What Is a Good API? - Learning Module

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API Design Process

From Principles to Practice

We've established the foundational principles: APIs as contracts, usability, and consistency. But knowing principles isn't the same as applying them. When you're staring at a blank interface definition, where do you start? How do you systematically apply these principles to produce excellent APIs?

This page presents a practical, step-by-step API design process—a methodology that guides you from requirements through design, implementation, and iteration. This process isn't just theory; it reflects how experienced API designers work, making their intuitive approach explicit and repeatable.

Following a structured process doesn't replace design skill, but it ensures you consider all important aspects, avoid common oversights, and produce APIs that serve consumers well.

What You Will Learn

By the end of this page, you will understand: (1) A complete API design methodology from requirements to iteration; (2) How to gather and analyze requirements from an API perspective; (3) Techniques for designing APIs that prioritize consumer needs; (4) Review and validation strategies before implementation; and (5) How to evolve APIs based on feedback while maintaining quality.

The API Design Lifecycle

API design isn't a single step—it's a cycle of activities that continues throughout the API's lifetime:

The Design Lifecycle Phases:

API Design Phases

•Requirements Gathering — Understand what consumers need and why. Identify use cases, constraints, and success criteria.
•API Modeling — Design the abstract API surface: resources, operations, relationships. Focus on consumer mental models.
•Interface Definition — Translate the model into concrete signatures: method names, parameters, types, return values, errors.
•Review and Validation — Evaluate the design against principles and consumer needs. Iterate before implementation.
•Implementation — Build the API according to the specification. Implementation shouldn't change the design.
•Documentation — Create comprehensive documentation that goes beyond auto-generated references.
•Feedback and Evolution — Gather usage data and feedback. Plan improvements for future versions.

The Front-Loading Principle:

API design follows a principle similar to software architecture: fix problems early. The cost of changing an API increases dramatically once consumers depend on it:

Changing design before implementation: Easy, cheap
Changing design during implementation: Moderate effort
Changing design after release: Expensive (versioning, migration)
Changing design with many consumers: Very expensive (coordination, support)

Invest heavily in the early phases. Every hour spent on requirements and design can save ten hours of implementation and a hundred hours of migration later.

Converting Mermaid diagram...

Design First, Implement Second

Resist the temptation to start coding immediately. Implementation details often leak into API design when you think about 'how' before fully understanding 'what.' Complete the design phase entirely before writing production code.

Phase 1: Requirements Gathering

Effective API design begins with deeply understanding consumer needs. This isn't about asking "What endpoints do you want?"—it's about understanding the problems consumers are trying to solve.

Key Questions to Answer:

Requirements Discovery Questions

•Who are the consumers? Internal teams, external developers, partner integrations, automated systems?
•What problems are they solving? What tasks do they need to accomplish? What's their workflow?
•What are their technical constraints? Languages, platforms, network conditions, security requirements?
•What's their expertise level? Are they API experts or occasional integrators?
•What are the performance requirements? Latency, throughput, availability targets?
•What are the security requirements? Authentication, authorization, data sensitivity?
•How will they discover and learn the API? Self-service, documentation, support?

Use Case Documentation:

Capture requirements as concrete use cases—specific scenarios that the API must support. Each use case should include:

UseCaseTemplate.md
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## Use Case: [Name]
 
### Summary
A brief description of what the consumer wants to accomplish.
 
### Actor
Who is performing this action? (External developer, internal service, etc.)
 
### Preconditions
What must be true before this use case begins?
 
### Basic Flow
Step-by-step description of the happy path:
1. Consumer does X
2. API responds with Y
3. Consumer does Z
4. ...
 
### Alternative Flows
What happens in alternative scenarios?
- If resource doesn't exist: ...
- If validation fails: ...
- If consumer lacks permission: ...
 
### Data Requirements
What data does the consumer need to provide?
What data does the consumer need to receive?
 
### Non-Functional Requirements
- Expected latency: < 100ms
- Expected throughput: 1000 requests/second
- Availability: 99.9%
 
### Priority
How critical is this use case? (P0, P1, P2)
 
### Example
Concrete example with sample data:
```
Consumer sends: { "email": "user@example.com", "name": "Jane Doe" }
API returns: { "id": "usr_123", "email": "user@example.com", ... }
```

Requirements Prioritization:

Not all use cases are equal. Prioritize ruthlessly:

Use Case Prioritization Matrix
Priority	Description	API Design Implication
P0 - Critical	API is useless without this	Must be simple, optimized, thoroughly tested
P1 - Important	Significant value, common need	Should be convenient, well-documented
P2 - Nice to Have	Adds value for some consumers	Can be more complex, use advanced patterns
P3 - Edge Case	Rare need or special case	Can require workarounds, lower priority

Talk to Real Consumers

If possible, talk to actual API consumers—not just product managers or architects. Developers who will write integration code have insights about pain points, workflow, and expectations that others lack. Their feedback is invaluable.

Phase 2: API Modeling

With requirements understood, the next phase is modeling the API at an abstract level—identifying resources, operations, and relationships before committing to specific syntax.

Resource Identification:

Start by identifying the core resources (nouns) your API exposes. Resources are the primary abstractions consumers interact with:

ResourceIdentification.ts
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// Example: E-commerce API Resource Identification
 
// Primary Resources (core domain entities)
// - User: represents a customer account
// - Product: represents an item for sale
// - Order: represents a purchase transaction
// - Cart: represents items selected for potential purchase
 
// Secondary Resources (supporting entities)
// - Address: shipping/billing location
// - Payment: payment method information
// - Review: product evaluation by user
// - Inventory: stock levels for products
 
// Derived/Computed Resources (not stored, but queryable)
// - Recommendation: suggested products for user
// - OrderSummary: aggregated order statistics
 
// Questions to ask for each resource:
// 1. What uniquely identifies this resource?
// 2. What attributes describe this resource?
// 3. What are the valid states/lifecycle?
// 4. Who can view/modify this resource?

Operation Identification:

For each resource, identify the operations (verbs) consumers need to perform:

Standard Operations Pattern
Operation Type	Description	Common Semantics
Create	Bring new resource into existence	Validates input, generates ID, returns created resource
Read (Single)	Retrieve one resource by identifier	Returns resource if found, error if not
Read (List)	Retrieve collection of resources	Supports filtering, sorting, pagination
Update (Full)	Replace entire resource	Requires complete resource data, overwrites all fields
Update (Partial)	Modify specific fields	Only specified fields change, others preserved
Delete	Remove resource from system	May be soft delete, may cascade
Action	Trigger domain-specific operation	Business logic beyond CRUD (e.g., 'submit', 'approve')

Relationship Mapping:

Identify how resources relate to each other:

RelationshipMapping.ts
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// Relationship types and their API implications
 
// One-to-One: User -> Profile
// API Design Options:
// - Embedded: User contains profile inline
// - Linked: User has profileId, separate Profile endpoint
// - Sub-resource: GET /users/:id/profile
 
// One-to-Many: User -> Orders
// API Design Options:
// - Linked: Order has userId, filter orders by user
// - Sub-resource: GET /users/:id/orders
// - Both: Support direct /orders?userId=X and /users/:id/orders
 
// Many-to-Many: Product <-> Category
// API Design Options:
// - Join resources: ProductCategory linking table
// - Embedded arrays: Product has categoryIds[]
// - Sub-resources: GET /products/:id/categories
 
// Hierarchical: Category -> SubCategory -> SubSubCategory
// API Design Options:
// - Flat with parent reference: Category has parentId
// - Nested paths: GET /categories/:id/subcategories
// - Tree operations: GET /categories/:id/ancestors, /descendants
 
// Key questions for relationship design:
// 1. How will consumers typically access related resources?
// 2. Should related data be embedded or linked?
// 3. What navigation paths make sense?
// 4. How do permissions flow across relationships?

Consumer Mental Model:

The most important aspect of API modeling is ensuring the API matches how consumers think about the domain—not how the system is implemented internally.

Implementation-Driven Model

•Exposes database tables directly
•Reflects internal service boundaries
•Uses internal terminology
•Requires knowledge of system internals

Consumer-Driven Model

•Expresses domain concepts
•Reflects consumer workflows
•Uses ubiquitous domain language
•Can be understood from domain knowledge

The Domain Language Test

Could a domain expert (non-technical) understand your API resources and operations by name alone? If your API references 'users' and 'orders' rather than 'user_records' and 'txn_logs', you're on the right track. Use ubiquitous language from the domain.

Phase 3: Interface Definition

With the abstract model defined, the next phase translates it into concrete interface definitions—specific method signatures, types, and contracts.

From Model to Interface:

Each resource and operation from the model becomes a concrete API element:

ModelToInterface.ts
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// Abstract Model: Order resource with CRUD operations
 
// Concrete Interface Definition:
interface OrderApi {
    /**
     * Creates a new order from the provided order data.
     * 
     * @param data - The order data including items and shipping info
     * @returns The created order with generated ID and status
     * 
     * @throws ValidationError - If order data is invalid
     * @throws InsufficientInventoryError - If items are out of stock
     * @throws PaymentRequiredError - If payment method is missing
     * 
     * Preconditions:
     *   - data.items is non-empty
     *   - data.shippingAddress is valid
     *   - Current user is authenticated
     * 
     * Postconditions:
     *   - Order is persisted with status 'pending'
     *   - Inventory is reserved for items
     *   - OrderCreated event is emitted
     * 
     * Idempotency:
     *   - Supports idempotency key in options
     *   - Duplicate key returns original order
     */
    createOrder(
        data: CreateOrderData,
        options?: CreateOrderOptions
    ): Promise<Order>;
    
    /**
     * Retrieves an order by its unique identifier.
     * 
     * @param orderId - The unique identifier of the order
     * @returns The order if found and accessible
     * 
     * @throws NotFoundError - If order doesn't exist
     * @throws ForbiddenError - If current user can't access this order
     */
    getOrder(orderId: string): Promise<Order>;
    
    // ... additional operations with same level of detail
}
 
// Supporting type definitions
interface CreateOrderData {
    items: OrderItem[];
    shippingAddress: Address;
    billingAddress?: Address;  // Optional, defaults to shipping
    notes?: string;
}
 
interface CreateOrderOptions {
    idempotencyKey?: string;
    notifyCustomer?: boolean;  // Default: true
}
 
interface Order {
    id: string;
    status: OrderStatus;
    items: OrderItem[];
    shippingAddress: Address;
    billingAddress: Address;
    totals: OrderTotals;
    createdAt: Date;
    updatedAt: Date;
}

Design Decisions Checklist:

For each operation, explicitly decide:

Operation Design Checklist

•Naming: Does the name clearly express what the operation does?
•Parameters: What inputs are required? What's optional? What are defaults?
•Return Type: What does the operation return on success? The full entity? Just an ID?
•Errors: What can go wrong? How is each error communicated?
•Idempotency: Is this operation safe to retry? How is idempotency implemented?
•Side Effects: What happens beyond the return value? Events? State changes?
•Authorization: Who can call this? How is access controlled?
•Validation: What constraints apply to inputs? How are violations reported?
•Performance: What are the expected latency and resource usage?

Type Design:

Pay special attention to the types used in your interface. They're part of the contract:

TypeDesign.ts
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// Type design principles
 
// 1. Make invalid states unrepresentable
type OrderStatus = 
    | 'pending'      // Awaiting payment
    | 'paid'         // Payment received
    | 'processing'   // Order being prepared
    | 'shipped'      // Order in transit
    | 'delivered'    // Order arrived
    | 'cancelled';   // Order cancelled
 
// Not: status: string (allows any string)
 
// 2. Use domain types, not primitives
type OrderId = string & { readonly __brand: 'OrderId' };
type UserId = string & { readonly __brand: 'UserId' };
type Money = { amount: number; currency: string };
 
// Not: orderId: string, userId: string, price: number
 
// 3. Separate input and output types when they differ
interface CreateUserInput {
    email: string;
    name: string;
    password: string;  // Only on input
}
 
interface User {
    id: UserId;
    email: string;
    name: string;
    createdAt: Date;   // Only on output
    // No password!
}
 
// 4. Use union types for polymorphic data
type PaymentMethod = 
    | { type: 'card'; cardToken: string; last4: string }
    | { type: 'bank'; accountId: string; routingNumber: string }
    | { type: 'wallet'; walletId: string; provider: string };
 
// 5. Document constraints in types when possible
interface PaginationOptions {
    limit?: number;   // 1-100, default 20
    offset?: number;  // >= 0, default 0
}
// Better with branded types or validation

Specification Formats

For REST APIs, consider using OpenAPI (Swagger) to formally specify your interface. For GraphQL, the schema is the specification. For RPC-style APIs, consider Protocol Buffers or similar IDL. Formal specifications enable code generation, validation, and automated documentation.

Phase 4: Review and Validation

Before implementation, rigorously review the design. This is your last cheap opportunity to catch issues.

Design Review Process:

1. Self-Review Against Principles:

Principle Checklist

•Contract clarity: Are preconditions, postconditions, and errors explicitly documented?
•Least astonishment: Does each operation behave as developers would expect?
•Discoverability: Can developers find what they need through names and types?
•Progressive disclosure: Is the simple case simple? Are advanced options accessible?
•Consistency: Do patterns apply uniformly across the API?
•Simplicity: Is complexity justified? Can anything be simplified?

2. Use Case Walkthrough:

For each documented use case, trace through exactly how a consumer would accomplish it:

UseCaseWalkthrough.ts
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// Use Case: Customer places an order
 
// Step through the actual consumer code:
 
// 1. Consumer is authenticated (precondition)
const api = new ApiClient(authToken);
 
// 2. Consumer adds items to cart
const cart = await api.getCart();
await api.addToCart(cart.id, { productId: 'prod_123', quantity: 2 });
await api.addToCart(cart.id, { productId: 'prod_456', quantity: 1 });
 
// Questions during walkthrough:
// - Is getCart() the right name? Should it create cart if none exists?
// - Should addToCart return the updated cart?
// - What if productId is invalid? What error?
 
// 3. Consumer provides shipping info
const address = { /* ... */ };
await api.setShippingAddress(cart.id, address);
 
// Questions:
// - Is this a cart operation or order operation?
// - Should validation happen now or at checkout?
 
// 4. Consumer submits order
const order = await api.checkout(cart.id, {
    paymentMethodId: 'pm_abc123'
});
 
// Questions:
// - What does checkout return? Order? Payment status?
// - What if payment fails? How is that communicated?
// - Is cart cleared automatically?
 
// 5. Consumer views order status
const orderStatus = await api.getOrder(order.id);
 
// Complete walkthrough reveals gaps and awkward patterns

3. Peer Review:

Have other engineers review the design. They bring fresh perspectives and catch blind spots. Focus areas:

Peer Review Focus Areas

•Usability: Would they understand this API without your explanation?
•Consistency: Does this match patterns in your existing APIs?
•Feasibility: Can this be implemented efficiently? Are there hidden complexities?
•Edge cases: What happens with empty inputs, nulls, large datasets, concurrent access?
•Security: Are there authorization gaps? Data exposure risks? Injection vulnerabilities?

4. Consumer Validation:

If possible, validate with actual consumers before implementation:

Consumer Validation Techniques

•Mock API: Create a non-functional mock that consumers can explore
•Code samples: Write sample integration code to see how the API feels
•API specification review: Share OpenAPI spec or documentation drafts
•Design partnership: Involve key consumers in design sessions

Don't Skip Review

Time pressure often leads teams to skip or rush design review. This is a false economy. Fixing design issues during review takes hours; fixing them after release takes weeks or months. Budget adequate time for review in your schedule.

Phase 5: Implementation Guidelines

With the design validated, implementation begins. The goal is to build exactly what was designed, without letting implementation concerns corrupt the API surface.

Implementation Principles:

1. The Specification is the Truth:

SpecIsTheTruth.ts
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// The design specifies: createOrder returns the created Order
 
// ❌ Wrong: Implementation convenience changes the contract
async createOrder(data: CreateOrderData): Promise<{ orderId: string }> {
    // "It's simpler to just return the ID"
    // But now consumers need another call to get the order!
}
 
// ✅ Right: Implementation matches the specification
async createOrder(data: CreateOrderData): Promise<Order> {
    const orderId = await this.repository.insert(data);
    return this.getOrder(orderId); // Extra query, but matches contract
}
 
// If the specification needs to change, go back to design phase,
// don't change it during implementation

2. Separate API Layer from Implementation:

The API layer should translate between external contracts and internal business logic:

ApiLayerSeparation.ts
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// API Layer: Handles contract, validation, transformation
class OrderApiController {
    constructor(private orderService: OrderService) {}
    
    async createOrder(request: CreateOrderRequest): Promise<OrderResponse> {
        // 1. Validate input against contract
        this.validateCreateOrderRequest(request);
        
        // 2. Transform external format to internal
        const internalData = this.mapToInternal(request.data);
        
        // 3. Call business logic
        const order = await this.orderService.createOrder(internalData);
        
        // 4. Transform internal result to external format
        return this.mapToResponse(order);
    }
    
    private mapToInternal(data: CreateOrderData): InternalOrderData {
        // External API uses camelCase; internal uses different conventions
        // External uses string IDs; internal uses typed IDs
        // This layer handles the translation
    }
    
    private mapToResponse(order: InternalOrder): OrderResponse {
        // Internal may have fields not exposed via API
        // This layer filters and transforms
    }
}
 
// Business Layer: Pure domain logic
class OrderService {
    async createOrder(data: InternalOrderData): Promise<InternalOrder> {
        // Business logic, unaware of API contracts
    }
}

3. Contract Testing:

Write tests that verify the implementation honors the contract:

ContractTests.ts
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describe('OrderApi contract tests', () => {
    
    describe('createOrder', () => {
        it('returns created order with all specified fields', async () => {
            const data: CreateOrderData = { /* valid data */ };
            
            const order = await api.createOrder(data);
            
            // Contract: Returns complete Order object
            expect(order.id).toBeDefined();
            expect(order.status).toBe('pending');
            expect(order.items).toEqual(data.items);
            expect(order.createdAt).toBeDefined();
        });
        
        it('throws ValidationError for empty items', async () => {
            const data: CreateOrderData = { items: [] };
            
            // Contract: Throws ValidationError for invalid input
            await expect(api.createOrder(data))
                .rejects.toThrow(ValidationError);
        });
        
        it('is idempotent with idempotency key', async () => {
            const data: CreateOrderData = { /* valid data */ };
            const options = { idempotencyKey: 'test-key-123' };
            
            const order1 = await api.createOrder(data, options);
            const order2 = await api.createOrder(data, options);
            
            // Contract: Duplicate key returns original order
            expect(order1.id).toBe(order2.id);
        });
    });
});

Contract Tests as Documentation

Contract tests serve double duty: they verify correctness and document expected behavior. When someone wonders 'what happens if X?', the contract test for that case provides the authoritative answer.

Phase 6: Documentation Strategy

Documentation is not an afterthought—it's an integral part of the API product. Even a well-designed API fails if consumers can't learn to use it.

Documentation Layers:

Documentation Types and Purposes
Layer	Purpose	Content
API Reference	Detailed spec of every operation	Parameters, types, errors, examples for each endpoint
Getting Started	First success experience	Quick start, authentication, first API call
Tutorials	Common task walkthroughs	Step-by-step guides for key use cases
Conceptual Guides	Deep understanding	How things work, design rationale, best practices
Changelog	Track evolution	Version history, breaking changes, migration guides

Reference Documentation Essentials:

For each operation, reference documentation should include:

Reference Documentation Checklist

•Description: What does this operation do? When would you use it?
•Authentication: What authentication is required?
•Parameters: Name, type, required/optional, constraints, defaults
•Request example: Complete, runnable example with realistic data
•Response structure: All fields with types and descriptions
•Response example: Complete example matching the request example
•Error responses: Each possible error with code, message, and cause
•Rate limits: If applicable, what limits apply?
•Related operations: Links to related or alternative operations

Example Quality:

Examples are often the first thing developers look at. Make them excellent:

DocumentationExamples.ts
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// ❌ Poor example: Minimal, unrealistic
const order = await api.createOrder({
    items: [{ productId: 'x', quantity: 1 }]
});
 
// ✅ Good example: Complete, realistic, annotated
// Create an order with multiple items
const order = await api.createOrder({
    // Items to purchase (at least one required)
    items: [
        { 
            productId: 'prod_laptop_pro_15',
            quantity: 1,
            customization: { color: 'space-gray' }
        },
        { 
            productId: 'prod_usbc_cable_2m',
            quantity: 2
        }
    ],
    // Shipping destination
    shippingAddress: {
        name: 'Jane Smith',
        line1: '123 Main Street',
        line2: 'Suite 456',
        city: 'San Francisco',
        state: 'CA',
        postalCode: '94105',
        country: 'US'
    },
    // Optional: custom notes for fulfillment
    notes: 'Please include gift receipt'
}, {
    // Enable idempotency for safe retries
    idempotencyKey: 'order_jane_2024_01_15_001'
});
 
// Response includes the complete order with generated fields
console.log(order.id);         // 'ord_abc123def456'
console.log(order.status);     // 'pending'
console.log(order.totals.total); // { amount: 1279.99, currency: 'USD' }

Living Documentation

Documentation should be generated from or validated against the actual implementation when possible. Tools like OpenAPI, Swagger, and API documentation generators help keep documentation in sync with code. Never let documentation drift from reality.

Phase 7: Feedback and Evolution

API design doesn't end at release. Gathering feedback and evolving the API is an ongoing process.

Feedback Channels:

API Feedback Sources

•Usage analytics: Which operations are used most/least? What error rates exist?
•Support tickets: What questions do developers ask? Where do they struggle?
•Community feedback: Forums, GitHub issues, social media, developer surveys
•Direct interviews: Scheduled conversations with key consumers
•Monitoring data: Latency distributions, error patterns, capacity trends
•Competitive analysis: What are alternatives doing that you could learn from?

Categorizing Feedback:

Not all feedback leads to immediate action. Categorize and prioritize:

Feedback Categorization
Category	Description	Action
Bugs	API doesn't match documented behavior	Fix immediately in current version
Usability issues	API works but is awkward to use	Consider for next minor version
Missing features	Consumers need capabilities not provided	Evaluate for roadmap
Breaking changes	Fundamental design needs revision	Plan for major version
Documentation gaps	Behavior is correct but not explained	Update documentation

Versioning Strategy:

Plan for evolution from the start. A clear versioning strategy enables improvement while maintaining stability:

VersioningStrategy.ts
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// Semantic versioning for APIs: MAJOR.MINOR.PATCH
 
// PATCH (v1.0.1): Bug fixes only
// - Behavior changes only to match documented contract
// - No new features, no breaking changes
// - Consumers update without code changes
 
// MINOR (v1.1.0): Backward-compatible additions
// - New operations, new optional parameters
// - New fields in responses (additive only)
// - Consumers update without code changes
// - New features require updates to use
 
// MAJOR (v2.0.0): Breaking changes
// - Changed signatures, removed operations
// - Changed response structures
// - Consumers must update code
// - Provide migration guide and overlap period
 
// Change types and their version impact:
// | Change                           | Version Impact |
// |----------------------------------|----------------|
// | Fix bug in implementation        | PATCH          |
// | Add new endpoint                 | MINOR          |
// | Add optional request parameter   | MINOR          |
// | Add new response field           | MINOR          |
// | Change request parameter type    | MAJOR          |
// | Remove response field            | MAJOR          |
// | Change error code for scenario   | MAJOR          |
// | Rename endpoint                  | MAJOR          |
 
// Deprecation process:
// 1. Mark as deprecated in v1.x (add @deprecated annotation)
// 2. Document migration path
// 3. Continue supporting for defined period (e.g., 6 months)
// 4. Remove in v2.0

The Feedback Loop

Each feedback cycle improves your understanding of consumer needs. Over time, you develop intuition for what makes APIs successful. This knowledge informs future designs, reducing iteration and increasing quality. The process gets easier with experience.

Summary: API Design Process

We've presented a complete, systematic API design process that transforms principles into practice. Following this process ensures you consider all important aspects and produce APIs that truly serve consumers.

Let's consolidate the key insights:

Key Takeaways

•Follow a structured lifecycle: Requirements → Modeling → Definition → Review → Implementation → Documentation → Evolution.
•Front-load design effort: Changes are cheap before implementation, expensive after release. Invest in early phases.
•Start with consumer needs: Gather requirements through use cases, prioritize by impact, and validate with real consumers.
•Model before defining: Identify resources, operations, and relationships at an abstract level before committing to signatures.
•Define contracts explicitly: Document preconditions, postconditions, errors, and side effects for every operation.
•Review rigorously: Use checklists, walkthroughs, peer review, and consumer validation before implementation.
•Implement to specification: The design is the truth. Don't let implementation convenience corrupt the API surface.
•Document as a product: Layer documentation from quick start to deep concepts. Make examples excellent.
•Embrace evolution: Gather feedback systematically, categorize it appropriately, and plan versions thoughtfully.

Module Complete:

You've now completed Module 1: What Is a Good API? You understand APIs as contracts, the usability principles that make APIs intuitive, the importance of consistency, and a systematic process for designing excellent APIs.

These foundations will serve you throughout the remainder of this chapter as we dive into specific aspects of API design: method signatures, error handling, versioning, REST principles, DTOs, and documentation.

Module Complete

Congratulations! You've completed the foundational module on API design principles. You now have a comprehensive framework for thinking about and creating excellent APIs—the contract mindset, usability principles, consistency requirements, and a systematic design process. Apply these principles to create APIs that developers love to use.