3.x: Route SELECT at SERIAL/LOCAL_SERIAL consistency like LWT

[nikagra]

Note

Fixes DRIVER-616. Related to: #886

Problem

Statements with SERIAL or LOCAL_SERIAL set as their main consistency level (via setConsistencyLevel) are serialised through the Paxos/serial path on the server side — the same path used by LWT writes. However, the 3.x driver only applied LWT load-balancing routing (e.g. PRESERVE_REPLICA_ORDER) to statements where isLWT() returned true, which is driven exclusively by Scylla's server-side prepare metadata.

A normal SELECT prepared or executed with LOCAL_SERIAL consistency:

SimpleStatement simpleSelect =
    new SimpleStatement("SELECT * FROM t WHERE pk=?")
        .setConsistencyLevel(ConsistencyLevel.LOCAL_SERIAL);
PreparedStatement ps = session.prepare(simpleSelect);
BoundStatement bs = ps.bind(1);
// bs.isLWT() == false  → routed as REGULAR, not PRESERVE_REPLICA_ORDER

was routed as REGULAR, ignoring the configured LWT routing method entirely.

Changes

• TokenAwarePolicy: getRequestRouting() now treats statements with SERIAL or LOCAL_SERIAL main consistency as LWT for routing method selection (same as isLWT()). In PreserveReplicaOrderIterator, remote-DC replicas are excluded when the consistency is LOCAL_SERIAL (preserving local-DC semantics).
• LatencyAwarePolicy: The latency-reordering bypass that already applied to isLWT() statements is extended to cover SERIAL/LOCAL_SERIAL consistency statements.
• RackAwareRoundRobinPolicy: The rack-prioritization bypass already in place for isLWT() is extended to cover SERIAL/LOCAL_SERIAL consistency statements.

What is NOT changed

• setSerialConsistencyLevel() (the paxos-phase serial CL option on LWT writes) is intentionally not affected — only the main setConsistencyLevel() triggers LWT routing.
• isLWT() semantics are unchanged; this is purely additive.

Tests

• TokenAwarePolicyTest: 7 new unit tests — serial consistency routed as LWT, LOCAL_SERIAL excludes remote replicas, plain SERIAL keeps remote replicas, serial-CL-option does not trigger LWT routing, null routing-method falls back to regular.
• LatencyAwarePolicyTest: 1 new unit test — serial consistency statements bypass latency reordering.
• RackAwareRoundRobinPolicyTest: 1 new unit test — serial consistency statements skip rack prioritization.
• LWTLoadBalancingIT: New CCM integration test — prepares a SimpleStatement with LOCAL_SERIAL/SERIAL consistency, binds and executes it, verifies coordinator is non-null.

All 67 unit tests pass (mvn test -pl driver-core -Dgroups=unit).

[Copilot]

Pull request overview

Extends LWT load-balancing behavior to non-LWT statements that use SERIAL/LOCAL_SERIAL as their main consistency level, so that they are routed through the Paxos-aware path (e.g. PRESERVE_REPLICA_ORDER) instead of as REGULAR queries. The change touches three load-balancing policies and adds unit + CCM integration coverage.

Changes:

• TokenAwarePolicy.getRequestRouting treats serial-CL statements as LWT for routing; PreserveReplicaOrderIterator filters out remote replicas for LOCAL_SERIAL.
• LatencyAwarePolicy and RackAwareRoundRobinPolicy extend their existing isLWT() bypasses to cover serial-CL statements via a duplicated hasSerialConsistency helper.
• New unit tests across the three policies and a new LWTLoadBalancingIT CCM integration test.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
driver-core/src/main/java/com/datastax/driver/core/policies/TokenAwarePolicy.java	Route serial-CL statements as LWT; restrict to local replicas only for LOCAL_SERIAL.
driver-core/src/main/java/com/datastax/driver/core/policies/LatencyAwarePolicy.java	Bypass latency reordering for serial-CL statements.
driver-core/src/main/java/com/datastax/driver/core/policies/RackAwareRoundRobinPolicy.java	Skip rack prioritization for serial-CL statements.
driver-core/src/test/java/com/datastax/driver/core/policies/TokenAwarePolicyTest.java	New unit tests covering serial routing, LOCAL_SERIAL local-only filtering, and null routing-method fallback.
driver-core/src/test/java/com/datastax/driver/core/policies/LatencyAwarePolicyTest.java	New test verifying serial-CL statements are not latency-reordered.
driver-core/src/test/java/com/datastax/driver/core/policies/RackAwareRoundRobinPolicyTest.java	New test verifying serial-CL statements skip rack prioritization.
driver-core/src/test/java/com/datastax/driver/core/policies/LWTLoadBalancingIT.java	New CCM integration test for SERIAL/LOCAL_SERIAL SELECT routing.