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fix(spanner): prioritize leader replica for read-write transactions in location-aware routing - #20620
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…n location-aware routing When location-aware routing is enabled, read-write requests that prefer the leader could be routed to a follower because a non-zero operation UID on the routing hint sent fillRoutingHintAttempt down the score-aware replica selection path before the local leader was considered. There the leader only receives a cost bias, so a sufficiently faster follower would win. Check for a healthy local leader first whenever preferLeader is set, and fall back to score-aware selection only when the leader is missing, remote, or unhealthy. This mirrors googleapis/google-cloud-java#14195.
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This pull request modifies the routing logic in spanner/key_range_cache.go to ensure that a healthy local leader is always selected when a request prefers the leader, even if latency scores are available. This prevents read-write transactions from being incorrectly routed to followers. Additionally, corresponding unit tests have been added in spanner/key_range_cache_test.go to verify this behavior and its fallback mechanism when the leader is unhealthy. There are no review comments to address, so no further feedback is provided.
sakthivelmanii
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Oct 1, 2026
rahul2393
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Oct 7, 2026
🤖 I have created a release *beep* *boop* --- ## [1.96.0](spanner/v1.95.1...spanner/v1.96.0) (2026-10-07) ### Features * **all:** Update supported go versions ([#20520](#20520)) ([90413d3](90413d3)) * **spanner:** Add experimental vtprotobuf decoding with ExperimentalBorrowRows under the spanner_vtproto build tag ([#20642](#20642)) ([fdbd4f4](fdbd4f4)) ### Bug Fixes * **spanner:** Keep DCP transaction on its channel while it drains ([#20618](#20618)) ([93dfc20](93dfc20)) * **spanner:** Prioritize leader replica for read-write transactions in location-aware routing ([#20620](#20620)) ([a524d66](a524d66)) * **spanner:** Record streaming built-in metrics per result stream ([#20596](#20596)) ([be75f7e](be75f7e)) * **spanner:** Retry and resume BatchWrite streams on UNAVAILABLE ([#20623](#20623)) ([a302277](a302277)) ### Performance Improvements * **spanner:** Skip builtin metrics tracer allocation when metrics are disabled ([#20582](#20582)) ([9971a0c](9971a0c)) * **spanner:** Skip reading response headers when no latency sink is enabled ([#20583](#20583)) ([3dcc2c3](3dcc2c3)) --- This PR was generated with [Release Please](https://github.com/googleapis/release-please). See [documentation](https://github.com/googleapis/release-please#release-please). Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>
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When location-aware routing is enabled, read-write requests that prefer the leader could be routed to a follower because a non-zero operation UID on the routing hint sent fillRoutingHintAttempt down the score-aware replica selection path before the local leader was considered. There the leader only receives a cost bias, so a sufficiently faster follower would win.
Check for a healthy local leader first whenever preferLeader is set, and fall back to score-aware selection only when the leader is missing, remote, or unhealthy. This mirrors googleapis/google-cloud-java#14195.