Architect Skill

Glossary

Precise vocabulary for every architectural decision. Use these terms exactly — consistency is the point.

• Module — anything with an interface and an implementation (function, class, package, layer). Router, Service, Entity, and Repository are all modules.
• Interface — everything a caller must know to use the module: types, invariants, error modes, ordering, config. Not just the type signature.
• Seam — where a module's interface lives; a place behaviour can be altered without editing in place. The functional core / effectful edge boundary is the primary seam.
• Depth — leverage at the interface. A module is deep when a lot of behaviour sits behind a small interface. A module is shallow when the interface is nearly as complex as the implementation.
• Adapter — a concrete thing satisfying an interface at a seam. A Postgres repository is an adapter; an in-memory fake for testing is another adapter at the same seam.
• Leverage — what callers get from depth: more capability per unit of interface they learn.
• Locality — what maintainers get from depth: change, bugs, and knowledge concentrate in one place.

Key principles (apply to every decision):

• The deletion test. Imagine deleting the module. If complexity vanishes, it was a pass-through. If complexity reappears across N callers, it was earning its keep.
• The interface is the test surface. Callers and tests cross the same seam. If you want to test past the interface, the module is probably the wrong shape.
• One adapter = hypothetical seam. Two adapters = real seam. Don't introduce a seam unless something actually varies across it.

Domain-Driven Design and hexagonal architecture with functional core pattern for feature design.

Architecture Model

Unified view of functional core and effectful edge:

          Effectful Edge (IO)              Functional Core (Pure)
┌─────────────────────────────────┐    ┌──────────────────────────┐
│  Router    → request parsing    │    │  Service  → orchestration│
│  Consumer  → event handling     │───▶│  Entity   → domain rules │
│  Client    → external APIs      │    │            → validation  │
│  Producer  → event publishing   │◀───│            → transforms  │
│  Repository→ data persistence   │    │                          │
└─────────────────────────────────┘    └──────────────────────────┘

Key Principle: Business logic lives in the functional core (Service + Entity). IO operations live in the effectful edge. Core defines interfaces; edge implements them (dependency inversion).

Evaluating Architecture Decisions

Before creating a new module, run these checks:

Deletion Test

Imagine deleting the module. If complexity vanishes, it was a pass-through. If complexity reappears across N callers, it was earning its keep.

When to apply: Before extracting a new Service, Repository, or utility class.

Example:

❌ Shallow: OrderFormatterService with 1 method per field
   → Deleting it just moves 6 lines into the Router
   → Interface is as complex as implementation

✅ Deep: OrderService with validate → reserve → save → publish
   → Deleting it replicates orchestration across N handlers
   → Small interface (createOrder), large implementation

Interface as Test Surface

Callers and tests cross the same seam. If you want to test past the interface, the module is probably the wrong shape.

Implication:

• Unit tests for Entities test the public interface (validate, transform)
• Unit tests for Services test through the seam with stub repositories
• Integration tests for Repositories test the actual adapter
• If you find yourself testing "internal" methods, the module needs redrawing

Adapter Rule

One adapter = hypothetical seam. Two adapters = real seam.

When to apply: Deciding whether to extract an interface.

Examples:

// ❌ Hypothetical seam — only one adapter exists
interface IEmailClient { send(email: Email): Promise<void>; }
class SendgridClient implements IEmailClient { ... }
// No second adapter. The interface adds indirection without value.

// ✅ Real seam — two adapters exist
interface IOrderRepository { save(order: Order): Promise<Order>; }
class PostgresOrderRepository implements IOrderRepository { ... }
class InMemoryOrderRepository implements IOrderRepository { ... } // for tests
// The seam earns its keep because callers vary (production vs test).

Depth Check

Prefer deep modules over shallow ones:

Rule of thumb: A module's interface should hide at least 3x the complexity it exposes.

Functional Core

Pure, deterministic components containing all business logic.

Service Layer

Responsibility: Orchestrate business operations, coordinate between entities and repositories.

Characteristics:

• Pure functions that take data and return results
• No IO operations (database, HTTP, file system)
• Calls repositories through interfaces (dependency injection)
• Composes entity operations into workflows
• Returns success/error results

Example:

class OrderService {
  async createOrder(request: CreateOrderRequest): Promise<Result<Order>> {
    // Validate with entity
    const order = Order.fromRequest(request);
    const validation = order.validate();
    if (!validation.ok) return validation;

    // Check business rules
    const inventory = await this.inventoryRepo.checkAvailability(order.items);
    if (!inventory.available) return Err('Items not available');

    // Coordinate persistence
    await this.inventoryRepo.reserve(order.items);
    const saved = await this.orderRepo.save(order.toRecord());

    return Ok(Order.fromRecord(saved));
  }
}

Entity Layer

Responsibility: Domain models, validation, business rules, data transformations.

Characteristics:

• Pure data structures with behavior
• All validation logic
• Data transformations (fromRequest, toRecord, toResponse)
• Business rules and invariants
• No IO, no framework dependencies

Example:

class Order {
  constructor(
    public readonly id: string,
    public readonly customerId: string,
    public readonly items: OrderItem[],
    public readonly status: OrderStatus,
    public readonly total: number
  ) {}

  static fromRequest(req: CreateOrderRequest): Order {
    return new Order(
      generateId(),
      req.customerId,
      req.items.map(i => new OrderItem(i)),
      'pending',
      req.items.reduce((sum, i) => sum + i.price * i.quantity, 0)
    );
  }

  toRecord(): OrderRecord {
    return {
      id: this.id,
      customer_id: this.customerId,
      items: JSON.stringify(this.items),
      status: this.status,
      total: this.total
    };
  }

  validate(): Result<Order> {
    if (this.items.length === 0) {
      return Err('Order must have at least one item');
    }
    if (this.total < 0) {
      return Err('Order total cannot be negative');
    }
    return Ok(this);
  }

  canCancel(): boolean {
    return ['pending', 'confirmed'].includes(this.status);
  }
}

Effectful Edge

IO-performing components that interact with the outside world.

Router

Responsibility: HTTP request handling, parsing, response formatting.

Characteristics:

• Parses HTTP requests into domain types
• Calls service layer with parsed data
• Formats service results into HTTP responses
• Handles HTTP-specific concerns (status codes, headers)
• No business logic

Example:

router.post('/orders', async (req, res) => {
  const result = await orderService.createOrder(req.body);

  if (result.ok) {