T4-W05 Bulk Write Contract

Combined multi-session group

Policy: ALL_OR_NOTHING

• Complete roster required across canonical sessions.
• Invalid/out-of-roster student fails before write.
• Child saves run under a DB savepoint; any child failure rolls back the group.

Single-session save

Policy: request-transaction atomic upsert (PARTIAL_UPSERT_ALLOWED for subset of roster)

• Only students in session.batch_id.student_ids may be written (invalid_student otherwise).
• Payload may omit some roster members; omitted students are left unchanged.
• Mid-request failure aborts the HTTP request (no per-row savepoint).
• Upsert prevents duplicate lines for submitted students.

Explicit non-policies

• Not silent skip of invalid students with overall success.
• Not row-atomic commit of valid rows while keeping invalid rows when validation fails early.