, since acknowledging that row would otherwise move the cursor back to it, and a NOTIFY for a row that was already read is skipped whatever its id.
There's no schema, token format, Helm, metric or interface change. A saved position can now have a lower eventID than rows already read, which is fine because resume compares (changed_at, id) as a pair. #2250 at fd305ca applies on top of this cleanly and the package tests pass with both.
Type of Change
🐛 Bug fix
✨ New feature
💥 Breaking change
📚 Documentation
🔧 Refactoring
🔨 Build/CI
Component(s) Affected
Health Monitor
Core Service
Fault Management
Janitor
Deployment/Config
API/Interface
Preflight
Plugins
Documentation/CI
New Component
Multiple Components
Other
Testing
Tests pass locally
Manual testing completed
No breaking changes (or documented)
Unit tests with sqlmock. TestFetchNewChanges_HighIDEarlyRow_CursorFollowsPollOrder, TestMarkProcessed_HighIDEarlyRow_DoesNotMoveCursorBack and TestHandleNotification_ReadRowAboveCursorID_IsSkipped fail on main and pass here. The last two also fail when only advancePosition changes, which is why MarkProcessed and handleNotification are part of this. TestHandleNotification_NewerNotifyFetchesExactRowBeforeTimestampCursor used to expect the late row to move the cursor to its older changed_at, now it checks that the row is delivered and the cursor stays. TestAdvancePosition_EqualChangedAt_OrdersByID and TestPositionAfter_OrdersByChangedAtThenID cover the tie break.
Real PostgreSQL 16.15. A scratch harness that isn't committed drove the real watcher, built the way fault-quarantine gets it and behind the real adapter, on the provider's own schema. The 5,000 event rows drain identical copies of one backlog, with one insert held 150 ms inside its statement so its row lands 382 positions out of order, and the consumer acknowledges every event.
Scenario
main
This PR
151 rows, row 151 has the earliest changed_at, polling, 5 s
101 of 151 delivered, 49,575 duplicates, 50 never
151 of 151, no duplicates
Same, hybrid as shipped, 50 s
101 of 151, 299 duplicates, 50 never
151 of 151, no duplicates
5,000 event backlog, polling
stuck at 2,600 of 5,000 after 120 s, 38,394 duplicates
5,000 of 5,000, no duplicates
Same backlog, hybrid
stuck at 2,600 of 5,000 after 420 s, 5,300 duplicates
5,000 of 5,000, no duplicates
5,000 event live flood from 16 writers, hybrid
5,000 of 5,000, no duplicates
5,000 of 5,000, no duplicates
Restarting after acknowledging part of a batch, and restarting behind a backlog with a live NOTIFY, give the same results as main.
On macOS I ran go test with the race detector three times over the postgresql package, make lint-test in store-client with golangci-lint v2.15.0 from .versions.yaml, and make license-headers-lint and make gomod-lint. All pass. make lint-test-all doesn't complete here because its first step deletes the generated protobuf files before calling protoc, which isn't installed on this Mac, so CI on Linux is the authoritative run.
Checklist
Self-review completed
Documentation updated (if needed)
Ready for review
Summary by CodeRabbit
Bug Fixes
Change stream resume positions now follow event timestamps, using event IDs to order events with matching timestamps.
Late-arriving events can still be delivered without moving the resume position backward.
Notifications for events already read are skipped, regardless of the notified event’s ID.
Acknowledgements for already-read events no longer require looking up their changelog rows, including rows that have been purged.
Acknowledgements for unread events still require a successful lookup before the resume position is updated.
…l order
The poll reads the changelog in (changed_at, id) order, but advancePosition
moved the cursor only to a row with a higher id and kept that row's
changed_at. changed_at is when the writer's transaction started and the id is
taken when the changelog trigger runs at the end of the statement, so under
concurrent inserts a row can have a higher id and an earlier changed_at than
rows read after it. Once the poll read such a row, the cursor sat behind rows
it had already delivered. Every later poll returned them again, and when 100
or more of them sorted after it, nothing past that batch was ever delivered.
The cursor now only moves forward in (changed_at, id) order. A late row
fetched from its NOTIFY is still delivered without moving the cursor back,
which keeps what NVIDIA#1469 fixed. MarkProcessed checks the same order before it
moves the cursor, so acknowledging such a row can't move it back either, and
a NOTIFY for a row that was already read is skipped whatever its id.
Signed-off-by: Saibernard Yogendran <bernie97@seas.upenn.edu>
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Walkthrough
The PostgreSQL change stream now advances its resume position by (changed_at, id) order and tracks the highest read ID separately. It also skips notifications for recently seen events. Tests cover polling, acknowledgements, resume behavior, and notification handling.
Changes: PostgreSQL change-stream cursor
Layer / File(s)
Summary
Polling position ordering store-client/pkg/datastore/providers/postgresql/changestream.go, store-client/pkg/datastore/providers/postgresql/changestream_resume_token_test.go
advancePosition compares timestamps first, then IDs. Polling tests cover out-of-order rows, batch delivery, and positionAfter.
MarkProcessed advances the saved position by tuple order. The watcher uses the highest read ID to determine when an acknowledgement needs a timestamp lookup. Tests cover resumed positions, lookup failures, and purged rows.
handleNotification skips recently seen events before comparing IDs with the cursor. Tests check that late and already-read rows leave the cursor unchanged.
NVIDIA/NVSentinel#2250: This PR builds on the acknowledgement timestamp lookup and resume-position behavior changed in #2250.
NVIDIA/NVSentinel#2275: This PR relies on the late-row handling rule in this change and also updates notification handling.
NVIDIA/NVSentinel#2234: Its proposed retention bounds depend on the saved-position semantics changed here.
Suggested reviewers:xrfxlp, dims
Merge Risk: Moderate · previous review 302fe
A restart can skip a delivered but unacknowledged change in a case this PR newly permits. Resolve or explicitly accept the checkpoint risk before merging.
Pre-merge checks: 4 · 1
Failed checks: 1 warning
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Resolution
Docstring Coverage
Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…
Write docstrings for the functions missing them to satisfy the coverage threshold.
Passed checks: 4 passed
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Description Check
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Title check
The title clearly and concisely describes the main change: advancing the PostgreSQL change-stream cursor in poll order.
Linked Issues check
Issue #2264 requires PostgreSQL change-stream delivery once per position and cursor advancement in (changed_at, id) order. advancePosition now uses positionAfter, which compares the timestamp fi…
Out of Scope Changes check
The reviewed changes are limited to PostgreSQL change-stream cursor state, acknowledgement handling, notification deduplication, resume loading, and focused tests. These changes directly support issue…
Full details: Linked Issues check
Explanation
Issue #2264 requires PostgreSQL change-stream delivery once per position and cursor advancement in (changed_at, id) order. advancePosition now uses positionAfter, which compares the timestamp first and the ID second. MarkProcessed uses the same ordering. highestReadEventID supports acknowledgements for rows already read, including rows whose IDs exceed the cursor. Notification handling skips recently read IDs before the cursor comparison. The updated sqlmock tests cover tuple ordering, tie-breaking, polling, acknowledgements, late notifications, and duplicate notification skips.
Full details: Out of Scope Changes check
Explanation
The reviewed changes are limited to PostgreSQL change-stream cursor state, acknowledgement handling, notification deduplication, resume loading, and focused tests. These changes directly support issue #2264. No unrelated schema, interface, metric, deployment, or provider changes appear in the reviewed diff.
Full details: Docstring Coverage
Explanation
Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 8 functions across 3 files.
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Write docstrings for the functions missing them to satisfy the coverage threshold.
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Thanks for picking this up, and for the backlog numbers. They show it's a stall, not just duplicates. I reviewed the diff against main and against #2250. I did not run the branch locally; CI here is gated on /ok to test.
The core change looks correct to me. Moving the cursor in (changed_at, id) order matches the poll's ORDER BY changed_at, id LIMIT 100. So the last row of each batch is the new cursor, the LIMIT-100 stall and the redelivery loop both go away, and ties break the same way the query does. Cursor timestamps come from the scanned changed_at or the saved RFC3339Nano token, so I don't see a precision mismatch. Both event paths (fetchChangeByID and fetchNewChanges) build events through buildEventDocument, which always sets clusterTime, so extractEventTimestamp doesn't reach its time.Now() fallback today.
One ordering concern with #2250. On main, MarkProcessed uses time.Now() when the changed_at lookup fails. With this PR the cursor id can sit below ids already delivered, so more acks take the processedEventID > lastEventID branch. If one of those lookups fails, positionAfter(now, …) is true and the cursor jumps to now, skipping undelivered rows. #2250 removes that fallback: the ack returns an error and saves nothing. Landing #2250 first, or together with this, closes it. As you noted, the two merge cleanly.
Non-blocking:
MarkProcessed guard:processedEventID > lastEventID && positionAfter(...) mixes two orderings. positionAfter alone may read more clearly.
processed flag: a couple of places do read it. GetUnprocessedEventCount counts id > cursor AND processed = FALSE, and cleanup_changelog() in docs/postgresql-schema.sql deletes processed = TRUE rows. With out-of-order rows, the cumulative UPDATE ... WHERE id <= $1 can mark an undelivered lower-id row processed. Today that only skews the metric, because cleanup_changelog() is never called (#2170). A short comment would help if retention ever wires it up; ADR-068 deliberately does not delete by processed.
NOTIFY recent-window check: checking the recent window first means a row whose id has been evicted can be redelivered. That needs a TTL of more than 5 minutes or more than 10,000 ids in the window. It's duplicates only, which seems fine.
Tests: a test with 100 or more out-of-order rows behind a high-id row would pin the stall itself. So would one where the ack lookup fails with the cursor below the acked id.
Follow-up on my side: if this lands, I'll update #2250 so that an ack for a row that was delivered but sits above the cursor id doesn't need the changed_at lookup. Right now such an ack would fail if the row had been purged.
Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.
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Actionable comments posted: 1
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Inline comments:
Review comments at
@store-client/pkg/datastore/providers/postgresql/changestream.go:
- Around line 1219-1225: Update advancePosition and MarkProcessed so the
position persisted across restarts never advances past delivered rows that have
not been acknowledged. Track or derive the acknowledged position separately from
the delivery cursor, and preserve the existing poll ordering so unacknowledged
rows are eligible for redelivery after a restart.
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@KaivalyaMDabhadkar's point on timestampForAck makes sense to me. It came from #2250, and its "skip the lookup at or below lastEventID" check assumed lastEventID only goes up. A separate highest-id-read field fixes that, and this PR is the natural place for it. @Saibernard, if you'd rather I do it as a follow-up after this merges, I'm happy to.
On CodeRabbit's "delivered rows ... skipped after a restart": I think that's a separate problem that already exists on main. MarkProcessed saves the read head (w.lastEventID), not the acknowledged position, so events still buffered in the 100-slot channel can be skipped on restart even when they arrive in id order. This PR widens it to lower-id rows, but doesn't cause it. ADR-068's protocol already requires acknowledgment-safe checkpoints. I've opened #2288 for it, so this PR can stay focused on the cursor order.
The existing cursor test has three rows behind the high-id row, which on
main only costs one batch of duplicates. With 100 or more rows behind it
the old id rule never got past them. The cursor stayed on the high id and
every poll returned the same 100 rows.
The new test runs three polls and answers each one the way the real query
would for the position the watcher asks from, the first 100 rows after it
in (changed_at, id) order. It checks that each poll starts after the last
row of the batch before it and that every row is delivered once. Copied
onto main it fails with polls two and three both starting at row 151,
101 of 151 rows delivered and 300 deliveries in all.
Signed-off-by: Saibernard Yogendran <bernie97@seas.upenn.edu>
…tcher has read
timestampForAck skipped the changed_at lookup for acks at or below
lastEventID, on the assumption that the cursor's id only goes up. With
the cursor moving in (changed_at, id) order it can drop below ids this
watcher has already read, so acks for those rows went back to the
lookup, and once the row had been purged the ack failed for good.
The watcher now keeps the highest id it has read, resumed from, or
acknowledged past in its own field, and timestampForAck compares
against that. An ack for a read row doesn't need the row's changed_at,
because the cursor is already at or past it. Acks for rows this
watcher hasn't read still look the row up and still fail if the row is
gone, as NVIDIA#2250 intended.
Asked for in review. The out-of-order ack test no longer expects the
lookup, and two tests pin the restart case and the failing lookup for
an unread row behind an out-of-order cursor.
Signed-off-by: Saibernard Yogendran <bernie97@seas.upenn.edu>
MarkProcessed moved the cursor only when the acked id was above the
cursor's id and the row also came after the cursor in (changed_at, id)
order. The order check alone is what's needed. advancePosition has
already put the cursor at or past every row the watcher delivered, so for
those rows neither version moves it. Only an ack for a row this watcher
never delivered can move the cursor, and now it does whenever that row
comes after the cursor, whatever its id.
The comment above the UPDATE said rows are delivered in order by id. They
are delivered in (changed_at, id) order, so the cumulative UPDATE can mark
a lower-id row processed before it is delivered. The comment now says
that, and that it only skews GetUnprocessedEventCount today, because
cleanup_changelog() is never called and ADR-068 doesn't delete by the
flag.
Signed-off-by: Saibernard Yogendran <bernie97@seas.upenn.edu>
Thanks both, this is pushed now as a few commits on top of the merge.
Kaivalya, I did it the way you suggested. The watcher keeps its own note of the highest id it's seen, whether that came from a read, the token or an ack, and timestampForAck checks that instead of the cursor. I reworked the out of order ack test so it runs with no changed_at lookup at all, which means a row that's been purged by the time its ack shows up doesn't matter anymore. I also put in one for acking on either side of the token right after a restart, and one where the lookup fails for a row we never actually read, that one just returns the error and leaves the cursor alone. So Sumedh, no need for the follow-up on your side, it's all in here.
The stall test is in too, a full batch of out of order rows sitting behind a single high id row. Without the fix the cursor parks on that row and the stream never drains. And the MarkProcessed guard is just positionAfter now, the mixed check was harder to read than it needed to be. While I was in there I rewrote the comment above the UPDATE so it spells out that the cumulative flag can mark a lower id row before it's delivered, and that only the unprocessed count notices that today.
The restart thing is older than this PR so I'm fine leaving it to #2288. I didn't touch the recent window either, worst case there is a duplicate.
@KaivalyaMDabhadkar@sumedh-aerram the highest read id field and the tests you asked for are in, whenever you have a minute for another look. @lalitadithya could you kick off CI on this when you get a chance?
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Thanks @Saibernard. I re-reviewed the new commits (7963e0fa, e2d3a8dc and 302fea36). My earlier points are all addressed:
Highest read id.highestReadEventID is updated from reads, the loaded token and acks, always under mu. It never decreases, and timestampForAck now compares against it. So an ack for any row already read skips the changed_at lookup, even if the row was purged. Thanks for the test that runs with no lookup at all.
Cursor can't jump to "now." Main (with #2250) no longer has the time.Now() fallback in MarkProcessed, and the combined code leaves the cursor unchanged when a lookup fails.
The stall test pins the bug. With the old id-only rule the cursor would stay on row 151 and polls 2 and 3 would re-read rows 1–100, so the test fails without the fix.
MarkProcessed guard. It is now positionAfter alone.
Two non-blocking notes:
positionAfter(zero, id, zero, 0) returns true. I couldn't find a path that reaches it, but a comment would help.
extractEventTimestamp still falls back to time.Now(). Every current event path sets clusterTime, so this is hardening only.
This was a static review; I didn't run the branch locally. Looks good to me. The acknowledgment-vs-read-head gap stays tracked in #2288, where I've posted a plan that builds on this PR.
Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.
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Summary
Fixes #2264. I found it while looking into #2171, and it's the polling side of what came up in the review of #2250.
The PostgreSQL change stream polls the changelog in (changed_at, id) order, but its cursor only moved to a row with a higher id. A row that gets a high id and an early changed_at, which the in line GIN pending list flush produces under load, left the cursor behind rows it had already delivered. Every later poll returned them again, and with 100 or more of them the consumer never got past that batch.
The cursor now only moves forward in (changed_at, id) order. A late row fetched from its NOTIFY is still delivered without moving the cursor back, which is the guarantee #1469 added. MarkProcessed gets the same check, since acknowledging that row would otherwise move the cursor back to it, and a NOTIFY for a row that was already read is skipped whatever its id.
There's no schema, token format, Helm, metric or interface change. A saved position can now have a lower eventID than rows already read, which is fine because resume compares (changed_at, id) as a pair. #2250 at fd305ca applies on top of this cleanly and the package tests pass with both.
Type of Change
Component(s) Affected
Testing
Unit tests with sqlmock. TestFetchNewChanges_HighIDEarlyRow_CursorFollowsPollOrder, TestMarkProcessed_HighIDEarlyRow_DoesNotMoveCursorBack and TestHandleNotification_ReadRowAboveCursorID_IsSkipped fail on main and pass here. The last two also fail when only advancePosition changes, which is why MarkProcessed and handleNotification are part of this. TestHandleNotification_NewerNotifyFetchesExactRowBeforeTimestampCursor used to expect the late row to move the cursor to its older changed_at, now it checks that the row is delivered and the cursor stays. TestAdvancePosition_EqualChangedAt_OrdersByID and TestPositionAfter_OrdersByChangedAtThenID cover the tie break.
Real PostgreSQL 16.15. A scratch harness that isn't committed drove the real watcher, built the way fault-quarantine gets it and behind the real adapter, on the provider's own schema. The 5,000 event rows drain identical copies of one backlog, with one insert held 150 ms inside its statement so its row lands 382 positions out of order, and the consumer acknowledges every event.
Restarting after acknowledging part of a batch, and restarting behind a backlog with a live NOTIFY, give the same results as main.
On macOS I ran go test with the race detector three times over the postgresql package, make lint-test in store-client with golangci-lint v2.15.0 from .versions.yaml, and make license-headers-lint and make gomod-lint. All pass. make lint-test-all doesn't complete here because its first step deletes the generated protobuf files before calling protoc, which isn't installed on this Mac, so CI on Linux is the authoritative run.
Checklist
Summary by CodeRabbit