separation-of-concerns

Separation of Concerns

Principles

1. Separate external clients from domain-specific code
2. Separate feature-specific from shared capabilities
3. Separate intent from execution
4. Separate functions that depend on different state
5. Separate functions that don't have related names

Mental Model: Verticals and Horizontals

Vertical = all code for ONE feature, grouped together Horizontal = capabilities used by MULTIPLE features

All three top-level folders are mandatory:

• features/ — verticals, each with entrypoint/, use-cases/, domain/
  • use-cases/ orchestrates workflow; domain/ contains business rules and behavior
• platform/ — horizontals, only contains domain/ and infra/ (nothing else)
• shell/ — thin wiring/routing only (no business logic)

infra/ lives in platform/infra/, not inside features.

features/              platform/              shell/
├── checkout/          ├── domain/            └── cli.ts
│   ├── entrypoint/    │   └── tax-calc/
│   ├── use-cases/     └── infra/
│   └── domain/            └── ext-clients/
└── refunds/
    ├── entrypoint/
    ├── use-cases/
    └── domain/

Entrypoint Responsibilities

What: Thin mapping layer between external world and use cases.

Pattern:

1. Parse external input into command/request object
2. Invoke use case
3. Map result to external response

// ✅ GOOD - thin mapping, no orchestration, no domain
class OrderController {
  constructor(private placeOrder: PlaceOrderUseCase) {}

  handle(req: HttpRequest): HttpResponse {
    const command = parseOrderCommand(req.body)
    const result = this.placeOrder.execute(command)
    return mapToHttpResponse(result)
  }
}

Dependency Rules:

• ✅ CAN depend on: use-cases/, platform/infra/
• ❌ FORBIDDEN: domain/ (direct domain imports are not allowed)

Behavioral Rules:

• ❌ NO orchestration (that's use-cases/)
• ❌ NO domain logic (that's domain/)
• ✅ Owns input parsing and output mapping

Principle 1: Separate external clients from domain-specific code

What: Generic wrappers for external services (APIs, databases, SDKs) live separately from code that uses them in domain-specific ways.

Why: Domain logic mixed with external service details is harder to understand and evolve. Separating them keeps domain logic pure and focused.

How:

• Ask: "Would the creators of this external service recognize this code?"
• YES → external-clients/
• NO → your domain code

❌ BAD:
platform/infra/external-clients/order-total.ts   ← domain logic in infra
features/checkout/stripe-api.ts                  ← external client in feature

✅ GOOD:
platform/infra/external-clients/stripe.ts        ← generic: charge, refund, subscribe
features/checkout/payment-processing.ts          ← OUR domain logic using stripe

Principle 2: Separate feature-specific from shared capabilities

What: Code that belongs to one feature stays in that feature's folder. Code used across features lives in a shared location named for what it IS.

Why: When shared logic is buried in one feature, other features either import across boundaries (coupling) or duplicate the logic (divergence). Both cause bugs.

How:

• Ask: "Does this conceptually belong to one feature?"
• YES → keep in features/
• NO → extract to platform/, name it for what it IS

❌ BAD - buried in one feature:
features/checkout/tax-calculator.ts
features/refunds/refund.ts           ← imports ../checkout/tax-calculator

❌ BAD - duplicated:
features/checkout/tax-calculator.ts
features/refunds/tax-calculator.ts   ← rules diverge over time

✅ GOOD - extracted to platform:
features/checkout/
features/refunds/
platform/domain/tax-calculation/     ← shared domain logic

Principle 3: Separate intent from execution

What: High-level flow visible at one abstraction level. Implementation details in lower levels.

Why: When intent and execution are mixed, you can't see what the code does without reading every line. Changes to one step's implementation ripple through unrelated code.

How:

• Ask: "Can I see the high-level flow without reading every line?"
• NO → extract details into named functions/methods

// ❌ BAD - can't see flow, details obscure intent
async function checkout(cart: Cart) {
  const ctx = new CheckoutContext()
  try {
    const validation = await validateCart(cart)
    if (!validation.success) { /* 10 lines of error handling */ }
    const payment = await processPayment(cart)
    if (!payment.success) { /* 10 lines of rollback */ }
    // ... 30 more lines
  } catch (e) { await cleanup(ctx); throw e }
}

// ✅ GOOD - flow visible, drill into details as needed
function checkout(cart: Cart, payment: PaymentDetails) {
  const validatedCart = cart.validate()
  const receipt = paymentService.process(validatedCart.total, payment)
  const order = Order.create(validatedCart, receipt)
  confirmationService.send(order)
  return order
}

Principle 4: Separate functions that depend on different state

What: Functions that depend on different state (different fields, databases, services, config) belong in different modules.

Why: Different state dependencies mean different reasons to change, different testing strategies, and different failure modes.

How:

• List the fields/dependencies in a class
• For each method, note which it uses
• Methods cluster around different state? → split into separate classes

❌ BAD:
class OrderService {
  db, emailClient, templateEngine

  save()  → uses db
  find()  → uses db
  sendConfirmation() → uses emailClient, templateEngine
}

✅ GOOD:
class OrderRepository { db }
class OrderNotifications { emailClient, templateEngine }

Principle 5: Separate functions that don't have related names

What: Functions in the same module should have names that relate to a common concept.

Why: Unrelated names signal unrelated responsibilities. If you can't name the module after what the functions have in common, they probably don't belong together.

How:

• Look at the function names in a module
• Can you describe what they have in common in one phrase?
• NO → split them into separate modules

❌ BAD - order-helpers.ts:
  calculateOrderTotal()
  formatOrderForInvoice()
  validateOrderForShipping()
  assessOrderFraudRisk()
  → all operate on "order" but change for different reasons:
    pricing rules, invoice formatting, shipping constraints, fraud detection

✅ GOOD - split by why they change:
  order-pricing.ts:      calculateTotal(), applyDiscounts()
  invoice-formatting.ts: formatForInvoice(), formatLineItems()
  shipping-validation.ts: validateForShipping(), checkWeightLimits()
  fraud-detection.ts:    assessFraudRisk(), flagSuspiciousPatterns()

Package Structure

/food-delivery/
├── features/
│   ├── order-placement/
│   │   ├── entrypoint/        ← thin, invokes use-case
│   │   ├── use-cases/         ← orchestration
│   │   └── domain/            ← business rules
│   │
│   └── driver-tracking/
│       ├── entrypoint/
│       ├── use-cases/
│       └── domain/
│
├── platform/
│   ├── domain/                ← shared business rules
│   └── infra/                 ← technical concerns (infra lives here, not in features)
│
└── shell/
    └── cli.ts

Mandatory Checklist

When designing, implementing, refactoring, or reviewing code, complete this checklist:

1. Verify features/, platform/, shell/ exist at the root of the package
2. Verify platform/ contains only domain/ and infra/
3. Verify each feature contains only entrypoint/, use-cases/, domain/
4. Verify shell/ contains no business logic
5. Verify code belonging to one feature is in features/[feature]/
6. Verify shared business logic is in platform/domain/ and no dependencies between features
7. Verify external service wrappers are in platform/infra/
8. Verify custom folders (steps/, handlers/) are inside domain/, not use-cases/
9. Verify each function relies on same state as others in its class/file and name aligns
10. Verify each file name relates to other files in its directory
11. Verify each directory name describes what all files inside have in common
12. Verify use-cases/ contains only use-case files (no nested folders, no helper files)
13. Verify no generic type-grouping files (types.ts, errors.ts, validators.ts) spanning multiple capabilities
14. Verify entrypoint/ is thin (parse input → invoke use-case → map output) and never imports from domain/

Do not proceed until all checks pass.