Momentum .NET

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Adding Domains ​

Guide for adding new business domains to your Momentum application, following Domain-Driven Design principles and CQRS patterns.

Overview ​

Adding domains in Momentum follows a structured approach that maintains consistency, testability, and performance. Each domain encapsulates related business logic and follows established patterns for commands, queries, and data access.

Key Architectural Principles:

  • Multi-Tenant by Design: All entities use composite primary keys with TenantId
  • Source Generation: Database access leverages compile-time code generation
  • Wolverine Message Bus: Uses Wolverine instead of MediatR for message handling
  • Result<T> Pattern: Commands and queries return strongly-typed results with error handling
  • Function-Based Database Access: PostgreSQL functions with $ prefix for source generation

Domain Structure ​

Standard Domain Layout ​

src/AppDomain/Orders/
├── Commands/
│   ├── CreateOrderCommand.cs
│   ├── UpdateOrderStatusCommand.cs
│   └── CancelOrderCommand.cs
├── Queries/
│   ├── GetOrderByIdQuery.cs
│   ├── GetOrdersByCustomerQuery.cs
│   └── GetOrderHistoryQuery.cs
├── Data/
│   ├── Entities/
│   │   ├── Order.cs
│   │   └── OrderItem.cs
│   └── DbMapper.cs
└── Contracts/
    ├── DomainEvents/
    │   ├── OrderCreated.cs
    │   └── OrderStatusChanged.cs
    └── IntegrationEvents/
        ├── OrderCreated.cs
        └── OrderCompleted.cs

Multi-Tenant Database Functions ​

infra/AppDomain.Database/Liquibase/app_domain/orders/functions/
├── orders_create.sql
├── orders_get_by_id.sql
├── orders_get_by_customer.sql
└── orders_update_status.sql

Step-by-Step Domain Creation ​

1. Define Domain Entities ​

csharp
// src/AppDomain/Orders/Data/Entities/Order.cs
using LinqToDB.Mapping;
using Momentum.Extensions.Common.Data;

[Table(Schema = "app_domain", Name = "orders")]
public record Order : DbEntity
{
    [PrimaryKey(Order = 0)]
    [Column("tenant_id")]
    public Guid TenantId { get; set; }

    [PrimaryKey(Order = 1)]
    [Column("order_id")]
    public Guid OrderId { get; set; }

    [Column("customer_id")]
    public Guid CustomerId { get; set; }

    [Column("order_date")]
    public DateTime OrderDate { get; set; }

    [Column("status")]
    public OrderStatus Status { get; set; }

    [Column("total_amount")]
    public decimal TotalAmount { get; set; }
}

// src/AppDomain/Orders/Data/Entities/OrderItem.cs
[Table(Schema = "app_domain", Name = "order_items")]
public record OrderItem : DbEntity
{
    [PrimaryKey(Order = 0)]
    [Column("tenant_id")]
    public Guid TenantId { get; set; }

    [PrimaryKey(Order = 1)]
    [Column("order_item_id")]
    public Guid OrderItemId { get; set; }

    [Column("order_id")]
    public Guid OrderId { get; set; }

    [Column("product_id")]
    public Guid ProductId { get; set; }

    [Column("product_name")]
    public string ProductName { get; set; } = string.Empty;

    [Column("quantity")]
    public int Quantity { get; set; }

    [Column("unit_price")]
    public decimal UnitPrice { get; set; }
}

public enum OrderStatus
{
    Pending,
    Confirmed,
    Shipped,
    Delivered,
    Cancelled
}

2. Create Commands ​

csharp
// src/AppDomain/Orders/Commands/CreateOrderCommand.cs
using Momentum.Extensions.Common.Messaging;
using Momentum.Extensions.Common.Results;
using Momentum.Extensions.Database;
using Wolverine;

public record CreateOrderCommand(
    Guid TenantId,
    Guid CustomerId,
    IReadOnlyList<CreateOrderItem> Items
) : ICommand<Result<Order>>;

public record CreateOrderItem(
    Guid ProductId,
    string ProductName,
    int Quantity,
    decimal UnitPrice
);

public static partial class CreateOrderCommandHandler
{
    [DbCommand(fn: "$app_domain.orders_create")]
    public partial record DbCommand(
        Guid TenantId,
        Guid CustomerId,
        string ItemsJson
    ) : ICommand<Result<Guid>>;

    public static async Task<Result<Order>> Handle(
        CreateOrderCommand command,
        IMessageBus messaging,
        CancellationToken cancellationToken)
    {
        // Business logic validation
        if (!command.Items.Any())
            return Result.Failure<Order>("Order must contain at least one item");

        if (command.Items.Any(item => item.Quantity <= 0))
            return Result.Failure<Order>("All items must have positive quantity");

        // Serialize items to JSON for database function
        var itemsJson = JsonSerializer.Serialize(command.Items);

        // Execute database command
        var dbCommand = new DbCommand(command.TenantId, command.CustomerId, itemsJson);
        var result = await messaging.InvokeAsync(dbCommand, cancellationToken);

        if (result.IsFailure)
            return Result.Failure<Order>(result.Error);

        // Fetch created order
        var getOrderQuery = new GetOrderByIdQuery.DbQuery(command.TenantId, result.Value);
        var order = await messaging.InvokeAsync(getOrderQuery, cancellationToken);

        return order is not null
            ? Result.Success(order)
            : Result.Failure<Order>("Failed to retrieve created order");
    }
}

3. Create Queries ​

csharp
// src/AppDomain/Orders/Queries/GetOrderByIdQuery.cs
using Momentum.Extensions.Common.Messaging;
using Momentum.Extensions.Database;
using Wolverine;

public record GetOrderByIdQuery(Guid TenantId, Guid OrderId) : IQuery<Order?>;

public static partial class GetOrderByIdQueryHandler
{
    [DbCommand(fn: "$app_domain.orders_get_by_id")]
    public partial record DbQuery(Guid TenantId, Guid OrderId) : IQuery<Order?>;

    public static async Task<Order?> Handle(
        GetOrderByIdQuery query,
        IMessageBus messaging,
        CancellationToken cancellationToken)
    {
        var dbQuery = new DbQuery(query.TenantId, query.OrderId);
        return await messaging.InvokeAsync(dbQuery, cancellationToken);
    }
}

// src/AppDomain/Orders/Queries/GetOrdersByCustomerQuery.cs
public record GetOrdersByCustomerQuery(Guid TenantId, Guid CustomerId, int Limit = 10, int Offset = 0)
    : IQuery<IEnumerable<GetOrdersByCustomerQuery.Result>>
{
    public record Result(Guid OrderId, DateTime OrderDate, decimal TotalAmount, OrderStatus Status);
}

public static partial class GetOrdersByCustomerQueryHandler
{
    [DbCommand(fn: "$app_domain.orders_get_by_customer")]
    public partial record DbQuery(Guid TenantId, Guid CustomerId, int Limit, int Offset)
        : IQuery<IEnumerable<Data.Entities.Order>>;

    public static async Task<IEnumerable<GetOrdersByCustomerQuery.Result>> Handle(
        GetOrdersByCustomerQuery query,
        IMessageBus messaging,
        CancellationToken cancellationToken)
    {
        var dbQuery = new DbQuery(query.TenantId, query.CustomerId, query.Limit, query.Offset);
        var orders = await messaging.InvokeAsync(dbQuery, cancellationToken);

        return orders.Select(o => new GetOrdersByCustomerQuery.Result(
            o.OrderId, o.OrderDate, o.TotalAmount, o.Status));
    }
}

4. Define Events ​

csharp
// src/AppDomain.Contracts/IntegrationEvents/OrderCreated.cs
using Momentum.Extensions.Kafka.Events;

[EventTopic("app_domain.orders.order-created")]
public record OrderCreated(
    Guid TenantId,
    Guid OrderId,
    Guid CustomerId,
    decimal TotalAmount,
    DateTime OrderDate,
    string Status
) : IIntegrationEvent;

// src/AppDomain/Orders/Contracts/DomainEvents/OrderStatusChanged.cs
using Momentum.Extensions.Common.Messaging;

public record OrderStatusChanged(
    Guid TenantId,
    Guid OrderId,
    OrderStatus OldStatus,
    OrderStatus NewStatus,
    DateTime ChangedAt
) : IDomainEvent;

// src/AppDomain/Orders/Contracts/IntegrationEvents/OrderCompleted.cs
[EventTopic("app_domain.orders.order-completed")]
public record OrderCompleted(
    Guid TenantId,
    Guid OrderId,
    Guid CustomerId,
    decimal TotalAmount,
    DateTime CompletedDate
) : IIntegrationEvent;

5. Database Schema ​

sql
-- infra/AppDomain.Database/Liquibase/app_domain/orders/changesets/001-orders-tables.sql
-- Multi-tenant table structure with composite primary keys
CREATE TABLE app_domain.orders (
    tenant_id UUID NOT NULL,
    order_id UUID NOT NULL,
    customer_id UUID NOT NULL,
    order_date TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT timezone('utc', now()),
    status VARCHAR(20) NOT NULL DEFAULT 'Pending',
    total_amount DECIMAL(12,2) NOT NULL,
    created_date_utc TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT timezone('utc', now()),
    updated_date_utc TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT timezone('utc', now()),

    -- Composite primary key ensures tenant isolation
    PRIMARY KEY (tenant_id, order_id),

    -- Check constraints for business rules
    CONSTRAINT chk_orders_total_amount_positive CHECK (total_amount >= 0),
    CONSTRAINT chk_orders_status_valid CHECK (status IN ('Pending', 'Confirmed', 'Shipped', 'Delivered', 'Cancelled'))
);

CREATE TABLE app_domain.order_items (
    tenant_id UUID NOT NULL,
    order_item_id UUID NOT NULL,
    order_id UUID NOT NULL,
    product_id UUID NOT NULL,
    product_name VARCHAR(255) NOT NULL,
    quantity INTEGER NOT NULL,
    unit_price DECIMAL(10,2) NOT NULL,
    created_date_utc TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT timezone('utc', now()),
    updated_date_utc TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT timezone('utc', now()),

    -- Composite primary key for tenant isolation
    PRIMARY KEY (tenant_id, order_item_id),

    -- Foreign key to orders table with tenant awareness
    FOREIGN KEY (tenant_id, order_id) REFERENCES app_domain.orders(tenant_id, order_id) ON DELETE CASCADE,

    -- Business rule constraints
    CONSTRAINT chk_order_items_quantity_positive CHECK (quantity > 0),
    CONSTRAINT chk_order_items_unit_price_positive CHECK (unit_price >= 0)
);

-- Indexes for efficient queries within tenant boundaries
CREATE INDEX idx_orders_customer_id ON app_domain.orders(tenant_id, customer_id);
CREATE INDEX idx_orders_status ON app_domain.orders(tenant_id, status);
CREATE INDEX idx_orders_order_date ON app_domain.orders(tenant_id, order_date DESC);
CREATE INDEX idx_order_items_order_id ON app_domain.order_items(tenant_id, order_id);
CREATE INDEX idx_order_items_product_id ON app_domain.order_items(tenant_id, product_id);

-- Add table comments for documentation
COMMENT ON TABLE app_domain.orders IS 'Orders table with multi-tenant architecture using composite primary keys';
COMMENT ON TABLE app_domain.order_items IS 'Order items table linked to orders with tenant-aware foreign keys';

6. Database Functions ​

sql
-- infra/AppDomain.Database/Liquibase/app_domain/orders/functions/orders_create.sql
/**
 * Creates a new order with items in a multi-tenant architecture.
 * Returns the newly created order ID or raises an exception on failure.
 */
CREATE OR REPLACE FUNCTION app_domain.orders_create(
    p_tenant_id UUID,
    p_customer_id UUID,
    p_items_json TEXT
) RETURNS UUID AS $$
DECLARE
    v_order_id UUID;
    v_total_amount DECIMAL(12,2) := 0;
    v_items JSONB;
    v_item JSONB;
BEGIN
    -- Parse items JSON
    v_items := p_items_json::JSONB;

    -- Validate inputs
    IF p_tenant_id IS NULL THEN
        RAISE EXCEPTION 'Tenant ID cannot be null';
    END IF;

    IF p_customer_id IS NULL THEN
        RAISE EXCEPTION 'Customer ID cannot be null';
    END IF;

    IF jsonb_array_length(v_items) = 0 THEN
        RAISE EXCEPTION 'Order must contain at least one item';
    END IF;

    -- Generate new order ID
    v_order_id := gen_random_uuid();

    -- Calculate total amount and validate items
    FOR v_item IN SELECT * FROM jsonb_array_elements(v_items)
    LOOP
        -- Validate item data
        IF (v_item->>'quantity')::INTEGER <= 0 THEN
            RAISE EXCEPTION 'Item quantity must be positive';
        END IF;

        IF (v_item->>'unitPrice')::DECIMAL < 0 THEN
            RAISE EXCEPTION 'Item unit price cannot be negative';
        END IF;

        v_total_amount := v_total_amount +
            ((v_item->>'quantity')::INTEGER * (v_item->>'unitPrice')::DECIMAL);
    END LOOP;

    -- Create order with tenant isolation
    INSERT INTO app_domain.orders (
        tenant_id, order_id, customer_id, total_amount, status
    ) VALUES (
        p_tenant_id, v_order_id, p_customer_id, v_total_amount, 'Pending'
    );

    -- Create order items with tenant isolation
    FOR v_item IN SELECT * FROM jsonb_array_elements(v_items)
    LOOP
        INSERT INTO app_domain.order_items (
            tenant_id, order_item_id, order_id, product_id,
            product_name, quantity, unit_price
        ) VALUES (
            p_tenant_id,
            gen_random_uuid(),
            v_order_id,
            (v_item->>'productId')::UUID,
            v_item->>'productName',
            (v_item->>'quantity')::INTEGER,
            (v_item->>'unitPrice')::DECIMAL
        );
    END LOOP;

    RETURN v_order_id;
EXCEPTION
    WHEN OTHERS THEN
        -- Re-raise with context
        RAISE EXCEPTION 'Failed to create order: %', SQLERRM;
END;
$$ LANGUAGE plpgsql;

-- infra/AppDomain.Database/Liquibase/app_domain/orders/functions/orders_get_by_id.sql
/**
 * Retrieves an order by ID within tenant boundaries.
 */
CREATE OR REPLACE FUNCTION app_domain.orders_get_by_id(
    p_tenant_id UUID,
    p_order_id UUID
) RETURNS TABLE(
    tenant_id UUID,
    order_id UUID,
    customer_id UUID,
    order_date TIMESTAMP WITH TIME ZONE,
    status VARCHAR(20),
    total_amount DECIMAL(12,2),
    created_date_utc TIMESTAMP WITH TIME ZONE,
    updated_date_utc TIMESTAMP WITH TIME ZONE
) AS $$
BEGIN
    RETURN QUERY
    SELECT o.tenant_id, o.order_id, o.customer_id, o.order_date,
           o.status, o.total_amount, o.created_date_utc, o.updated_date_utc
    FROM app_domain.orders o
    WHERE o.tenant_id = p_tenant_id
      AND o.order_id = p_order_id;
END;
$$ LANGUAGE plpgsql;

-- infra/AppDomain.Database/Liquibase/app_domain/orders/functions/orders_get_by_customer.sql
/**
 * Retrieves orders for a specific customer with pagination and tenant isolation.
 */
CREATE OR REPLACE FUNCTION app_domain.orders_get_by_customer(
    p_tenant_id UUID,
    p_customer_id UUID,
    p_limit INTEGER DEFAULT 10,
    p_offset INTEGER DEFAULT 0
) RETURNS TABLE(
    tenant_id UUID,
    order_id UUID,
    customer_id UUID,
    order_date TIMESTAMP WITH TIME ZONE,
    status VARCHAR(20),
    total_amount DECIMAL(12,2),
    created_date_utc TIMESTAMP WITH TIME ZONE,
    updated_date_utc TIMESTAMP WITH TIME ZONE
) AS $$
BEGIN
    RETURN QUERY
    SELECT o.tenant_id, o.order_id, o.customer_id, o.order_date,
           o.status, o.total_amount, o.created_date_utc, o.updated_date_utc
    FROM app_domain.orders o
    WHERE o.tenant_id = p_tenant_id
      AND o.customer_id = p_customer_id
    ORDER BY o.order_date DESC
    LIMIT p_limit
    OFFSET p_offset;
END;
$$ LANGUAGE plpgsql;

7. API Endpoints ​

csharp
// src/AppDomain.Api/Orders/OrdersEndpoints.cs
using Microsoft.AspNetCore.Mvc;
using Wolverine;
using Momentum.Extensions.Api.Results;
using AppDomain.Orders.Commands;
using AppDomain.Orders.Queries;

public static class OrdersEndpoints
{
    public static void MapOrdersEndpoints(this IEndpointRouteBuilder app)
    {
        var group = app.MapGroup("/orders")
            .WithTags("Orders")
            .WithOpenApi()
            .RequireAuthorization(); // Ensure authentication for tenant context

        group.MapPost("/", CreateOrder)
            .WithSummary("Create a new order")
            .WithDescription("Creates a new order with the specified items for the authenticated tenant");

        group.MapGet("/{id:guid}", GetOrder)
            .WithSummary("Get order by ID")
            .WithDescription("Retrieves a specific order by ID within the tenant context");

        group.MapGet("/customer/{customerId:guid}", GetOrdersByCustomer)
            .WithSummary("Get orders by customer")
            .WithDescription("Retrieves all orders for a specific customer with pagination");

        group.MapPut("/{id:guid}/status", UpdateOrderStatus)
            .WithSummary("Update order status")
            .WithDescription("Updates the status of an existing order");
    }

    private static async Task<IResult> CreateOrder(
        [FromBody] CreateOrderRequest request,
        IMessageBus messageBus,
        HttpContext context)
    {
        // Extract tenant ID from authenticated user context
        var tenantId = context.GetTenantId(); // Extension method for tenant extraction

        var command = new CreateOrderCommand(
            tenantId,
            request.CustomerId,
            request.Items.Select(i => new CreateOrderItem(
                i.ProductId, i.ProductName, i.Quantity, i.UnitPrice
            )).ToList()
        );

        var result = await messageBus.InvokeAsync(command);

        return result.Match(
            onSuccess: order => Results.Created($"/orders/{order.OrderId}", order),
            onFailure: error => Results.BadRequest(new { Error = error })
        );
    }

    private static async Task<IResult> GetOrder(
        Guid id,
        IMessageBus messageBus,
        HttpContext context)
    {
        var tenantId = context.GetTenantId();
        var query = new GetOrderByIdQuery(tenantId, id);

        var order = await messageBus.InvokeAsync(query);
        return order is not null ? Results.Ok(order) : Results.NotFound();
    }

    private static async Task<IResult> GetOrdersByCustomer(
        Guid customerId,
        [FromQuery] int limit = 10,
        [FromQuery] int offset = 0,
        IMessageBus messageBus,
        HttpContext context)
    {
        var tenantId = context.GetTenantId();
        var query = new GetOrdersByCustomerQuery(tenantId, customerId, limit, offset);

        var orders = await messageBus.InvokeAsync(query);
        return Results.Ok(new { Orders = orders, Limit = limit, Offset = offset });
    }

    private static async Task<IResult> UpdateOrderStatus(
        Guid id,
        [FromBody] UpdateOrderStatusRequest request,
        IMessageBus messageBus,
        HttpContext context)
    {
        var tenantId = context.GetTenantId();
        var command = new UpdateOrderStatusCommand(tenantId, id, request.Status);

        var result = await messageBus.InvokeAsync(command);

        return result.Match(
            onSuccess: _ => Results.NoContent(),
            onFailure: error => Results.BadRequest(new { Error = error })
        );
    }
}

public record CreateOrderRequest(
    Guid CustomerId,
    IReadOnlyList<CreateOrderItemRequest> Items
);

public record CreateOrderItemRequest(
    Guid ProductId,
    string ProductName,
    int Quantity,
    decimal UnitPrice
);

public record UpdateOrderStatusRequest(OrderStatus Status);

8. Register Endpoints ​

csharp
// src/AppDomain.Api/Program.cs
using AppDomain.Api.Orders;

// Register domain endpoints
app.MapOrdersEndpoints();

// Extension method for tenant context extraction
public static class HttpContextExtensions
{
    public static Guid GetTenantId(this HttpContext context)
    {
        var tenantClaim = context.User.FindFirst("tenant_id")?.Value;

        if (string.IsNullOrEmpty(tenantClaim) || !Guid.TryParse(tenantClaim, out var tenantId))
        {
            throw new UnauthorizedAccessException("Invalid or missing tenant context");
        }

        return tenantId;
    }
}

Domain Patterns ​

Aggregate Roots ​

Define clear aggregate boundaries:

csharp
public record Order
{
    public void AddItem(OrderItem item)
    {
        // Business logic for adding items
        // Validate business rules
        // Raise domain events
    }

    public void ChangeStatus(OrderStatus newStatus)
    {
        // Validate status transitions
        // Apply business rules
        // Raise domain events
    }
}

Value Objects ​

Encapsulate related values:

csharp
public record Money(decimal Amount, string Currency)
{
    public static Money Zero(string currency) => new(0, currency);

    public Money Add(Money other)
    {
        if (Currency != other.Currency)
            throw new InvalidOperationException("Cannot add different currencies");

        return new Money(Amount + other.Amount, Currency);
    }
}

Domain Services ​

Complex business logic that doesn't belong to entities:

csharp
public class OrderPricingService
{
    public decimal CalculateTotal(IEnumerable<OrderItem> items, Customer customer)
    {
        var subtotal = items.Sum(item => item.Quantity * item.UnitPrice);
        var discount = CalculateDiscount(subtotal, customer);
        var tax = CalculateTax(subtotal - discount);

        return subtotal - discount + tax;
    }
}

Testing Domains ​

Unit Tests ​

csharp
public class CreateOrderHandlerTests
{
    [Fact]
    public async Task CreateOrder_WithValidData_ReturnsOrderId()
    {
        // Arrange
        var command = new CreateOrder(
            CustomerId: Guid.NewGuid(),
            Items: [new CreateOrderItem(Guid.NewGuid(), 2, 19.99m)]
        );

        // Act
        var orderId = await _mediator.Send(command);

        // Assert
        orderId.Should().NotBeEmpty();
    }
}

Integration Tests ​

csharp
public class OrdersIntegrationTests : IntegrationTestFixture
{
    [Fact]
    public async Task CreateOrder_EndToEnd_Success()
    {
        // Arrange
        var createCommand = new CreateOrder(
            CustomerId: Guid.NewGuid(),
            Items: [new CreateOrderItem(Guid.NewGuid(), 1, 29.99m)]
        );

        // Act
        var orderId = await SendAsync(createCommand);
        var order = await SendAsync(new GetOrderById(orderId));

        // Assert
        order.Should().NotBeNull();
        order!.TotalAmount.Should().Be(29.99m);
    }
}

Source Generation and Partial Classes ​

Understanding Source Generation ​

Momentum uses source generators to eliminate boilerplate code and ensure type safety. The [DbCommand] attribute triggers compile-time code generation:

csharp
// Your partial class definition
[DbCommand(fn: "$app_domain.orders_create")]
public partial record DbCommand(Guid TenantId, Guid CustomerId, string ItemsJson)
    : ICommand<Result<Guid>>;

// Generated code (simplified view)
public partial record DbCommand
{
    public static async Task<Result<Guid>> Handle(
        DbCommand command,
        IDbConnection connection,
        CancellationToken cancellationToken)
    {
        // Generated SQL execution code
        var result = await connection.QuerySingleAsync<Guid>(
            "SELECT * FROM app_domain.orders_create(@TenantId, @CustomerId, @ItemsJson)",
            command,
            cancellationToken);
        return Result.Success(result);
    }
}

Function Prefix Convention ​

The $ prefix in fn: "$app_domain.orders_create" tells the generator to:

  1. Auto-generate SQL: SELECT * FROM app_domain.orders_create(...)
  2. Map parameters: Automatically map record properties to function parameters
  3. Handle results: Convert database results to strongly-typed objects
  4. Error handling: Wrap exceptions in Result<T> pattern

Partial Class Requirements ​

csharp
// ✅ Correct: Partial class with static handler
public static partial class CreateOrderCommandHandler
{
    [DbCommand(fn: "$app_domain.orders_create")]
    public partial record DbCommand(...) : ICommand<Result<Guid>>;

    // Your business logic handler
    public static async Task<Result<Order>> Handle(
        CreateOrderCommand command,
        IMessageBus messaging,
        CancellationToken cancellationToken)
    {
        // Business validation
        // Database call via DbCommand
        // Result transformation
    }
}

// ❌ Incorrect: Missing partial keyword
public class CreateOrderCommandHandler { ... }

// ❌ Incorrect: Non-static class
public partial class CreateOrderCommandHandler { ... }

Result<T> Pattern and Error Handling ​

Understanding Result<T> ​

Momentum uses the Result<T> pattern for explicit error handling without exceptions:

csharp
// `Result<T>` represents either success with a value or failure with an error
public record Result<T>
{
    public bool IsSuccess { get; }
    public bool IsFailure => !IsSuccess;
    public T Value { get; } // Only available when IsSuccess = true
    public string Error { get; } // Only available when IsFailure = true
}

// Creating results
var success = Result.Success(order); // `Result<Order>`
var failure = Result.Failure<Order>("Order not found");

Command Result Patterns ​

csharp
public static async Task<Result<Order>> Handle(
    CreateOrderCommand command,
    IMessageBus messaging,
    CancellationToken cancellationToken)
{
    // Validation with early returns
    if (!command.Items.Any())
        return Result.Failure<Order>("Order must contain at least one item");

    if (command.Items.Any(item => item.Quantity <= 0))
        return Result.Failure<Order>("All items must have positive quantity");

    // Database operation
    var dbResult = await messaging.InvokeAsync(new DbCommand(...));
    if (dbResult.IsFailure)
        return Result.Failure<Order>(dbResult.Error);

    // Success path
    var order = await GetCreatedOrder(dbResult.Value);
    return order is not null
        ? Result.Success(order)
        : Result.Failure<Order>("Failed to retrieve created order");
}

API Result Handling ​

csharp
// Extension method for `Result<T>` in APIs
public static IResult ToApiResult<T>(this Result<T> result)
{
    return result.Match(
        onSuccess: value => Results.Ok(value),
        onFailure: error => Results.BadRequest(new { Error = error })
    );
}

// Usage in endpoints
private static async Task<IResult> CreateOrder(
    CreateOrderRequest request,
    IMessageBus messageBus,
    HttpContext context)
{
    var command = new CreateOrderCommand(...);
    var result = await messageBus.InvokeAsync(command);

    return result.Match(
        onSuccess: order => Results.Created($"/orders/{order.OrderId}", order),
        onFailure: error => Results.BadRequest(new { Error = error })
    );
}

Query Result Patterns ​

csharp
// Queries typically return nullable results instead of `Result<T>`
public static async Task<Order?> Handle(
    GetOrderByIdQuery query,
    IMessageBus messaging,
    CancellationToken cancellationToken)
{
    var dbQuery = new DbQuery(query.TenantId, query.OrderId);
    return await messaging.InvokeAsync(dbQuery, cancellationToken);
}

// API handling for nullable queries
private static async Task<IResult> GetOrder(
    Guid id,
    IMessageBus messageBus,
    HttpContext context)
{
    var order = await messageBus.InvokeAsync(new GetOrderByIdQuery(...));
    return order is not null ? Results.Ok(order) : Results.NotFound();
}

Best Practices ​

Naming Conventions ​

  • Commands: Use action verbs with "Command" suffix (CreateOrderCommand, UpdateOrderStatusCommand)
  • Queries: Use "Get" prefix with "Query" suffix (GetOrderByIdQuery, GetOrdersByCustomerQuery)
  • Events: Use past tense (OrderCreated, OrderStatusChanged)
  • Entities: Use singular nouns (Order, OrderItem)
  • Database Functions: Use underscore notation (orders_create, orders_get_by_id)

Multi-Tenant Architecture ​

  • Always include TenantId: Every entity, command, and query must include tenant context
  • Composite Primary Keys: Use (tenant_id, entity_id) pattern for all tables
  • Tenant Isolation: Ensure all queries filter by tenant_id
  • Index Strategy: Create indexes with tenant_id as the first column

Performance Considerations ​

  • Pagination: Use limit/offset pattern for queries returning multiple records
  • Tenant-Aware Indexing: All indexes should start with tenant_id
  • Function-Based Access: Use PostgreSQL functions instead of raw SQL for better performance
  • Source Generation: Leverage compile-time generation to eliminate runtime reflection

Error Handling Strategy ​

  • Commands: Return Result<T> for operations that can fail
  • Queries: Return nullable types for single entities, collections for lists
  • Database Errors: Let functions handle validation and constraints
  • API Layer: Convert Result<T> to appropriate HTTP responses

Testing Strategy ​

  • Unit Tests: Test business logic in command/query handlers
  • Integration Tests: Test database functions and complete workflows
  • Architecture Tests: Enforce domain boundaries and tenant isolation
  • API Tests: Test complete request/response cycles with tenant context

Common Patterns ​

Validation with FluentValidation ​

csharp
// FluentValidation with Wolverine integration
public class CreateOrderCommandValidator : AbstractValidator<CreateOrderCommand>
{
    public CreateOrderCommandValidator()
    {
        RuleFor(x => x.TenantId).NotEmpty().WithMessage("Tenant ID is required");
        RuleFor(x => x.CustomerId).NotEmpty().WithMessage("Customer ID is required");
        RuleFor(x => x.Items).NotEmpty().WithMessage("Order must contain at least one item");

        RuleForEach(x => x.Items).ChildRules(item => {
            item.RuleFor(x => x.ProductId).NotEmpty();
            item.RuleFor(x => x.ProductName).NotEmpty().MaximumLength(255);
            item.RuleFor(x => x.Quantity).GreaterThan(0).WithMessage("Quantity must be positive");
            item.RuleFor(x => x.UnitPrice).GreaterThanOrEqualTo(0).WithMessage("Unit price cannot be negative");
        });
    }
}

// Validation is automatically executed by Wolverine before handler execution

Event Publishing Patterns ​

csharp
// In command handler - two-tier event publishing
public static async Task<Result<Order>> Handle(
    CreateOrderCommand command,
    IMessageBus messaging,
    CancellationToken cancellationToken)
{
    // Execute database operation
    var result = await ExecuteCreateOrder(command, messaging);
    if (result.IsFailure) return result;

    var order = result.Value;

    // Publish domain event (internal to bounded context)
    var domainEvent = new OrderStatusChanged(
        order.TenantId, order.OrderId, OrderStatus.Pending,
        OrderStatus.Pending, DateTime.UtcNow);
    await messaging.PublishAsync(domainEvent, cancellationToken);

    // Publish integration event (cross-bounded context)
    var integrationEvent = new OrderCreated(
        order.TenantId, order.OrderId, order.CustomerId,
        order.TotalAmount, order.OrderDate, order.Status.ToString());
    await messaging.PublishAsync(integrationEvent, cancellationToken);

    return Result.Success(order);
}

Error Handling Strategies ​

csharp
// Domain-specific exceptions for `Result<T>` pattern
public static class OrderErrors
{
    public static string OrderNotFound(Guid orderId) => $"Order {orderId} not found";
    public static string InvalidOrderStatus(OrderStatus current, OrderStatus requested) =>
        $"Cannot change order status from {current} to {requested}";
    public static string InsufficientInventory(Guid productId, int requested, int available) =>
        $"Insufficient inventory for product {productId}: requested {requested}, available {available}";
}

// Usage in handlers
if (order is null)
    return Result.Failure<Order>(OrderErrors.OrderNotFound(command.OrderId));

if (!IsValidStatusTransition(order.Status, command.NewStatus))
    return Result.Failure<Order>(OrderErrors.InvalidOrderStatus(order.Status, command.NewStatus));

// Global error handling middleware for unhandled exceptions
app.UseExceptionHandler(exceptionHandlerApp =>
{
    exceptionHandlerApp.Run(async context =>
    {
        var exception = context.Features.Get<IExceptionHandlerPathFeature>()?.Error;

        var response = exception switch
        {
            UnauthorizedAccessException => (StatusCodes.Status401Unauthorized, "Unauthorized access"),
            ArgumentException argEx => (StatusCodes.Status400BadRequest, argEx.Message),
            _ => (StatusCodes.Status500InternalServerError, "An error occurred")
        };

        context.Response.StatusCode = response.Item1;
        await context.Response.WriteAsJsonAsync(new { Error = response.Item2 });
    });
});

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