# Changelog: Unified Builders Implementation

## Version: Breaking Change

### Summary

This release introduces mandatory use of unified builders (`RdbcItemReaderBuilder` and `RdbcItemWriterBuilder`) for all RDBC readers and writers. Direct instantiation is no longer possible.

## Breaking Changes

### 1. Removed Database-Specific Builder Types

**Removed:**
- `PostgresRdbcItemReaderBuilder`
- `MySqlRdbcItemReaderBuilder`
- `SqliteRdbcItemReaderBuilder`

**Replaced with:**
- `RdbcItemReaderBuilder` (unified, works for all database types)

### 2. Private Constructors and Fields

All reader and writer structs now have:
- `pub(crate)` fields (not accessible from outside the crate)
- `pub(crate) fn new()` methods (not accessible from outside the crate)
- `pub(crate)` builder methods (not accessible from outside the crate)

**Affected Structures:**
- `PostgresRdbcItemReader`
- `MySqlRdbcItemReader`
- `SqliteRdbcItemReader`
- `PostgresItemWriter`
- `MySqlItemWriter`
- `SqliteItemWriter`

### 3. Mandatory Builder Usage

**Before (No Longer Works):**
```rust
// ❌ Direct constructor - REMOVED
let reader = PostgresRdbcItemReader::new(pool, query, Some(100));

// ❌ Database-specific builder - REMOVED
let reader = PostgresRdbcItemReaderBuilder::new()
    .pool(pool)
    .query(query)
    .build();

// ❌ Direct writer construction - REMOVED
let writer = PostgresItemWriter::new()
    .pool(&pool)
    .table("users")
    .add_column("id");
```

**After (Required):**
```rust
// ✅ Unified reader builder - REQUIRED
let reader = RdbcItemReaderBuilder::<User>::new()
    .postgres(pool)
    .query(query)
    .with_page_size(100)
    .build_postgres();

// ✅ Unified writer builder - REQUIRED
let writer = RdbcItemWriterBuilder::<User>::new()
    .postgres(&pool)
    .table("users")
    .add_column("id")
    .postgres_binder(&binder)
    .build_postgres();
```

## Migration Guide

### For Readers

```rust
// Old code
use spring_batch_rs::item::rdbc::postgres_reader::PostgresRdbcItemReaderBuilder;

let reader = PostgresRdbcItemReaderBuilder::new()
    .pool(pool)
    .query("SELECT * FROM users")
    .with_page_size(100)
    .build();

// New code
use spring_batch_rs::item::rdbc::RdbcItemReaderBuilder;

let reader = RdbcItemReaderBuilder::<User>::new()
    .postgres(pool)
    .query("SELECT * FROM users")
    .with_page_size(100)
    .build_postgres();
```

### For Writers

```rust
// Old code
use spring_batch_rs::item::rdbc::mysql_writer::MySqlItemWriter;

let writer = MySqlItemWriter::new()
    .pool(&pool)
    .table("products")
    .add_column("id")
    .add_column("name")
    .item_binder(&binder);

// New code
use spring_batch_rs::item::rdbc::RdbcItemWriterBuilder;

let writer = RdbcItemWriterBuilder::<Product>::new()
    .mysql(&pool)
    .table("products")
    .add_column("id")
    .add_column("name")
    .mysql_binder(&binder)
    .build_mysql();
```

## Key Changes Summary

| Component | Status | Details |
|-----------|--------|---------|
| Database-specific builders | ❌ Removed | No longer exported or accessible |
| Direct `new()` constructors | 🔒 Private | Changed to `pub(crate)` |
| Struct fields | 🔒 Private | Changed to `pub(crate)` |
| Unified builders | ✅ Required | Only way to create readers/writers |
| Type safety | ✅ Improved | Compile-time database type checking |
| API consistency | ✅ Improved | Same pattern for all databases |

## Files Modified

### Core Implementation
- ✅ `src/item/rdbc/database_type.rs` - Added database type enum
- ✅ `src/item/rdbc/unified_reader_builder.rs` - New unified reader builder
- ✅ `src/item/rdbc/unified_writer_builder.rs` - New unified writer builder
- ✅ `src/item/rdbc/mod.rs` - Updated exports
- ✅ `src/item/rdbc/postgres_reader.rs` - Fields and methods now `pub(crate)`
- ✅ `src/item/rdbc/mysql_reader.rs` - Fields and methods now `pub(crate)`
- ✅ `src/item/rdbc/sqlite_reader.rs` - Fields and methods now `pub(crate)`
- ✅ `src/item/rdbc/postgres_writer.rs` - Fields and methods now `pub(crate)`
- ✅ `src/item/rdbc/mysql_writer.rs` - Fields and methods now `pub(crate)`
- ✅ `src/item/rdbc/sqlite_writer.rs` - Fields and methods now `pub(crate)`

### Tests
- ✅ `tests/rdbc_postgres.rs` - Updated to use unified builder
- ✅ `tests/rdbc_mysql.rs` - Updated to use unified builder
- ✅ `tests/rdbc_sqlite.rs` - Updated to use unified builder

### Examples
- ✅ `examples/log_records_from_postgres_database.rs` - Updated
- ✅ `examples/unified_rdbc_builder_example.rs` - Already using unified builder
- ✅ `examples/sqlite_writer_example.rs` - Updated
- ✅ All other example files - Updated

### Documentation
- ✅ `docs/unified_rdbc_builders.md` - Comprehensive usage guide
- ✅ `docs/migration_to_unified_builders.md` - Migration guide
- ✅ `docs/builder_enforcement.md` - Architecture documentation
- ✅ `docs/CHANGELOG_unified_builders.md` - This file

## Benefits

### For Users
1. **Consistent API** - Same pattern for all databases
2. **Type Safety** - Compile-time validation of database types
3. **Discoverability** - IDE autocomplete shows all options
4. **Clear Intent** - Database type explicit in code
5. **Easier Migration** - Simple to switch between databases

### For Maintainers
1. **Single Point of Change** - Updates in one place
2. **Validation Centralization** - All validation in builder
3. **Future Flexibility** - Easy to add features
4. **Quality Control** - Enforces best practices
5. **Reduced Code Duplication** - Shared builder logic

## Compiler Guarantees

With these changes, the Rust compiler now prevents:

✅ Creating readers/writers with invalid configurations
✅ Bypassing validation logic
✅ Using deprecated or removed APIs
✅ Mixing database types incorrectly
✅ Partial or invalid construction

## Compatibility

- **Rust Version:** No change in minimum supported Rust version
- **Dependencies:** No new dependencies added
- **Runtime Behavior:** No changes to runtime behavior
- **Performance:** Zero overhead - same performance as before
- **Binary Size:** Negligible change in binary size

## Testing

All tests updated and passing:
- ✅ Unit tests in reader/writer modules
- ✅ Integration tests in `tests/` directory
- ✅ Example code updated
- ✅ Documentation examples verified

## Rollout Recommendation

This is a **breaking change** that requires code updates. Recommended approach:

1. **Review migration guide** in `docs/migration_to_unified_builders.md`
2. **Update imports** to use `RdbcItemReaderBuilder` / `RdbcItemWriterBuilder`
3. **Update instantiation code** to use unified builders
4. **Test thoroughly** before deploying
5. **Update documentation** and internal guides

## Support

For questions or issues:
1. Check `docs/unified_rdbc_builders.md` for usage examples
2. Review `docs/builder_enforcement.md` for architecture details
3. See `examples/` directory for working code samples

## Future Enhancements

The unified builder architecture enables:
- Adding new database support easily
- Implementing common configuration across databases
- Adding validation and safety checks centrally
- Providing migration helpers for database switching
- Implementing database-agnostic code patterns

---

**Note:** This is a one-time breaking change that establishes a solid foundation for future development and provides long-term benefits for code quality and maintainability.
