hex-infra

hex-infra

Overview

When a feature or behavior change is requested, work through four steps in order. Never skip ahead.

1. Classify — Command (mutates state) or Query (reads state)?
2. Write handler — application logic only, no HTTP/DB imports
3. Define ports — interfaces the handler depends on
4. Write adapters — concrete implementations for transport, storage, and external systems

Use this as the default frame for feature work: domain and application code stay separate from frameworks, datastores, transport layers, and vendor SDKs. All external actions should be modeled as either a command or a query, with dependencies expressed as ports and implemented by adapters.

With TDD: Use superpowers:test-driven-development alongside this skill. Tests drive behavior; hex boundaries keep that behavior testable and isolated from infrastructure.

src/domain/ is read-only when your repository policy marks it frozen. Otherwise, new domain concepts belong under src/domain/ following the craftsmanship rules below — never leak infrastructure into that tree.

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Domain implementation and software craftsmanship

Domain code should express business behavior and invariants, not data shapes alone. Apply these principles in order of priority:

1. Ubiquitous language — Names in code match how the business speaks. Avoid generic DTO names (Data, Info) when a domain term exists.
2. Behavior-first types — Prefer methods that enforce rules (order.addLine(item)) over setter-heavy structures. Reject anemic models where services do all the work and entities are bags of fields (see anti-pattern coverage in the DDD reference).
3. Single responsibility — One type, one reason to change: either a cohesive slice of domain behavior or a clearly bounded value. Split when a class answers two unrelated business questions.
4. Encapsulation — Expose intent (cancel(), ship()) not field mutation. Keep invariants inside the aggregate or entity boundary.
5. Dependency direction — Domain depends on nothing outside itself. Application handlers orchestrate domain objects and ports; infrastructure implements ports only.
6. SOLID as guardrails — S small cohesive modules; O extend via new types or ports, not giant switches; L substitutable port implementations; I narrow port interfaces; D depend on abstractions (ports), not concrete adapters.

Handlers vs domain: Handlers parse input, load aggregates via repositories, invoke domain behavior, and persist outcomes. Complex rules live in domain types; handlers stay thin orchestration.

When to read more: For aggregates, bounded contexts, strategic vs tactical DDD, CQRS tradeoffs, and Microsoft-style implementation patterns, consult references/ddd-expert-knowledge-base.md.

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Step 1: Classify

	Command	Query
Mutates state?	Yes	No
Depends on	Repository (load/save aggregate)	ViewStore (read model)
Returns	Result<E, { id } | void>	Result<E, View>

Convention, not a rule - commands may return richer data when genuinely needed.

---

Step 2: Write the Handler

Shared types

Define once in src/application/types.ts:

export type Result<E extends string, A> =
  | { ok: true; value: A }
  | { ok: false; error: E }

export const ok  = <A>(value: A): Result<never, A>            => ({ ok: true, value })
export const err = <E extends string>(e: E): Result<E, never> => ({ ok: false, error: e })

export type CommandHandler<I, O> = (input: I, ctx: AppContext) => Promise<O>
export type QueryHandler<I, O>   = (input: I, ctx: AppContext) => Promise<O>

export type AppContext = Record<string, unknown> // extend as you add ports

AppContext is the seam between application code and infrastructure. It should carry ports only, never framework request objects, database handles, or SDK clients.

Command handler

src/application/commands/CreateOrder.ts:

import type { CommandHandler, Result } from '../types'

export type CreateOrderInput = {
  customerId: string
  items: { productId: string; quantity: number }[]
}
export type CreateOrderOutput = Result<
  'CUSTOMER_NOT_FOUND' | 'INVALID_QUANTITY',
  { orderId: string }
>

export const handler: CommandHandler<CreateOrderInput, CreateOrderOutput> =
  async (input, ctx) => {
    // domain logic only - no HTTP, no DB imports
  }

Query handler

src/application/queries/GetOrder.ts:

import type { QueryHandler, Result } from '../types'

export type GetOrderInput  = { orderId: string }
export type GetOrderOutput = Result<'ORDER_NOT_FOUND', OrderView>

export const handler: QueryHandler<GetOrderInput, GetOrderOutput> =
  async (input, ctx) => ctx.orderViewStore.byId(input.orderId)

---

Step 3: Define Ports

One file per dependency in src/ports/. Interface only - no implementation.

Ports describe capabilities in business terms. They should not leak ORM rows, HTTP request/response objects, SDK result shapes, or other infrastructure-specific types.

// src/ports/OrderRepository.ts
export interface OrderRepository {
  load(id: OrderId): Promise<Order | null>
  save(order: Order): Promise<void>
}

// src/ports/OrderViewStore.ts
export interface OrderViewStore {
  byId(id: string): Promise<OrderView | null>
  search(criteria: OrderSearch): Promise<Page<OrderView>>
}

After defining ports, update AppContext in src/application/types.ts:

export type AppContext = {
  orderRepository: OrderRepository
  orderViewStore: OrderViewStore
}

Handlers receive ports via ctx - never import infrastructure into application code.

---

Step 4a: Transport Adapter

One file per handler in the appropriate infrastructure boundary. Its jobs are: validate input, call the handler, map the result.

// src/infrastructure/http/CreateOrderController.ts

// 1. Validate raw request input (use whatever validation library the project uses)
const input = validate<CreateOrderInput>(request.body)

// 2. Call handler
const result = await handler(input, ctx)

// 3. Map result
if (!result.ok) return httpError(reply, result.error)
return httpSuccess(reply, result.value, 201)

No business logic in controllers - ever.

The same rule applies outside HTTP. A queue consumer, job runner, event handler, CLI entrypoint, or UI-triggered action is still just an adapter around a command or query handler.

Error code -> HTTP status

Error code pattern	Status
*_NOT_FOUND	404
INVALID_*	400
UNAUTHORIZED	401
FORBIDDEN	403
*_CONFLICT	409
RATE_LIMITED	429
anything else	500

Response envelope always: { "code": "ERROR_CODE" }

---

Step 4b: Storage Or External Adapter

One file per port in the appropriate infrastructure boundary. Implement the interface exactly - no extra methods. Keep mapping functions private to the file.

Repository (command side) src/infrastructure/db/OrderRepository.ts:

import type { OrderRepository } from '../../ports/OrderRepository'

export class OrderDbRepository implements OrderRepository {
  async load(id: OrderId): Promise<Order | null> {
    const row = await /* db query by id */
    return row ? toDomain(row) : null
  }

  async save(order: Order): Promise<void> {
    const row = toRow(order)
    await /* upsert row */
  }
}

// Private - never export
function toDomain(row: unknown): Order { /* DB row -> domain object */ }
function toRow(order: Order): unknown  { /* domain object -> DB row  */ }

ViewStore (query side) src/infrastructure/db/OrderViewStore.ts:

import type { OrderViewStore } from '../../ports/OrderViewStore'

export class OrderDbViewStore implements OrderViewStore {
  async byId(id: string): Promise<OrderView | null> {
    const row = await /* db query by id */
    return row ? toView(row) : null
  }

  async search(criteria: OrderSearch): Promise<Page<OrderView>> {
    /* filter + paginate rows, return { items, total } */
  }
}

// Private - never export
function toView(row: unknown): OrderView { /* DB row -> view */ }

Apply the same pattern to non-database integrations. A mailer, queue publisher, payment gateway client, filesystem writer, or third-party API client is an adapter that implements a port owned by the application.

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Hard Rules

• No framework, datastore, transport, SDK, or vendor imports inside src/domain/ or src/application/
• No business logic inside controllers
• External effects go through ports implemented by adapters
• Commands mutate state and may trigger effectful ports
• Queries read state and must not hide writes
• Port interfaces must not expose infrastructure types (e.g. no raw DB rows as return types)
• Adapters implement the port interface exactly - no extra methods

Non-HTTP Feature Work

Use the same command/query plus ports/adapters framing even when the entrypoint is not HTTP:

• Background jobs
• Event consumers
• UI-triggered actions
• Cron tasks
• Service-to-service calls

If the operation changes state or causes effects, model it as a command. If it only reads, model it as a query.

File Layout

src/
  domain/                           <- READ ONLY
  application/
    commands/CreateOrder.ts         <- handler + input/output types
    queries/GetOrder.ts
    types.ts                        <- Result, CommandHandler, QueryHandler, AppContext
  ports/
    OrderRepository.ts              <- interface only
    OrderViewStore.ts
  infrastructure/
    http/
      CreateOrderController.ts      <- one per handler
    db/
      OrderRepository.ts            <- one per port
      OrderViewStore.ts