fix(sessions): use read-only transactions for get/list operations#4810
fix(sessions): use read-only transactions for get/list operations#4810giulio-leone wants to merge 1 commit intogoogle:mainfrom
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@rohityan Thanks for the heads-up! I'll get the Google CLA signed. Will follow up once it's done. |
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DatabaseSessionService.get_session() and list_sessions() are pure-read operations but currently open regular read-write transactions via _rollback_on_exception_session(). On Cloud Spanner this causes RetryAborted errors when a concurrent write commits during the read, because Spanner's OCC layer sees the read-write transaction as conflicting. Add _readonly_session() context manager that: - Marks the connection as read-only (postgresql_readonly=True) which also benefits Cloud Spanner's sqlalchemy-spanner dialect - Never commits, avoiding unnecessary write-path overhead - Still rolls back on exception to release the connection cleanly Switch get_session() and list_sessions() to use _readonly_session(). Write operations (create_session, delete_session, append_event) continue to use _rollback_on_exception_session() with explicit commits. Fixes google#4771
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Hi @rohityan — the CLA is now signed and passing ✅ (it was a |
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Summary
Fixes #4771
DatabaseSessionService.get_session()andlist_sessions()are pure-read operations but currently open regular read-write transactions via_rollback_on_exception_session(). On Cloud Spanner this causesRetryAbortederrors when a concurrent write commits during the read, because Spanner's OCC (Optimistic Concurrency Control) layer sees the read-write transaction as conflicting.Root Cause
Spanner uses OCC for read-write transactions. Even a
SELECT-only transaction opened in read-write mode will be aborted if a concurrent write touches the same data. Since reads never mutate data, they should use read-only transactions which are not subject to OCC aborts.Fix
Added a
_readonly_session()context manager that:execution_options(postgresql_readonly=True)sqlalchemy-spannerdialect to open read-only transactionsdatabase_session_factoryand connection poolSwitched
get_session()andlist_sessions()to use_readonly_session().Write operations (
create_session,delete_session,append_event) continue to use_rollback_on_exception_session()with explicit commits — unchanged.Testing
All 128 session tests pass. Full suite: 4725 passed, 0 failures, 0 regressions.