Stage 10 · L34

Give async sessions and serialization a clear owner

Optional extension · extended visuals/current Equipment runtime; not original Session 8 implementation

Optional resource implementation checked in isolated PostgreSQL/HTTP fixtures.

This optional extension changes the database execution model deliberately. async_resources owns an async engine and AsyncSession factory using the installed psycopg async dialect. It does not use asyncpg and does not convert the core provider preparation to async. The caller enters the resource context and awaits disposal at exit.

A factory can be shared; a mutable AsyncSession cannot be shared across concurrent units of work. export_rental creates one session for one operation. The test runs two exports concurrently through the same factory and confirms independent results. Sharing a session would mix transaction state and can fail under concurrent driver activity.

Load everything required before serialization. The query joinedloads equipment, constructs RentalDetail inside the session and returns the DTO after cleanup. A lazy relationship accessed afterward would either fail or attempt I/O from an unsupported serialization context. expire_on_commit=False can prevent scalar expiration but is not a substitute for a relationship load plan.

The guided create_customer uses async with session.begin, awaits flush for its UUID, validates CustomerOut, and lets successful exit commit. A duplicate email raises a real PostgreSQL IntegrityError and rolls back that operation; a later create still succeeds. A fresh synchronous observer verifies the committed row count. The value is an inactive customer profile, not a fabricated verified identity.

The independent transfer implements scoped async customer export and public output only. An ordinary actor can read its own UUID; another customer is masked404; an admin has the deliberate broader read scope. The DTO must serialize after session/engine cleanup without password hash, role, credential version or an ORM reference.

async with factory() as session:
    result = await session.execute(statement_with_eager_load)
    output = RentalDetail.model_validate(result.scalar_one())
# output is plain/public and usable after owned cleanup.

Tasks may share the engine/session factory, not one mutable AsyncSession. Each operation owns a new session, applies current actor scope, constructs CustomerOut inside it and returns a public DTO after cleanup. Another customer is masked404; the deliberate admin scope is wider. Async PostgreSQL is optional; core provider preparation remains synchronous.