github-actions[bot] commented on code in PR #64647:
URL: https://github.com/apache/doris/pull/64647#discussion_r3441137024
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cloud/src/meta-service/meta_service_txn.cpp:
##########
@@ -1853,7 +1853,11 @@ void MetaServiceImpl::commit_txn_immediately(
int64_t table_id = i.first;
std::string ver_key = table_version_key({instance_id, db_id,
table_id});
std::string ver_val;
- err = txn->get(ver_key, &ver_val);
+ // snapshot read: the returned table version is only a hint
for FE's version
+ // cache; the real increment is done by update_table_version()
via atomic_add.
+ // A non-snapshot read would add ver_key to the read-conflict
set and make
+ // concurrent commits on the same table conflict
(KV_TXN_CONFLICT).
+ err = txn->get(ver_key, &ver_val, true);
Review Comment:
This snapshot read makes the response version come from a pre-commit value
that is no longer protected by a read conflict. Two concurrent commits on the
same table can both read table version N, both apply `update_table_version()`
via `atomic_add`, and both return N+1 in `TableStatsPB`. FE uses that value as
the table invalidation version (`CloudGlobalTransactionMgr.updateVersion` ->
`OlapTable.setCachedTableVersion`), and SQL cache/MTMV/statistics consumers
compare `OlapTable.getVisibleVersion()`. A query cache entry created after the
first response at N+1 can therefore remain valid after the second response,
because the table version does not change even though another commit became
visible. Existing concurrent lazy-commit coverage also expects the two returned
table versions to be distinct. Please preserve a unique monotonic response
version per committed table update, or avoid updating FE caches from an
advisory stale value and force a safe sync, while still addressing the KV
conflict.
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