kvclient: serve read-your-own-writes from the write buffer #139054
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A-buffered-writes
Related to the introduction of buffered writes
A-kv-transactions
Relating to MVCC and the transactional model.
C-enhancement
Solution expected to add code/behavior + preserve backward-compat (pg compat issues are exception)
T-kv
KV Team
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arulajmani
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A-buffered-writes
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This patch teaches the txnWriteBuffer to recognize when a point-read (i.e. a Get request) should be served from its local buffer. In such cases, we eschew sending the request to the KV layer, and instead stitch the result back on the response path. The approach above is fairly straightforward. The only thing we must take care of is ensuring the correct value is returned based on the Get request's sequence number. The logic here mirrors that of `getOne` in `pebble_mvcc_scanner.go`. It's a bit simpler, as we don't have to account for sequence number rollbacks here. Unlike on the server, we'll proactively rollback writes belonging to sequence numbers that have been ignored as part of a savepoint rollback. As a result, we can be sure that any writes in the buffer aren't rolled back. In a followup patch, we'll add support for Scan/Reverse requests that overlap with a part of the butffer. Informs cockroachdb#139054 Release note: None
arulajmani
added a commit
to arulajmani/cockroach
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this issue
Feb 7, 2025
This patch teaches the txnWriteBuffer to recognize when a point-read (i.e. a Get request) should be served from its local buffer. In such cases, we eschew sending the request to the KV layer, and instead stitch the result back on the response path. The approach above is fairly straightforward. The only thing we must take care of is ensuring the correct value is returned based on the Get request's sequence number. The logic here mirrors that of `getOne` in `pebble_mvcc_scanner.go`. It's a bit simpler, as we don't have to account for sequence number rollbacks here. Unlike on the server, we'll proactively rollback writes belonging to sequence numbers that have been ignored as part of a savepoint rollback. As a result, we can be sure that any writes in the buffer aren't rolled back. In a followup patch, we'll add support for Scan/Reverse requests that overlap with a part of the butffer. Informs cockroachdb#139054 Release note: None
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In service of #72614
Once we introduce a write buffer, transactions will buffer writes on the client until commit time. That means if a transaction wants to read its own writes, it must do so from the buffer (instead of going to the leaseholder).
Jira issue: CRDB-46540
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