feat(dbapi): use inline begin to eliminate BeginTransaction RPC

[waiho-gumloop]

Fixes #1503

Summary

Connection.transaction_checkout() currently calls Transaction.begin() explicitly before the first query, which sends a standalone BeginTransaction gRPC RPC. This is unnecessary because the Transaction class already supports inline begin — piggybacking BeginTransaction onto the first ExecuteSql/ExecuteBatchDml request via TransactionSelector(begin=...).

This PR removes the explicit begin() call, letting the existing inline begin logic in execute_sql(), execute_update(), and batch_update() handle transaction creation. This saves one gRPC round-trip per transaction (~16ms measured on the emulator).

What changed

google/cloud/spanner_dbapi/connection.py — transaction_checkout():

• Removed self._transaction.begin() on L413
• The transaction is now returned with _transaction_id=None
• Updated docstring to explain the inline begin behavior

tests/unit/spanner_dbapi/test_connection.py:

• Added test_transaction_checkout_does_not_call_begin to assert begin() is not called

tests/mockserver_tests/test_dbapi_inline_begin.py (new):

• 9 mockserver tests verifying inline begin behavior for read-write transactions
• Covers: no BeginTransactionRequest sent, first ExecuteSqlRequest uses TransactionSelector(begin=...), transaction ID reuse on second statement, rollback, read-only unaffected, retry after abort

tests/mockserver_tests/test_tags.py:

• Updated 4 read-write tag tests: removed BeginTransactionRequest from expected RPC sequences, adjusted tag index offsets

tests/mockserver_tests/test_dbapi_isolation_level.py:

• Updated 4 isolation level tests: verify isolation level on ExecuteSqlRequest.transaction.begin.isolation_level instead of BeginTransactionRequest.options.isolation_level

Why this is safe

The inline begin code path already exists and is battle-tested — Session.run_in_transaction() creates a Transaction without calling begin() and relies on the same inline begin logic (session.py L566).

Specific safety analysis:

1. transaction_checkout() callers always execute SQL immediately: It's only called from run_statement() → execute_sql() and batch_dml_executor → batch_update(). Both set _transaction_id via inline begin before any commit/rollback path.

2. execute_sql/execute_update/batch_update handle _transaction_id is None: They acquire a lock, use _make_txn_selector() which returns TransactionSelector(begin=...), and store the returned _transaction_id (transaction.py L612-L623).

3. rollback() handles _transaction_id is None: Skips the RPC — correct when no server-side transaction exists (transaction.py L163).

4. commit() handles _transaction_id is None: Falls back to _begin_mutations_only_transaction() for mutation-only transactions (transaction.py L263-L267).

5. Retry mechanism is compatible: _set_connection_for_retry() resets _spanner_transaction_started=False, so replayed statements go through transaction_checkout() again, create a fresh Transaction, and use inline begin.

PEP 249 conformance

This change is fully conformant with PEP 249 (DB-API 2.0). The spec does not define a begin() method — transactions are implicit. The PEP author clarified on the DB-SIG mailing list that "transactions start implicitly after you connect and after you call .commit() or .rollback()", and the mechanism by which the driver starts the server-side transaction is an implementation detail. Deferring the server-side begin to the first SQL execution (as psycopg2 and other mature DB-API drivers do) is the standard approach. The observable transactional semantics — atomicity between commit()/rollback() calls — are unchanged.

Performance impact

Before (4 RPCs per read-write transaction):

BeginTransaction → ExecuteSql (read) → ExecuteSql (write) → Commit

After (3 RPCs per read-write transaction):

ExecuteSql (read, with inline begin) → ExecuteSql (write) → Commit

Measured ~16ms savings per transaction on the Spanner emulator.

Context

This optimization was identified while profiling SQLAlchemy/DBAPI performance against Spanner. The DBAPI was created ~2 years before inline begin support was added to the Python client library (inline begin landed in PR #740, Dec 2022). The run_in_transaction path was updated to use inline begin, but transaction_checkout was not.

Test plan

• All 198 DBAPI unit tests pass (tests/unit/spanner_dbapi/)
• New unit test verifies begin() is not called by transaction_checkout()
• 9 new mockserver tests verify inline begin RPC behavior (tests/mockserver_tests/test_dbapi_inline_begin.py)
• 8 existing mockserver tests updated for inline begin expectations (test_tags.py, test_dbapi_isolation_level.py)
• All 53 DBAPI system tests pass against Spanner emulator (tests/system/test_dbapi.py)
• CI system tests (will run automatically)

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[gemini-code-assist]

Summary of Changes

Hello @waiho-gumloop, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request optimizes transaction handling in the Spanner DBAPI by leveraging the existing inline begin mechanism. It removes an unnecessary explicit BeginTransaction gRPC call, allowing transactions to be initiated implicitly with the first ExecuteSql or ExecuteBatchDml request. This change streamlines the transaction workflow, reduces network overhead, and improves performance by saving one RPC round-trip per transaction.

Highlights

• Eliminated BeginTransaction RPC: Removed the explicit _transaction.begin() call in Connection.transaction_checkout(), allowing the first SQL execution to initiate the transaction via inline begin.
• Performance Improvement: Reduced one gRPC round-trip per transaction, resulting in approximately 16ms savings on the Spanner emulator.
• Docstring Update: Updated the transaction_checkout() docstring to explain the new inline begin behavior.
• New Unit Test: Added test_transaction_checkout_does_not_call_begin to verify that begin() is no longer explicitly called.

Changelog

• google/cloud/spanner_dbapi/connection.py
  • Removed the explicit call to self._transaction.begin() within the transaction_checkout method.
  • Updated the docstring for transaction_checkout to describe the inline begin behavior and the elimination of a separate BeginTransaction RPC.
• tests/unit/spanner_dbapi/test_connection.py
  • Added a new unit test test_transaction_checkout_does_not_call_begin to assert that Transaction.begin() is not called by transaction_checkout.

Activity

• All 198 existing DBAPI unit tests have passed.
• All 53 existing DBAPI system tests have passed against the Spanner emulator.
• A new unit test was added to specifically verify the absence of the begin() call in `transaction_checkout().
• CI system tests are configured to run automatically.

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[gemini-code-assist]

Code Review

This pull request introduces a valuable optimization by removing the explicit BeginTransaction RPC call in transaction_checkout. Instead, it leverages the existing inline begin functionality, where the transaction is initiated with the first SQL execution. This change effectively reduces latency by saving one network round-trip per transaction. The modification is well-supported by a new unit test ensuring begin() is not called, and the detailed description provides a thorough safety analysis, confirming the change is sound.