feat(spanner): define IsolationLevel enum for Spanner transactions by prathmeshcc · Pull Request #15853

feat(spanner): define IsolationLevel enum for Spanner transactions by prathmeshcc · Pull Request #15853 · googleapis/google-cloud-cpp

bot added the api: spanner

Issues related to the Spanner API.

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Jan 5, 2026

This commit improves the API design by integrating `IsolationLevel` directly into `Transaction::ReadWriteOptions`.

Previously, `IsolationLevel` was passed as a separate argument to some `Transaction` constructors, even though it is semantically relevant only to read-write transactions. This could lead to ambiguity or misuse.

By making `IsolationLevel` a member of `ReadWriteOptions`, the API more accurately reflects the underlying Spanner transaction semantics, enforcing that isolation levels are configured solely within the context of read-write transactions. This enhances type safety and improves the clarity of the client library's API.

This change addresses reviewer feedback regarding the logical grouping of `IsolationLevel` within the `ReadWriteOptions` message.#

This commit refines the API design by integrating `IsolationLevel` directly into `Transaction::ReadWriteOptions`.

Previously, `IsolationLevel` was passed as a separate argument to some `Transaction` constructors, even though it is semantically relevant only to read-write
transactions. This could lead to ambiguity or misuse.

These changes address reviewer feedback, enforcing that isolation levels are configured solely within the context of read-write transactions. This enhances type safe
and improves the clarity of the client library's API.

Key changes include:
 - Removing the `IsolationLevel` parameter from `Transaction` constructors, as it is now configured via `ReadWriteOptions`.
 - Enhancing `Transaction::ReadWriteOptions` with a `WithIsolationLevel` method, allowing for a fluent configuration of the desired isolation level (e.g.,
  `kSerializable`, `kRepeatableRead`).
 - Updating internal transaction handling logic to correctly propagate the selected isolation level from `ReadWriteOptions` to the Spanner backend.

Changed "Note:" to "@note" in the Doxygen documentation for `Transaction::IsolationLevel` to use the correct Doxygen tag. This improves consistency and ensures proper
rendering of the documentation.

This change modifies the `MakeOpts` function in `transaction.cc` to ensure that an explicitly provided `Transaction::IsolationLevel::kUnspecified` value is honored,
rather than falling back to a client-level default.

Previously, if `kUnspecified` was passed to `WithIsolationLevel()`, it would be treated as if no isolation level was specified, potentially leading to an unexpected
fallback. This update ensures that if a user explicitly requests `kUnspecified`, the `TransactionOptions` proto will reflect `ISOLATION_LEVEL_UNSPECIFIED`.

The `ReadWriteOptions` class in `transaction.h` was updated to use `absl::optional<IsolationLevel>` to correctly represent the presence or absence of an explicitly se
isolation level.
A new test case `IsolationLevelNotSpecified` was added to `transaction_test.cc` to verify this behavior, ensuring that when no isolation level is explicitly set, the default `ISOLATION_LEVEL_UNSPECIFIED` is used.