---
name: nextjs-data-access-layer
description: Build secure, reusable data access patterns with DTOs, taint checks, and colocated authorization in Next.js. Use when centralizing database queries, transforming raw data to DTOs, adding server-only guards, or preventing sensitive data from reaching Client Components.
metadata:
  triggers:
    files:
    - '**/lib/data.ts'
    - '**/dal/**'
    keywords:
    - DAL
    - Data Access Layer
    - server-only
    - DTO
---
# Data Access Layer (DAL)

## **Priority: P1 (HIGH)**

Centralize all data access (Database & External APIs) to ensure consistent security, authorization, and caching.

## Workflow

1. **Create DAL module** in `services/` or `lib/data.ts` with `import 'server-only'`.
2. **Verify auth** inside every DAL function using `await auth()`.
3. **Transform** raw DB/API data into DTOs before returning to components.
4. **Wrap** with `cache()` from React to deduplicate requests within render cycle.
5. **Taint-check** sensitive objects to prevent accidental client exposure.

See [implementation examples](references/implementation.md)

## Implementation Guidelines

- **DTOs**: Always transform raw data into plain objects. Never return ORM model instances.
- **Security**: Use `taintObjectReference` or `taintUniqueValue` from experimental taint API to guard sensitive data.
- **Authorization**: Colocate auth checks inside every DAL function. Never rely on UI layer.
- **Caching**: Wrap DAL functions in `cache()` to deduplicate within single render.
- **Error Handling**: Throw standardized errors (`NotFoundError`, `UnauthorizedError`) caught by `error.tsx` or `notFound()`.

## Limitations

- **Client Components** cannot import DAL files. Use Server Actions or Route Handlers as bridges.

## Anti-Patterns

- **No auth checks outside DAL**: Auth verification must live inside DAL functions.
- **No raw ORM instances returned**: Transform to plain DTO objects before returning.
- **No `fetch('localhost/api')` in Server Components**: Call DAL functions directly.
- **No DAL imports in Client Components**: Use Server Actions or Route Handlers as bridges.

## Canonical response anchors

When this skill applies, preserve the following domain terminology or equivalent concrete examples in the answer when relevant:
- transform