Task 1.13: Dependency Injection

Summary

Implement proper dependency injection pattern for buylater.email to improve testability, maintainability, and separation of concerns by cleanly managing application dependencies.

Motivation

Following Let’s Go Chapter 3.3, we need to formalize our dependency injection approach to make the application more modular, testable, and maintainable. This builds on our application struct pattern to properly inject dependencies throughout the system.

Acceptance Criteria

Refactor application struct to be the central dependency container
Implement dependency injection for all handlers
Create interfaces for external dependencies (future database, email service)
Update handler methods to receive dependencies through application struct
Remove global variables and direct dependency access
Implement clean dependency initialization in main.go
Prepare structure for future database and email service injection
Add dependency validation and initialization checks
Document dependency injection patterns used

Technical Requirements

Application Structure

type application struct {
    config config
    logger struct {
        info  *log.Logger
        error *log.Logger
    }
    // Future dependencies will be added here:
    // db     database.DB
    // mailer mailer.Service
}

Implementation Details

Make application struct the single source of dependencies
Pass application instance to all handlers that need dependencies
Create initialization functions for complex dependencies
Implement clean separation between dependency creation and usage
Add validation for required dependencies
Prepare interfaces for future external services

Dependencies

Task 1.12 completed (structured logging system)

Testing Strategy

Unit Tests

Test dependency initialization and validation
Verify proper dependency injection throughout handlers
Test application struct methods

Integration Tests

All handlers work with injected dependencies
Dependency initialization is robust
Application starts correctly with all dependencies

Manual Testing

Application starts without dependency errors
All handlers function correctly with injected dependencies
Logging works through injected loggers
Configuration is properly available throughout app

Definition of Done

All acceptance criteria met
All specified tests pass
Code follows project conventions (go fmt, go vet)
Clean dependency injection pattern implemented
No global variables or direct dependency access
Dependencies are properly initialized and validated
Human verification completed successfully
Git commit created with proper message format

Implementation Notes

Approach

Formalize the dependency injection pattern started in Task 1.10 by making the application struct a proper dependency container and ensuring all dependencies flow cleanly through the application.

Key Files to Modify

cmd/web/main.go - Dependency initialization and injection
cmd/web/handlers.go - Update to use injected dependencies
Future: Add interfaces for external services (database, email)

Dependency Injection Patterns

Constructor Pattern: Initialize dependencies in main()
Method Injection: Pass application struct to handler methods
Interface Segregation: Define interfaces for external services
Dependency Container: Application struct holds all dependencies

Current Dependencies to Inject

Configuration settings
Structured loggers (info, error)
Future: Database connection, email service, template cache

Potential Risks

Over-engineering simple dependency needs
Circular dependency issues
Complex initialization order requirements

Success Metrics

Clean, maintainable dependency injection system that makes buylater.email more testable and modular while preparing for future service integrations.

Blocks: Database integration, email service, testing improvements
Blocked by: Task 3.2 (needs structured logging)
Related: Testing, modularity, and future service integrations

Implementation Log

Implementation details will be recorded here during development.

Final Verification

Human Tester: [Name]
Date Completed: [YYYY-MM-DD]
Verification Result: [Pass/Fail]
Notes: [Any issues found or additional observations]

task_id: “1.13” title: “Dependency Injection” status: “pending” priority: “high” estimated_effort: “medium” created: “2025-07-24” assigned_to: “pair” related_rfds: “RFD 004”