policies: treat SERIAL/LOCAL_SERIAL consistency as LWT for routing#887
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mykaul wants to merge 1 commit into
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policies: treat SERIAL/LOCAL_SERIAL consistency as LWT for routing#887mykaul wants to merge 1 commit into
mykaul wants to merge 1 commit into
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Statements with SERIAL or LOCAL_SERIAL consistency level are serialized through the Paxos path on the server, but TokenAwarePolicy only checked is_lwt() (from server prepare metadata) when deciding whether to skip replica shuffling. This meant serial-consistency reads could be routed with shuffled replicas instead of the deterministic order needed for optimal Paxos coordination. Now TokenAwarePolicy also checks the statement's consistency level and skips shuffling for SERIAL/LOCAL_SERIAL, matching LWT routing behavior. Fixes: scylladb#886
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Pull request overview
Updates TokenAwarePolicy routing behavior so statements executed with SERIAL / LOCAL_SERIAL consistency are treated like LWT-routing candidates (replica order preserved), matching expected Paxos/serial-path coordinator selection and fixing the replica-shuffle behavior described in #886.
Changes:
- Skip replica shuffling in
TokenAwarePolicywhenquery.consistency_levelisSERIALorLOCAL_SERIAL. - Add a unit test ensuring
shuffle()is not called for serial-consistency statements on both vnode and tablet-style metadata setups.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.
| File | Description |
|---|---|
cassandra/policies.py |
Adjusts shuffle condition to preserve replica order for serial-consistency statements (in addition to LWT). |
tests/unit/test_policies.py |
Adds regression test asserting shuffle() is not invoked for SERIAL / LOCAL_SERIAL statements. |
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sylwiaszunejko
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May 20, 2026
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Looks good to me, @dkropachev could you also take a look?
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Summary
TokenAwarePolicynow skips replica shuffling for statements withSERIALorLOCAL_SERIALconsistency level, treating them as LWT-routing candidatesis_lwt()(from server prepare metadata) was checked, meaning serial-consistency reads got shuffled replicas instead of deterministic Paxos-optimal orderingFixes: #886
Related: scylladb/java-driver#885