Route SELECT at SERIAL/LOCAL_SERIAL consistency like LWT#227
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sylwiaszunejko wants to merge 4 commits into
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Route SELECT at SERIAL/LOCAL_SERIAL consistency like LWT#227sylwiaszunejko wants to merge 4 commits into
sylwiaszunejko wants to merge 4 commits into
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dkropachev
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May 20, 2026
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| internal static RetryDecisionWithReason GuardSerialConsistencyDowngrade(RetryDecisionWithReason result, ConsistencyLevel originalCl) | ||
| { | ||
| if (originalCl.IsSerialConsistencyLevel() | ||
| && result.Decision.DecisionType == RetryDecision.RetryDecisionType.Retry | ||
| && result.Decision.RetryConsistencyLevel.HasValue | ||
| && !result.Decision.RetryConsistencyLevel.Value.IsSerialConsistencyLevel()) | ||
| { | ||
| return new RetryDecisionWithReason(RetryDecision.Rethrow(), result.Reason); | ||
| } | ||
| return result; | ||
| } |
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There are two competing constraints in LWT case:
- We really really don't want to retry on antoher host
- There are cases when node is gone, so we have no choice
So, probably we need to decide based on case:
- If node is up and running we should make sure that cl is not degraded and we don't retry on next node
- For critical cases, when say taget node is down (at least UnavailableException),then we should throw
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Drivers rely on server-provided prepared metadata to detect LWT statements. However, the server cannot set the LWT flag for SELECT statements during PREPARE because the consistency level is only known at EXECUTE time. This change ensures that any statement executed with regular consistency SERIAL or LOCAL_SERIAL is treated as LWT-equivalent for routing.
When a statement is executed with SERIAL or LOCAL_SERIAL as regular consistency (serial/Paxos read), retry policies must not downgrade it to a non-serial consistency level like ONE or QUORUM, as that would break the serial read guarantees. Add GuardSerialConsistencyDowngrade() which intercepts retry decisions for ReadTimeout, WriteTimeout, and Unavailable errors: if the original CL was serial and the policy suggests a non-serial CL, the decision is overridden to Rethrow.
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Pull request overview
This PR adds client-side handling so that statements executed with regular consistency SERIAL / LOCAL_SERIAL are routed like LWT requests (i.e., deterministically to a single replica owner), compensating for the fact that the server can’t mark these as LWT during PREPARE because consistency is only known at EXECUTE time.
Changes:
- Treat
ConsistencyLevel.Serial/LocalSerialas “LWT-like” for routing purposes (including effective consistency coming from request options/execution profiles). - Add retry-guard logic intended to prevent unsafe retry behaviors for serial-consistency requests.
- Add unit/integration tests around serial-consistency LWT detection and routing behavior.
Reviewed changes
Copilot reviewed 10 out of 10 changed files in this pull request and generated 5 comments.
Show a summary per file
| File | Description |
|---|---|
| src/Cassandra/Statement.cs | Changes default IsLwt() behavior to treat serial regular CL as LWT-like. |
| src/Cassandra/BoundStatement.cs | Extends LWT detection to include serial regular consistency on bound statements. |
| src/Cassandra/Requests/RequestHandler.cs | Uses effective consistency (statement vs request options) and decorates statements to force LWT-like routing when needed. |
| src/Cassandra/Requests/RequestExecution.cs | Wraps retry decisions with a new guard for serial-consistency retry safety. |
| src/Cassandra.Tests/SerialConsistencyLwtTests.cs | Adds unit tests for serial-consistency LWT detection and guarded retry behavior. |
| src/Cassandra.IntegrationTests/ScyllaLwtTests.cs | Adds Scylla real-cluster tests asserting serial-consistency SELECTs stick to a single coordinator. |
| src/Cassandra.IntegrationTests/OpenTelemetry/OpenTelemetryTests.cs | Gates one OTEL integration test behind a Scylla version attribute. |
| src/Cassandra.IntegrationTests/Mapping/Tests/Update.cs | Gates UpdateIf_Applied_Test behind a Scylla version attribute. |
| src/Cassandra.IntegrationTests/Mapping/Tests/InsertTests.cs | Gates InsertIfNotExists_Applied_Test behind a Scylla version attribute. |
| src/Cassandra.IntegrationTests/Mapping/Tests/Delete.cs | Gates DeleteIf_Applied_Test behind a Scylla version attribute. |
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| /// Guards LWT (serial consistency) requests against unsafe retry behavior. | ||
| /// For LWT requests: | ||
| /// - UnavailableException (node is down): always rethrow, since retrying on another node is unsafe | ||
| /// - ReadTimeout/WriteTimeout (node is up): prevent CL downgrade and force retry on the same host |
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| [Category(TestCategory.RealCluster)] | ||
| [Test] | ||
| [Test, TestScyllaVersion(2026, 1)] | ||
| public async Task AddOpenTelemetry_MapperAndMapperAsync_SessionRequestIsParentOfNodeRequest() |
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| [Test, TestScyllaVersion(2026, 1)] | ||
| public void UpdateIf_Applied_Test() | ||
| { |
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| [Test, TestScyllaVersion(2026, 1)] | ||
| public void InsertIfNotExists_Applied_Test() | ||
| { |
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| [Test, TestScyllaVersion(2026, 1)] | ||
| public void DeleteIf_Applied_Test() | ||
| { |
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The server cannot mark SELECT statements as LWT during PREPARE because the consistency level is only provided at EXECUTE time. This PR adds client-side detection so that any statement executed with regular consistency SERIAL or LOCAL_SERIAL is routed like an LWT (to a single replica owner).
Fixes: https://scylladb.atlassian.net/browse/DRIVER-617