API Design

Overview

Based on "A Philosophy of Software Design" by John Ousterhout. Ousterhout's key principle: modules should be deep - simple interface, powerful implementation. Applied to APIs: the surface area exposed to callers should be as small as possible, while the behavior provided should be as rich as possible. A leaky abstraction (where callers need to understand implementation details) is a design failure.

Workflow

Step 1: Define the API's job

Before designing any endpoint, state what the API enables:

• What is the caller trying to accomplish?
• What information does the caller have?
• What should the API hide from the caller?

Ousterhout: if callers need to know how the API works internally to use it correctly, the API is leaky.

Step 2: Design resources (REST) or schema (GraphQL)

REST:

• Resources are nouns, not verbs: /users, /orders not /getUsers, /createOrder
• Use HTTP verbs for actions: GET (read), POST (create), PUT (replace), PATCH (update), DELETE (remove)
• Nest for ownership: /users/{id}/orders (orders owned by a user)
• Don't nest beyond 2 levels deep

GraphQL:

• Types model your domain, not your database schema
• Queries fetch data, mutations change it, subscriptions stream it
• Design the schema from the client's perspective: what does the UI need?

Step 3: Design request and response contracts

For each endpoint:

Endpoint: [METHOD] /path/{param}
Purpose: [one sentence]
Auth: [required scope or token type]

Request:
  Path params: [required identifiers]
  Query params: [optional filters, pagination]
  Body: [JSON schema with required/optional fields]

Response (200):
  [JSON schema]

Errors:
  400 - [validation failure - which fields, why]
  401 - [unauthenticated]
  403 - [unauthorized - caller lacks permission]
  404 - [resource not found]
  409 - [conflict - e.g. duplicate]
  422 - [unprocessable entity - valid syntax, invalid semantics]
  500 - [internal error]

Step 4: Apply consistency rules

Pick conventions and apply them everywhere:

• Naming: snake_case or camelCase for fields (pick one)
• Dates: ISO 8601 (2026-03-03T14:30:00Z)
• IDs: UUID or integer (pick one, apply everywhere)
• Pagination: cursor-based or page/limit (pick one)
• Errors: consistent error response shape { error: { code, message, details } }

Inconsistency is the top complaint from API consumers.

Step 5: Design for the caller's mental model

Ousterhout's information hiding principle: don't expose implementation artifacts.

• Don't expose database table names or internal IDs in API paths
• Don't return internal error codes that callers can't act on
• Don't require callers to make multiple requests for data they logically need together

Step 6: Version the API

Every public API needs a versioning strategy before it ships:

• URL versioning: /v1/users (most common, most visible)
• Header versioning: API-Version: 2026-01-01
• Pick a strategy before v1 ships - changing it later breaks all callers

Anti-Patterns

1. Verbs in resource paths Bad: POST /createUser, GET /getUserById Good: POST /users, GET /users/{id}

2. Inconsistent error shapes Bad: Some endpoints return { error: "message" }, others return { message: "error" }. Good: One error schema, applied everywhere.

3. Leaking implementation details Bad: /api/db/users_table/row/42 Good: /api/users/42

4. No versioning plan on day 1 Bad: Ship v1 with no versioning, then break callers when you need to change the contract. Good: Version from the first public release. Breaking changes require a new version.

Quality Checklist

• Resources are nouns, HTTP verbs are the actions
• Request and response contracts documented for every endpoint
• All error codes documented with when each occurs
• Naming convention consistent across all endpoints (snake_case or camelCase)
• Dates in ISO 8601, IDs consistent type throughout
• No implementation details exposed to callers
• Versioning strategy defined before first public release
• Pagination strategy defined and consistent