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Posted to reviews@iotdb.apache.org by "Caideyipi (via GitHub)" <gi...@apache.org> on 2026/08/04 08:33:50 UTC
[PR] Fix repeated WAL scans during subscription catch-up [iotdb]
Caideyipi opened a new pull request, #18392:
URL: https://github.com/apache/iotdb/pull/18392
## Description
### Reuse the WAL iterator during historical catch-up
A bounded consensus-subscription prefetch round recreated `ProgressWALIterator` at `nextExpectedSearchIndex` before reading each batch. The iterator still opens retained WAL files from the beginning and applies `startSearchIndex` only after request deserialization, so every batch repeatedly scanned all earlier WAL entries. The resulting read amplification grew with catch-up progress and made consumption progressively slower.
This change keeps the existing iterator, including its WAL reader cursor and buffered look-ahead request, across normal bounded prefetch rounds. Explicit resets remain in place for seek, WAL-gap recovery, exhausted live WAL refresh, and memory rollback.
### Regression coverage
A real sealed-WAL test limits each batch to one entry and verifies that three consecutive catch-up rounds:
- reuse the same `ProgressWALIterator`;
- advance the read search index contiguously;
- deliver writer local sequences 1, 2, and 3 exactly once.
### Verification
```text
mvn -o test -pl iotdb-core/datanode -am -Dtest=ProgressWALIteratorTest,ConsensusPrefetchingQueueTest,ConsensusPrefetchingQueueWalBackpressureTest -Dsurefire.failIfNoSpecifiedTests=false -DfailIfNoTests=false
```
Tests run: 29, Failures: 0, Errors: 0, Skipped: 0.
This PR has:
- [x] been self-reviewed.
- [x] added comments explaining the why and intent of the code.
- [x] added unit tests covering the regression.
##### Key changed/added classes
- `ConsensusPrefetchingQueue`
- `ConsensusPrefetchingQueueWalBackpressureTest`
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Re: [PR] Fix repeated WAL scans during subscription catch-up [iotdb]
Posted by "Caideyipi (via GitHub)" <gi...@apache.org>.
Caideyipi commented on PR #18392:
URL: https://github.com/apache/iotdb/pull/18392#issuecomment-5290653354
Closing as superseded by #18402, which already includes this WAL iterator reuse fix and a more complete solution for refresh handling, pending-cursor fast-forwarding, and late-ACK lock contention.
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Re: [PR] Fix repeated WAL scans during subscription catch-up [iotdb]
Posted by "Caideyipi (via GitHub)" <gi...@apache.org>.
Caideyipi closed pull request #18392: Fix repeated WAL scans during subscription catch-up
URL: https://github.com/apache/iotdb/pull/18392
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