# E-Commerce Example: Domain Requirements

Define the requirements for a complete e-commerce order management system.

## Business Requirements

### Order Management

**As a customer, I want to:**
- ✅ Place orders with multiple items
- ✅ View my order history
- ✅ Track order status (Placed, Paid, Shipped, Delivered, Cancelled)
- ✅ Cancel orders before shipment
- ✅ Receive email notifications for order events

**As an admin, I want to:**
- ✅ View all orders
- ✅ Filter orders by status, customer, date range
- ✅ Process payments
- ✅ Mark orders as shipped
- ✅ View order analytics (total revenue, order count)

### Inventory Management

**As a system, I need to:**
- ✅ Reserve inventory when orders are placed
- ✅ Release inventory when orders are cancelled
- ✅ Prevent overselling (no negative inventory)
- ✅ Track inventory levels per product

### Payment Processing

**As a system, I need to:**
- ✅ Charge customers when orders are placed
- ✅ Refund customers when orders are cancelled
- ✅ Handle payment failures gracefully
- ✅ Prevent double-charging

### Analytics

**As an admin, I want to:**
- ✅ View total revenue
- ✅ View order count by status
- ✅ View top-selling products
- ✅ View customer purchase history

## Domain Model

### Entities

**Order (Aggregate Root)**
```csharp
public class Order
{
    public string OrderId { get; set; }
    public string CustomerId { get; set; }
    public List<OrderLine> Lines { get; set; }
    public decimal TotalAmount { get; set; }
    public OrderStatus Status { get; set; }
    public DateTimeOffset PlacedAt { get; set; }
    public DateTimeOffset? PaidAt { get; set; }
    public DateTimeOffset? ShippedAt { get; set; }
    public DateTimeOffset? DeliveredAt { get; set; }
    public DateTimeOffset? CancelledAt { get; set; }
}

public enum OrderStatus
{
    Placed,
    Paid,
    Shipped,
    Delivered,
    Cancelled
}

public class OrderLine
{
    public string ProductId { get; set; }
    public string ProductName { get; set; }
    public int Quantity { get; set; }
    public decimal UnitPrice { get; set; }
    public decimal LineTotal => Quantity * UnitPrice;
}
```

**Customer**
```csharp
public class Customer
{
    public string CustomerId { get; set; }
    public string Name { get; set; }
    public string Email { get; set; }
    public decimal TotalSpent { get; set; }
    public int OrderCount { get; set; }
}
```

**Product**
```csharp
public class Product
{
    public string ProductId { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public decimal Price { get; set; }
    public int AvailableStock { get; set; }
}
```

## Domain Events

### Order Events

```csharp
public record OrderPlacedEvent
{
    public string OrderId { get; init; }
    public string CustomerId { get; init; }
    public List<OrderLineDto> Lines { get; init; }
    public decimal TotalAmount { get; init; }
    public DateTimeOffset PlacedAt { get; init; }
}

public record OrderPaidEvent
{
    public string OrderId { get; init; }
    public string PaymentId { get; init; }
    public decimal Amount { get; init; }
    public DateTimeOffset PaidAt { get; init; }
}

public record OrderShippedEvent
{
    public string OrderId { get; init; }
    public string ShipmentId { get; init; }
    public string TrackingNumber { get; init; }
    public DateTimeOffset ShippedAt { get; init; }
}

public record OrderDeliveredEvent
{
    public string OrderId { get; init; }
    public DateTimeOffset DeliveredAt { get; init; }
}

public record OrderCancelledEvent
{
    public string OrderId { get; init; }
    public string Reason { get; init; }
    public DateTimeOffset CancelledAt { get; init; }
}
```

### Inventory Events

```csharp
public record InventoryReservedEvent
{
    public string OrderId { get; init; }
    public string ReservationId { get; init; }
    public List<InventoryItemDto> Items { get; init; }
    public DateTimeOffset ReservedAt { get; init; }
}

public record InventoryReleasedEvent
{
    public string ReservationId { get; init; }
    public List<InventoryItemDto> Items { get; init; }
    public DateTimeOffset ReleasedAt { get; init; }
}

public record InventoryDepletedEvent
{
    public string ProductId { get; init; }
    public DateTimeOffset DepletedAt { get; init; }
}
```

### Payment Events

```csharp
public record PaymentProcessedEvent
{
    public string PaymentId { get; init; }
    public string OrderId { get; init; }
    public decimal Amount { get; init; }
    public DateTimeOffset ProcessedAt { get; init; }
}

public record PaymentRefundedEvent
{
    public string PaymentId { get; init; }
    public string OrderId { get; init; }
    public decimal Amount { get; init; }
    public DateTimeOffset RefundedAt { get; init; }
}

public record PaymentFailedEvent
{
    public string OrderId { get; init; }
    public string Reason { get; init; }
    public DateTimeOffset FailedAt { get; init; }
}
```

## User Stories

### Story 1: Place Order

**Given** I am a customer
**When** I place an order with 2 items
**Then** The system should:
1. Create an order in "Placed" status
2. Reserve inventory for the items
3. Publish OrderPlacedEvent
4. Send order confirmation email

### Story 2: Process Payment

**Given** An order has been placed
**When** The payment is processed successfully
**Then** The system should:
1. Update order status to "Paid"
2. Publish PaymentProcessedEvent and OrderPaidEvent
3. Send payment confirmation email

### Story 3: Ship Order

**Given** An order has been paid
**When** The order is shipped
**Then** The system should:
1. Update order status to "Shipped"
2. Create shipment record with tracking number
3. Publish OrderShippedEvent
4. Send shipment notification email

### Story 4: Cancel Order

**Given** An order has been placed but not shipped
**When** The customer cancels the order
**Then** The system should:
1. Update order status to "Cancelled"
2. Release reserved inventory
3. Refund payment if already paid
4. Publish OrderCancelledEvent
5. Send cancellation confirmation email

### Story 5: View Order History

**Given** I am a customer
**When** I view my order history
**Then** I should see:
- All my orders sorted by date
- Order status for each order
- Items in each order
- Order totals

## Success Criteria

✅ **Functional:**
- Orders can be placed, paid, shipped, and cancelled
- Inventory is accurately tracked and reserved
- Payments are processed and refunded correctly
- Order history is queryable

✅ **Non-Functional:**
- System handles 100 orders/second
- Order history queries return in < 100ms
- Events are processed in < 1 second
- System is resilient to failures (retry logic)

## Architecture Decisions

### CQRS Pattern

- **Commands**: PlaceOrder, ProcessPayment, ShipOrder, CancelOrder
- **Queries**: GetOrder, ListOrders, GetOrderHistory, GetAnalytics
- **Events**: All state changes emit domain events
- **Projections**: Order summary, customer analytics, product analytics

### Event Sourcing

- Order aggregate state is rebuilt from events
- Enables full audit trail
- Allows rebuilding projections
- Supports temporal queries

### Saga Pattern

- OrderFulfillmentSaga coordinates order processing:
  1. Reserve inventory
  2. Process payment
  3. Ship order
- Compensation logic for failures

## Next Steps

- [02-domain-events.md](02-domain-events.md) - Define domain events in detail
- [03-commands.md](03-commands.md) - Implement commands and handlers
- [04-queries.md](04-queries.md) - Build queries and projections

## See Also

- [Event Sourcing Tutorial](../event-sourcing/README.md)
- [Sagas](../../event-streaming/sagas/README.md)
- [Best Practices](../../best-practices/README.md)