# Bulk Merge Helper Design

**Date:** 2026-01-01
**Status:** Draft - Pending Review

## Overview

Replace the current `StagingTableManager` approach with a streamlined `IBulkMergeHelper` backed by source-generated `IDataReader` converters for efficient `SqlBulkCopy` operations.

## Goals

1. Simplify bulk merge operations to a single method call with expression-based configuration
2. Generate efficient `IAsyncEnumerable<T>` to `IDataReader` converters at compile time
3. Provide better error diagnostics with optional pre-validation
4. Remove manual staging table management code

## Architecture

```
┌─────────────────────────────────────────────────────────────────┐
│                    JdeScoping.DataSync                          │
│  ┌─────────────────────┐    ┌─────────────────────────────────┐ │
│  │ BulkCopyTypeRegistry│    │      IBulkMergeHelper           │ │
│  │ - Lists types to    │    │  - MergeAsync<T>(...)           │ │
│  │   generate for      │    │  - Uses IDataReaderFactory      │ │
│  └─────────────────────┘    │  - Builds MERGE SQL from exprs  │ │
│            │                └─────────────────────────────────┘ │
│            │ (analyzed by)              │                       │
│            ▼                            │ (uses)                │
│  ┌─────────────────────┐                ▼                       │
│  │  Source Generator   │    ┌─────────────────────────────────┐ │
│  │  - Generates        │───▶│  Generated Code:                │ │
│  │    IDataReader      │    │  - WorkOrderDataReader          │ │
│  │    wrappers         │    │  - LotDataReader                │ │
│  │  - Generates DI     │    │  - DataReaderFactory impl       │ │
│  │    registration     │    │  - AddBulkCopyConverters()      │ │
│  └─────────────────────┘    └─────────────────────────────────┘ │
└─────────────────────────────────────────────────────────────────┘
```

## Design Decisions

| Decision | Choice | Rationale |
|----------|--------|-----------|
| Type identification | Explicit list in `BulkCopyTypeRegistry.cs` | Keeps Core project free of bulk copy concerns |
| Registry location | DataSync project | Consolidates bulk copy knowledge in one place |
| API style | Single method with expression parameters | Simple, all config visible in one place |
| Conditional updates | Explicit `updateWhen` expression | Flexible, not tied to property naming conventions |
| Error handling | Hybrid - context wrapping + optional validation | Balances performance with debuggability |
| Transactions | None - each batch independent | Matches current behavior, idempotent syncs |
| Generator project | Single `JdeScoping.DataSync.SourceGenerators` | Simple, can extract later if needed |
| DI pattern | Generic `IDataReaderFactory` | Single injection point, easy to mock |
| DELETE support | None | YAGNI, matches current behavior |
| Migration config | Convention + override | Less boilerplate, explicit when needed |

## Component Details

### 1. BulkCopyTypeRegistry

Location: `JdeScoping.DataSync/BulkCopyTypeRegistry.cs`

```csharp
namespace JdeScoping.DataSync;

public static class BulkCopyTypeRegistry
{
    public static readonly Type[] Types =
    [
        typeof(WorkOrder),
        typeof(Lot),
        typeof(LotUsage),
        typeof(Item),
        typeof(WorkCenter),
        typeof(ProfitCenter),
        typeof(JdeUser),
        typeof(Branch),
        typeof(MisData),
    ];
}
```

### 2. Source Generator

Project: `JdeScoping.DataSync.SourceGenerators`

**Generated DataReader wrapper (per type):**
```csharp
public sealed class WorkOrderDataReader : IDataReader
{
    private readonly IAsyncEnumerator<WorkOrder> _enumerator;
    private WorkOrder? _current;

    private static readonly string[] _columnNames =
        ["WorkOrderNumber", "BranchCode", "LotNumber", ...];

    public object GetValue(int i) => i switch
    {
        0 => _current!.WorkOrderNumber,
        1 => _current!.BranchCode,
        // ... generated for each property
    };

    public bool Read()
    {
        return _enumerator.MoveNextAsync().AsTask().GetAwaiter().GetResult();
    }

    // IDataReader implementation...
}
```

**Generated factory:**
```csharp
public sealed class DataReaderFactory : IDataReaderFactory
{
    public IDataReader CreateReader<T>(IAsyncEnumerable<T> source)
    {
        return source switch
        {
            IAsyncEnumerable<WorkOrder> wo => new WorkOrderDataReader(wo),
            IAsyncEnumerable<Lot> lot => new LotDataReader(lot),
            _ => throw new NotSupportedException($"No converter for {typeof(T).Name}")
        };
    }
}
```

**Generated DI extension:**
```csharp
public static class BulkCopyServiceCollectionExtensions
{
    public static IServiceCollection AddBulkCopyConverters(this IServiceCollection services)
    {
        services.AddSingleton<IDataReaderFactory, DataReaderFactory>();
        return services;
    }
}
```

### 3. IBulkMergeHelper Interface

Location: `JdeScoping.DataSync/Contracts/IBulkMergeHelper.cs`

```csharp
namespace JdeScoping.DataSync.Contracts;

public interface IBulkMergeHelper
{
    Task<MergeResult> MergeAsync<T>(
        IAsyncEnumerable<T> data,
        string destinationTable,
        Expression<Func<T, object>> matchOn,
        Expression<Func<T, object>>? updateColumns = null,
        Expression<Func<T, T, bool>>? updateWhen = null,
        Expression<Func<T, object>>? insertColumns = null,
        string? tempTableName = null,
        int batchSize = 0,
        bool validateBeforeCopy = false,
        CancellationToken cancellationToken = default);
}

public record MergeResult(
    int TotalRowsProcessed,
    int RowsInserted,
    int RowsUpdated,
    int BatchCount,
    TimeSpan Elapsed);
```

**Parameters:**

| Parameter | Purpose | Default |
|-----------|---------|---------|
| `data` | Source records to merge | required |
| `destinationTable` | Target SQL table name | required |
| `matchOn` | PK expression for MERGE ON clause | required |
| `updateColumns` | Columns to SET on match | null = all non-PK |
| `updateWhen` | Condition for UPDATE | null = always update |
| `insertColumns` | Columns for INSERT | null = all columns |
| `tempTableName` | Staging table name | `#TEMP_{table}` |
| `batchSize` | Rows per batch | 0 = all at once |
| `validateBeforeCopy` | Pre-validate data against schema | false |

### 4. BulkMergeHelper Implementation

Location: `JdeScoping.DataSync/Services/BulkMergeHelper.cs`

**Processing flow:**
```
1. Parse expressions → extract column names
   matchOn: x => new { x.A, x.B }  →  ["A", "B"]

2. Get destination table schema (for temp table creation)
   SELECT TOP 0 * FROM WorkOrder → column types/lengths

3. Create temp table matching destination schema
   CREATE TABLE #TEMP_WorkOrder (... same columns ...)

4. If validateBeforeCopy: load schema constraints

5. Stream data in batches:
   foreach batch:
     a. Collect batchSize records from IAsyncEnumerable
     b. If validate: check each row against schema
     c. Create IDataReader via IDataReaderFactory
     d. SqlBulkCopy to temp table
     e. Execute MERGE statement
     f. TRUNCATE temp table
     g. Accumulate inserted/updated counts

6. DROP temp table (in finally block)

7. Return MergeResult with totals
```

**Generated MERGE SQL:**
```sql
MERGE INTO [WorkOrder] AS target
USING [#TEMP_WorkOrder] AS source
ON target.[WorkOrderNumber] = source.[WorkOrderNumber]
   AND target.[BranchCode] = source.[BranchCode]

WHEN MATCHED AND source.[LastUpdateDt] > target.[LastUpdateDt] THEN
    UPDATE SET
        target.[StatusCode] = source.[StatusCode],
        target.[OrderQuantity] = source.[OrderQuantity],
        target.[LastUpdateDt] = source.[LastUpdateDt]

WHEN NOT MATCHED THEN
    INSERT ([WorkOrderNumber], [BranchCode], [StatusCode], ...)
    VALUES (source.[WorkOrderNumber], source.[BranchCode], ...);

SELECT @@ROWCOUNT;
```

### 5. Error Handling

**Exception hierarchy:**
```csharp
public class BulkMergeException : Exception
{
    public string TableName { get; init; }
    public int BatchNumber { get; init; }
    public int RowsInBatch { get; init; }
    public string? SqlStatement { get; init; }
}

public class BulkMergeValidationException : BulkMergeException
{
    public IReadOnlyList<ValidationError> Errors { get; init; }
}

public record ValidationError(
    int RowIndex,
    string ColumnName,
    object? Value,
    string Message);
```

**Validation checks (when `validateBeforeCopy: true`):**

| Check | Example Error |
|-------|---------------|
| String length | `"Column 'StatusCode' value 'TOOLONG' exceeds max length 5 at row 42"` |
| Null in non-nullable | `"Column 'WorkOrderNumber' cannot be null at row 17"` |
| Type mismatch | `"Column 'OrderQuantity' expected int, got string at row 89"` |
| Decimal precision | `"Column 'Amount' value 12345.6789 exceeds precision(10,2) at row 5"` |

### 6. DI Registration

```csharp
public static class ServiceCollectionExtensions
{
    public static IServiceCollection AddDataSync(
        this IServiceCollection services,
        IConfiguration configuration)
    {
        // Existing registrations...

        // Add bulk copy converters (generated)
        services.AddBulkCopyConverters();

        // Add bulk merge helper
        services.AddScoped<IBulkMergeHelper, BulkMergeHelper>();

        return services;
    }
}
```

## Testing Strategy

### Unit Tests (JdeScoping.DataSync.Tests)

| Test Class | Coverage |
|------------|----------|
| `BulkMergeHelperTests` | Expression parsing, SQL generation, batch splitting |
| `ExpressionParserTests` | Column name extraction from expressions |
| `MergeSqlBuilderTests` | Generated MERGE SQL correctness |
| `DataReaderFactoryTests` | Factory type resolution |
| `ValidationTests` | Schema validation logic |
| `BulkMergeExceptionTests` | Exception properties and formatting |

### Integration Tests (JdeScoping.DataSync.IntegrationTests)

| Test Class | Coverage |
|------------|----------|
| `BulkMergeHelperIntegrationTests` | End-to-end merge against SQL Server |
| `BatchingIntegrationTests` | Large datasets, multiple batches |
| `ValidationIntegrationTests` | Schema validation against real table |

**Key scenarios:**
- Insert new records (WHEN NOT MATCHED)
- Update existing records (WHEN MATCHED)
- Conditional update respects `updateWhen`
- Composite primary key matching
- Batch processing (10k+ records across multiple batches)
- Temp table cleanup on success and failure
- Validation catches truncation before SQL error

## Migration Plan

### Code to Replace

| File | Change |
|------|--------|
| `StagingTableManager.cs` | Delete - replaced by `BulkMergeHelper` |
| `TableSyncOperation.cs` | Simplify to use `IBulkMergeHelper` |
| `LotFinderRepository.DataSync.cs` | Remove bulk-related methods |

### Before/After

**Before:**
```csharp
await _stagingTableManager.CreateStagingTableAsync(...);
await _stagingTableManager.BulkCopyToStagingAsync(...);
await _stagingTableManager.MergeFromStagingAsync(...);
await _stagingTableManager.DropStagingTableAsync(...);
```

**After:**
```csharp
var result = await _bulkMergeHelper.MergeAsync(
    data: fetcher.FetchAsync(lastUpdate),
    destinationTable: config.TableName,
    matchOn: config.MatchExpression,
    updateColumns: config.UpdateExpression,
    updateWhen: config.UpdateCondition,
    batchSize: _options.BatchSize);
```

### Tests to Remove

- `StagingTableManagerTests.cs` (unit)
- `StagingTableManagerTests.cs` (integration)

## Example Usage

```csharp
// Simple case - match on single PK, update all columns
var result = await _bulkMergeHelper.MergeAsync(
    data: workOrders,
    destinationTable: "WorkOrder",
    matchOn: x => x.WorkOrderNumber);

// Full configuration
var result = await _bulkMergeHelper.MergeAsync(
    data: workOrders,
    destinationTable: "WorkOrder",
    matchOn: x => new { x.WorkOrderNumber, x.BranchCode },
    updateColumns: x => new { x.StatusCode, x.OrderQuantity, x.LastUpdateDt },
    updateWhen: (src, tgt) => src.LastUpdateDt > tgt.LastUpdateDt,
    batchSize: 10000,
    validateBeforeCopy: true);
```