Added AI skills
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Jose Selesan
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.agents/skills/clean-code-principles/rules/solid-isp-clients.md
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.agents/skills/clean-code-principles/rules/solid-isp-clients.md
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---
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id: solid-isp-clients
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title: SOLID - Interface Segregation (Client-Specific)
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category: solid-principles
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priority: critical
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tags: [SOLID, ISP, interface-segregation, client-design]
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related: [solid-isp-interfaces, solid-srp-class, solid-lsp-contracts]
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---
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# Interface Segregation Principle - Client-Specific Interfaces
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Clients should not be forced to depend on interfaces they do not use. Design interfaces from the client's perspective, not the implementation's.
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## Bad Example
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```typescript
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// Anti-pattern: Fat interface that forces clients to depend on methods they don't use
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interface UserService {
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// Authentication
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login(email: string, password: string): Promise<AuthToken>;
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logout(userId: string): Promise<void>;
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refreshToken(token: string): Promise<AuthToken>;
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validateToken(token: string): Promise<boolean>;
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// User management
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createUser(data: CreateUserData): Promise<User>;
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updateUser(id: string, data: UpdateUserData): Promise<User>;
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deleteUser(id: string): Promise<void>;
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getUser(id: string): Promise<User>;
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listUsers(filters: UserFilters): Promise<User[]>;
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// Profile
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updateProfile(userId: string, profile: ProfileData): Promise<Profile>;
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uploadAvatar(userId: string, image: Buffer): Promise<string>;
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getProfile(userId: string): Promise<Profile>;
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// Notifications
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sendNotification(userId: string, notification: Notification): Promise<void>;
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getNotificationPreferences(userId: string): Promise<NotificationPrefs>;
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updateNotificationPreferences(userId: string, prefs: NotificationPrefs): Promise<void>;
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// Analytics
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trackUserEvent(userId: string, event: AnalyticsEvent): Promise<void>;
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getUserAnalytics(userId: string): Promise<UserAnalytics>;
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}
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// Login page only needs authentication but must depend on entire interface
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class LoginPage {
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constructor(private userService: UserService) {} // Depends on 15+ methods!
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async handleLogin(email: string, password: string): Promise<void> {
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// Only uses 1 method
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const token = await this.userService.login(email, password);
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this.storeToken(token);
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}
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}
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// Profile component forced to depend on authentication and analytics
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class ProfileEditor {
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constructor(private userService: UserService) {} // Depends on 15+ methods!
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async saveProfile(userId: string, data: ProfileData): Promise<void> {
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// Only uses 2 methods
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await this.userService.updateProfile(userId, data);
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const profile = await this.userService.getProfile(userId);
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}
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}
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// Mock implementation nightmare for testing
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class MockUserService implements UserService {
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// Must implement ALL methods even for simple tests
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login = jest.fn();
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logout = jest.fn();
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refreshToken = jest.fn();
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validateToken = jest.fn();
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createUser = jest.fn();
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updateUser = jest.fn();
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deleteUser = jest.fn();
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getUser = jest.fn();
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listUsers = jest.fn();
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updateProfile = jest.fn();
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uploadAvatar = jest.fn();
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getProfile = jest.fn();
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sendNotification = jest.fn();
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getNotificationPreferences = jest.fn();
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updateNotificationPreferences = jest.fn();
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trackUserEvent = jest.fn();
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getUserAnalytics = jest.fn();
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}
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```
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## Good Example
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```typescript
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// Correct approach: Client-specific interfaces
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// Authentication client interface
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interface Authenticator {
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login(email: string, password: string): Promise<AuthToken>;
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logout(userId: string): Promise<void>;
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refreshToken(token: string): Promise<AuthToken>;
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validateToken(token: string): Promise<boolean>;
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}
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// User management client interface
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interface UserManager {
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createUser(data: CreateUserData): Promise<User>;
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updateUser(id: string, data: UpdateUserData): Promise<User>;
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deleteUser(id: string): Promise<void>;
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getUser(id: string): Promise<User>;
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listUsers(filters: UserFilters): Promise<User[]>;
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}
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// Profile client interface
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interface ProfileManager {
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updateProfile(userId: string, profile: ProfileData): Promise<Profile>;
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uploadAvatar(userId: string, image: Buffer): Promise<string>;
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getProfile(userId: string): Promise<Profile>;
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}
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// Notification client interface
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interface NotificationManager {
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sendNotification(userId: string, notification: Notification): Promise<void>;
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getNotificationPreferences(userId: string): Promise<NotificationPrefs>;
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updateNotificationPreferences(userId: string, prefs: NotificationPrefs): Promise<void>;
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}
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// Analytics client interface
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interface UserAnalyticsTracker {
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trackUserEvent(userId: string, event: AnalyticsEvent): Promise<void>;
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getUserAnalytics(userId: string): Promise<UserAnalytics>;
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}
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// Read-only user lookup for components that only need to fetch
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interface UserLookup {
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getUser(id: string): Promise<User>;
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getProfile(userId: string): Promise<Profile>;
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}
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// Login page depends only on what it needs
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class LoginPage {
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constructor(private auth: Authenticator) {} // Only 4 methods
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async handleLogin(email: string, password: string): Promise<void> {
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const token = await this.auth.login(email, password);
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this.storeToken(token);
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}
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}
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// Profile editor depends only on profile operations
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class ProfileEditor {
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constructor(private profileManager: ProfileManager) {} // Only 3 methods
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async saveProfile(userId: string, data: ProfileData): Promise<void> {
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await this.profileManager.updateProfile(userId, data);
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}
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async loadProfile(userId: string): Promise<Profile> {
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return this.profileManager.getProfile(userId);
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}
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}
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// User display component only needs read access
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class UserCard {
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constructor(private userLookup: UserLookup) {} // Only 2 methods
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async render(userId: string): Promise<void> {
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const user = await this.userLookup.getUser(userId);
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const profile = await this.userLookup.getProfile(userId);
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// Render user card
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}
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}
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// Admin panel needs user management
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class AdminUserPanel {
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constructor(private userManager: UserManager) {} // Only 5 methods
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async deleteUserAccount(id: string): Promise<void> {
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await this.userManager.deleteUser(id);
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}
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}
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// Implementation can still implement multiple interfaces
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class UserServiceImpl implements
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Authenticator,
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UserManager,
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ProfileManager,
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NotificationManager,
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UserAnalyticsTracker,
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UserLookup
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{
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// Implement all methods...
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async login(email: string, password: string): Promise<AuthToken> { /* ... */ }
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async logout(userId: string): Promise<void> { /* ... */ }
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// ... rest of implementation
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}
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// Testing is now simple - only mock what you need
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describe('LoginPage', () => {
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it('should login user', async () => {
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const mockAuth: Authenticator = {
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login: jest.fn().mockResolvedValue({ token: 'abc123' }),
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logout: jest.fn(),
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refreshToken: jest.fn(),
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validateToken: jest.fn()
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};
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const loginPage = new LoginPage(mockAuth);
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await loginPage.handleLogin('test@example.com', 'password');
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expect(mockAuth.login).toHaveBeenCalledWith('test@example.com', 'password');
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});
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});
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describe('ProfileEditor', () => {
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it('should save profile', async () => {
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const mockProfileManager: ProfileManager = {
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updateProfile: jest.fn().mockResolvedValue({}),
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uploadAvatar: jest.fn(),
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getProfile: jest.fn()
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};
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const editor = new ProfileEditor(mockProfileManager);
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await editor.saveProfile('user1', { name: 'John' });
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expect(mockProfileManager.updateProfile).toHaveBeenCalled();
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});
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});
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```
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## Why
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1. **Minimal Dependencies**: Each client depends only on the methods it actually uses, reducing coupling.
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2. **Easier Testing**: Mocks are small and focused. Testing `LoginPage` requires mocking 4 methods, not 15+.
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3. **Better Encapsulation**: Clients can't accidentally call methods they shouldn't have access to.
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4. **Clearer Intent**: Interface names describe what clients need: `Authenticator`, `ProfileManager`, `UserLookup`.
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5. **Independent Evolution**: Changes to analytics don't affect authentication clients. Interfaces can evolve separately.
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6. **Flexible Composition**: Different implementations can provide different subsets of functionality.
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7. **Single Responsibility**: Each interface has a focused responsibility, making the system easier to understand.
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