Design Patterns

Some common use cases

Creational

Singleton

• DB connection pools - thread safety, max connections
• App config - env vars, feature flags, settings
• System logger - centralized logging, rotation
• Hardware interface - serial port, USB access
• Cache manager - in-memory LRU, size limits

Factory

• DB drivers - MySQL/PostgreSQL instances, configs
• Payment services - merchant credentials, API keys
• Document parsers - format-specific validation
• Auth providers - OAuth clients, scopes, endpoints
• UI components - platform-specific widgets, styling

Builder

• HTTP client - timeout, retries, headers, middleware
• Server config - ports, SSL, rate limits, CORS
• Report generator - headers, sections, charts
• Email composer - templates, attachments, tracking
• Query builder - SQL joins, conditions, sorting

Prototype

• Doc templates - invoice/contract cloning, fields
• Game objects - enemy/item spawning, modifiers
• Test data - sample users/orders generation
• UI components - widget cloning, state preservation
• Env configs - dev/staging/prod settings, overrides

Structural

Adapter

• API compatibility: SOAP→REST, XML-RPC→gRPC, old→new
• DB abstraction: SQL/NoSQL ops, query translation
• Payment gateways: Stripe/PayPal/Square unified interface
• Storage: S3/GCS/Azure common operations
• Auth: OAuth/SAML/JWT standardization

Decorator

• HTTP middleware - compression, rate limiting, auth, metrics
• Stream processing - encryption, buffering, checksums
• Data validation - schema checking, sanitization, transformation
• UI components - theming, accessibility, analytics tracking
• Database queries - caching, retry logic, connection pooling

Facade

• Payment processing - abstract order/payment/shipping/inventory
• ML model deployment - simplify training/validation/inference
• Multi-API operations - combine auth/user/billing services
• DevOps workflows - unite build/test/deploy/monitor
• Media processing - abstract encode/transform/store/serve

Composite

• Document object model - HTML/XML tree structures
• UI component trees - nested containers/layouts
• File system operations - recursive directory handling
• Expression trees - AST for query/rule engines
• Graphics scenes - hierarchical transformations

Proxy

• Image loading - lazy load high-res versions
• API gateway - rate limiting, caching, auth
• Database - connection pooling, query monitoring
• Virtual lists - paginated data loading
• Service mesh - traffic management, security

Flyweight

• Text rendering - shared font/style objects
• Game assets - pooled particles/sprites
• Data tables - shared cell formats
• Map tiles - cached terrain chunks
• UI themes - shared style definitions

Behavioral

Observer

• Event tracking system - analytics, user behavior monitoring
• Trading platform - price alerts, portfolio updates
• IoT sensor network - temperature/humidity monitoring
• CI/CD pipeline - build status notifications
• Cache invalidation - distributed cache updates

Strategy

• Image compression - JPEG/PNG/WebP with quality settings
• Sort algorithms - quicksort/mergesort based on data size
• Tax calculation - country-specific tax rules
• Path finding - A*/Dijkstra based on graph properties
• Data storage - in-memory/disk/remote based on size

Command

• Video editor - cut/trim/merge operations
• Drawing app - brush strokes, shape creation
• Shell commands - command history, script execution
• Database transactions - atomic operations
• GUI actions - keyboard shortcuts, menu items

Template Method

• API response handling - parse/validate/transform/cache
• PDF generation - header/content/footer/metadata
• Data import - validate/clean/transform/load
• Unit test setup - prepare/execute/verify/cleanup
• Email processing - scan/filter/categorize/store

Mediator

• Smart home system - device coordination
• Trading platform - order matching engine
• Form validation - cross-field dependencies
• Multiplayer game - player interaction/combat
• Workflow engine - task dependencies

Memento

• Code editor - snapshot/restore text state
• Image editor - operation history
• Shopping cart - session state management
• Browser history - navigation state
• Game engine - save/load system

Chain of Responsibility

• Input validation - type/format/business rules
• Image processing - resize/filter/compress/optimize
• Payment processing - fraud/balance/limit checks
• Access control - authentication/role/permission
• Request handling - rate limit/cache/auth/route

State

• Media player - play/pause/stop/seeking
• Job scheduler - pending/running/completed/failed
• User session - guest/authenticated/admin
• File transfer - init/transfer/verify/complete
• Order system - created/paid/shipped/delivered

Visitor

• Code formatter - different language styles
• JSON/XML parser - custom node processing
• Compiler - AST optimization passes
• Report generator - HTML/PDF/CSV export
• File system - size/permission scanning

Interpreter

• Search query parser - elasticsearch/solr syntax
• Template engine - variable/loop/conditional
• Math expression parser - arithmetic operations
• Regex engine - pattern matching rules
• Shell script interpreter - command execution