# BAT-011 Design

## Architecture summary

- Introduce dedicated write-oriented application modules under `jarca_ledger_mcp/application/` for accounts, transactions, budgets, and category rules/categories.
- Keep broad modules (`system_services.py`, `asset_account_services.py`, `transaction_report_services.py`, `categorization_services.py`) as compatibility aggregation layers for read/report/mixed responsibilities that are not yet in scope.
- Preserve `service.py` as a thin re-export facade and avoid MCP/Streamlit/dashboard call-site churn.

## Changed areas

- `jarca_ledger_mcp/application/accounts_write_services.py`
- `jarca_ledger_mcp/application/transactions_write_services.py`
- `jarca_ledger_mcp/application/budgets_write_services.py`
- `jarca_ledger_mcp/application/category_rules_write_services.py`
- Existing application modules that previously held these implementations
- `jarca_ledger_mcp/application/__init__.py`
- `jarca_ledger_mcp/service.py`

## Key decisions

- Move only mutation implementations in BAT-011; read/report/import/advanced workflow logic stays in place.
- Favor direct function relocation plus compatibility re-export over new abstraction layers to minimize contract risk.
- Keep shared helpers in `application/shared.py`; do not introduce storage-layer refactors in this batch.

## Edge cases

- `create_ledger_account` remains paired with `update_ledger_account` even though the old implementations lived in different modules.
- Category rule preview/apply remains out of BAT-011 scope if moving it would force read/workbench coupling; mutation extraction takes precedence.
- Any dashboard/MCP references must continue resolving through `service.py` without signature changes.

## Architecture Impact

- none

## Reference Doc Update Plan

- No baseline doc rewrite in BAT-011 unless the final module tree differs materially from the existing dcy-011/REQ-015 description.