modular-monolith

name	modular-monolith
description	Use when structuring a modular monolith with module isolation and inter-module communication.
metadata	{"category":"architecture","agent":"dotnet-architect","when-to-use":"When designing modular monolith structure, module isolation, or inter-module communication"}

Modular Monolith

Core Principles

• Each module is a self-contained unit with its own domain, data, and API surface
• Modules communicate through well-defined contracts — never share database tables
• Each module owns its data: separate DbContext per module, potentially separate schemas
• Shared kernel contains only truly cross-cutting types (base classes, common interfaces)
• A modular monolith is the ideal stepping stone toward microservices

Project Structure

src/
  {Company}.{Domain}.WebApi/              # Host — composition root
  {Company}.{Domain}.SharedKernel/        # Base classes, common interfaces

  Modules/
    Orders/
      {Company}.{Domain}.Orders.Api/       # Public contracts (DTOs, interfaces)
      {Company}.{Domain}.Orders.Core/      # Domain + Application logic
      {Company}.{Domain}.Orders.Infrastructure/  # Data access, external services
    Inventory/
      {Company}.{Domain}.Inventory.Api/
      {Company}.{Domain}.Inventory.Core/
      {Company}.{Domain}.Inventory.Infrastructure/
    Shipping/
      {Company}.{Domain}.Shipping.Api/
      {Company}.{Domain}.Shipping.Core/
      {Company}.{Domain}.Shipping.Infrastructure/

Patterns

Module Public API (Contracts)

// Modules/Orders/Orders.Api/IOrderModule.cs
namespace {Company}.{Domain}.Orders.Api;

// Only this project is referenced by other modules
public interface IOrderModule
{
    Task<OrderSummaryDto?> GetOrderSummaryAsync(
        Guid orderId, CancellationToken ct);
    Task<bool> OrderExistsAsync(Guid orderId, CancellationToken ct);
}

public sealed record OrderSummaryDto(
    Guid Id, string CustomerName, decimal Total, string Status);

Module Internal Implementation

// Modules/Orders/Orders.Core/OrderModule.cs
namespace {Company}.{Domain}.Orders.Core;

internal sealed class OrderModule(OrderDbContext db) : IOrderModule
{
    public async Task<OrderSummaryDto?> GetOrderSummaryAsync(
        Guid orderId, CancellationToken ct)
    {
        return await db.Orders
            .Where(o => o.Id == orderId)
            .Select(o => new OrderSummaryDto(
                o.Id, o.CustomerName, o.Total, o.Status.ToString()))
            .FirstOrDefaultAsync(ct);
    }

    public async Task<bool> OrderExistsAsync(
        Guid orderId, CancellationToken ct)
    {
        return await db.Orders.AnyAsync(o => o.Id == orderId, ct);
    }
}

Separate DbContext Per Module

// Modules/Orders/Orders.Infrastructure/OrderDbContext.cs
namespace {Company}.{Domain}.Orders.Infrastructure;

internal sealed class OrderDbContext(
    DbContextOptions<OrderDbContext> options) : DbContext(options)
{
    public DbSet<Order> Orders => Set<Order>();
    public DbSet<OrderItem> OrderItems => Set<OrderItem>();

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.HasDefaultSchema("orders"); // schema isolation
        modelBuilder.ApplyConfigurationsFromAssembly(
            typeof(OrderDbContext).Assembly);
    }
}

Inter-Module Communication via Events

// SharedKernel/Events/IIntegrationEvent.cs
public interface IIntegrationEvent : INotification
{
    Guid EventId { get; }
    DateTimeOffset OccurredAt { get; }
}

// Orders module publishes
public sealed record OrderPlacedIntegrationEvent(
    Guid OrderId,
    string CustomerName,
    decimal Total) : IIntegrationEvent
{
    public Guid EventId { get; } = Guid.NewGuid();
    public DateTimeOffset OccurredAt { get; } = DateTimeOffset.UtcNow;
}

// Inventory module handles
internal sealed class ReserveInventoryOnOrderPlaced(
    InventoryDbContext db)
    : INotificationHandler<OrderPlacedIntegrationEvent>
{
    public async Task Handle(
        OrderPlacedIntegrationEvent notification,
        CancellationToken ct)
    {
        // Reserve inventory for the order
        var reservation = InventoryReservation.Create(
            notification.OrderId, notification.OccurredAt);
        db.Reservations.Add(reservation);
        await db.SaveChangesAsync(ct);
    }
}

Module Registration

// Each module has a registration extension
// Modules/Orders/Orders.Infrastructure/OrderModuleRegistration.cs
public static class OrderModuleRegistration
{
    public static IServiceCollection AddOrderModule(
        this IServiceCollection services, IConfiguration configuration)
    {
        services.AddDbContext<OrderDbContext>(options =>
            options.UseSqlServer(
                configuration.GetConnectionString("Default"),
                sql => sql.MigrationsHistoryTable(
                    "__EFMigrationsHistory", "orders")));

        services.AddScoped<IOrderModule, OrderModule>();

        services.AddMediatR(cfg =>
            cfg.RegisterServicesFromAssembly(
                typeof(OrderModule).Assembly));

        return services;
    }
}

// WebApi/Program.cs
builder.Services
    .AddOrderModule(builder.Configuration)
    .AddInventoryModule(builder.Configuration)
    .AddShippingModule(builder.Configuration);

Shared Kernel

// SharedKernel/BaseEntity.cs
public abstract class BaseEntity
{
    public Guid Id { get; protected set; } = Guid.NewGuid();

    private readonly List<IDomainEvent> _domainEvents = [];
    public IReadOnlyList<IDomainEvent> DomainEvents =>
        _domainEvents.AsReadOnly();

    protected void AddDomainEvent(IDomainEvent e) => _domainEvents.Add(e);
    public void ClearDomainEvents() => _domainEvents.Clear();
}

// SharedKernel/IUnitOfWork.cs
public interface IUnitOfWork
{
    Task<int> SaveChangesAsync(CancellationToken ct = default);
}

Anti-Patterns

• Modules sharing database tables or DbContext instances
• Module A directly querying Module B's DbContext
• Circular dependencies between modules
• Putting business logic in the shared kernel (keep it minimal)
• Making module internals public (use InternalsVisibleTo for tests only)
• Synchronous cross-module calls during write operations

Detect Existing Patterns

1. Look for Modules/ folder structure in the solution
2. Check for multiple DbContext classes with separate schemas
3. Look for integration event types and handlers
4. Check for module API/contracts projects referenced by other modules
5. Verify modules have their own DI registration extension methods

Adding to Existing Project

1. Identify bounded contexts — group related features into modules
2. Create module structure — Api (contracts), Core (logic), Infrastructure (data)
3. Separate DbContexts — one per module with schema isolation
4. Define module interfaces — public contracts in the Api project
5. Implement integration events for cross-module communication
6. Create module registration extension methods for the host

Decision Guide

Question	Answer
How do modules communicate?	Integration events (async) or module interfaces (sync reads)
Can modules share a database?	Same server is OK, but use separate schemas
What goes in SharedKernel?	Base classes, common interfaces, integration event contracts
When to split into microservices?	When independent deployment or scaling is needed
How to handle transactions across modules?	Eventual consistency via events; avoid distributed transactions

References

• https://learn.microsoft.com/en-us/dotnet/architecture/modern-web-apps-azure/
• https://www.youtube.com/watch?v=5OjqD-ow8GE (Modular Monolith with DDD)