Async Transaction Boundaries

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Status: adopted Decision level: mandatory Applies to: api-runtime, workers, tests Last reviewed: 2026-06-17

Purpose

Define consistent transaction demarcation for async SQLAlchemy so write behavior is predictable, rollback semantics are clear, and concurrent request flows remain safe.

Scope and Non-Goals

In scope: transaction ownership, write/read policy, exception and rollback behavior, nested transaction guidance.
Out of scope: business-domain validation rules and cross-service distributed transactions.

Rules

Every mutating use case must run inside an explicit transaction boundary.
Prefer async with session.begin(): for write units.
Keep transaction ownership at service/use-case boundary, not deep in helper internals.
Read paths should not auto-upgrade into hidden write behavior.
On exception in a transaction block, rely on rollback semantics and propagate or map exceptions intentionally.

Recommended Patterns

Pattern A: Single write unit

async def create_order(session: AsyncSession, payload: OrderIn) -> Order:
    async with session.begin():
        order = Order(...)
        session.add(order)
        # additional writes...
    return order

Pattern B: Explicit read flow

async def get_order(session: AsyncSession, order_id: UUID) -> Order | None:
    stmt = select(Order).where(Order.id == order_id)
    return await session.scalar(stmt)

Pattern C: Nested transaction (only when required)

async with session.begin():
    # outer transaction
    async with session.begin_nested():
        # savepoint-scoped operation
        ...

Use nested transactions only when partial failure semantics are explicitly required.

Exception and Rollback Policy

Write block fails: transaction context rolls back.
Caller decides whether to translate exception (for example to domain/API errors).
Do not swallow DB exceptions silently; map or re-raise intentionally.

Anti-Patterns

Multiple commits scattered across one logical use case.
Helper functions that commit/rollback without caller awareness.
Mixing implicit and explicit transaction styles in confusing ways.
Using savepoints as a default pattern rather than a targeted tool.

Operational Checks

All mutating service functions declare one clear transaction boundary.
No repository/helper performs hidden commit calls.
Transaction style is consistent across handlers and workers.

Testing Checks

Success path test verifies expected durable writes.
Failure path test verifies rollback behavior.
Tests cover concurrency-sensitive write flows.
Savepoint usage (if present) has dedicated behavior tests.

Migration Notes

First stabilize session scope, then normalize transaction ownership.
Replace ad hoc commit patterns incrementally with bounded write units.

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