[ADR] Define API Evolution Strategy for Long-term Compatibility

[groldan]

Decision Title

Define API Evolution Strategy for Long-term Compatibility

Status

• Proposed (under consideration)
• Accepted (decision made)
• Superseded (replaced by newer decision)
• Deprecated (no longer relevant)

Context

As the library evolves from version 1.0 through future releases, we need a clear strategy for API evolution that balances innovation with backward compatibility. Without a defined approach, API changes could break existing integrations and reduce confidence in the library's stability.

Decision

Implement a comprehensive API evolution strategy using semantic versioning, deprecation cycles, and compatibility layers to ensure predictable API evolution while enabling innovation and improvement.

Problem Statement

Current challenges in API evolution planning:

• No formal process for introducing breaking changes
• Uncertainty about backward compatibility guarantees
• Lack of clear migration paths for API improvements
• Difficulty balancing innovation with stability
• No guidelines for deprecation and removal timelines

Considered Options

Option 1: Strict Backward Compatibility

Description: Never break backward compatibility, only add new APIs

Pros:

• Maximum stability for existing users
• No migration effort required
• Predictable upgrade path

Cons:

• Accumulation of technical debt
• Inability to fix design mistakes
• API bloat from deprecated methods
• Limited innovation capability

Option 2: Frequent Breaking Changes

Description: Prioritize optimal API design over backward compatibility

Pros:

• Clean, optimal API design
• Rapid innovation and improvement
• No technical debt accumulation

Cons:

• High migration costs for users
• Reduced adoption due to instability
• Fragmented ecosystem versions
• Poor enterprise suitability

Option 3: Managed Evolution with Deprecation Cycles

Description: Balanced approach with planned deprecation and migration support

Pros:

• Balance between stability and innovation
• Clear migration paths and timelines
• Predictable evolution process
• Enterprise-friendly stability

Cons:

• Complexity in managing multiple API versions
• Development overhead for compatibility layers
• Longer development cycles

Decision Rationale

Option 3 (Managed Evolution with Deprecation Cycles) is selected because:

• Provides predictable API evolution for enterprise users
• Enables innovation while maintaining reasonable stability
• Offers clear migration paths and support for API changes
• Aligns with industry best practices for library evolution
• Balances developer productivity with user stability needs

Architecture Impact

Affected Components

• Core API design and interfaces
• All provider implementations
• Configuration system evolution
• Error handling and exception hierarchy
• Documentation and migration guides
• Build system versioning
• Testing and compatibility validation

Breaking Changes

• No breaking changes
• Minor breaking changes (patch release)
• Major breaking changes (major version bump)

Performance Impact

• Performance improvement expected
• Performance neutral
• May degrade performance (requires optimization)
• Unknown performance impact (requires analysis)

Security Impact

• Improves security
• Security neutral
• Potential security implications (requires review)

Implementation Plan

Phase 1: API Stability Framework

• Define API stability annotations and contracts
• Establish semantic versioning guidelines
• Create deprecation policy and procedures
• Implement compatibility testing infrastructure

Phase 2: Evolution Tooling

• Develop API compatibility checking tools
• Create migration guide generation automation
• Implement deprecated API tracking and warnings
• Add version compatibility validation

Phase 3: Documentation and Process

• Create API evolution documentation
• Establish change review and approval process
• Implement community feedback mechanisms
• Create long-term API roadmap

Phase 4: Monitoring and Maintenance

• Monitor deprecated API usage in the wild
• Track migration progress and user feedback
• Refine evolution process based on experience
• Plan and execute API cleanup cycles

Dependencies

• Semantic versioning infrastructure
• API compatibility testing tools
• Documentation generation and maintenance
• Community feedback and communication channels

Effort Estimate

• Large (1-3 months)

Quality Attributes Impact

Performance

Impact: Neutral
Details: Evolution strategy focuses on compatibility, not performance optimization

Reliability

Impact: Positive
Details: Predictable evolution increases confidence and reduces integration risks

Security

Impact: Neutral
Details: No direct security implications from evolution strategy

Maintainability

Impact: Positive
Details: Clear evolution guidelines simplify long-term maintenance decisions

Usability

Impact: Positive
Details: Predictable evolution with migration support improves developer experience

Portability

Impact: Positive
Details: Compatibility layers ensure smooth transitions across versions

Consequences

Positive Consequences

• Predictable API evolution timeline for enterprise planning
• Clear migration paths reducing upgrade friction
• Maintained ecosystem stability while enabling innovation
• Industry-standard practices increasing library credibility
• Long-term sustainability of API design decisions

Negative Consequences

• Additional development overhead for compatibility maintenance
• Longer development cycles for major API improvements
• Complexity in managing multiple API versions simultaneously
• Potential API bloat during transition periods

Risks

• Risk: Compatibility layers introduce performance overhead

  • Impact: Low
  • Probability: Medium
  • Mitigation: Performance testing, efficient compatibility implementations

• Risk: Deprecated APIs remain in use longer than planned

  • Impact: Medium
  • Probability: High
  • Mitigation: Usage analytics, community engagement, clear incentives

Validation & Verification

How will this decision be validated?

• Community feedback
• Expert review
• Proof of concept implementation
• Performance benchmarks
• Production testing

Success Criteria

1. 95% of users successfully migrate during deprecation cycles
2. API evolution decisions approved by architectural review board
3. Compatibility testing prevents accidental breaking changes
4. Community satisfaction with evolution process >4.0/5

Rollback Plan

If evolution strategy proves problematic:

1. Revert to strict backward compatibility for critical APIs
2. Accelerate deprecation cycles for non-critical APIs
3. Provide comprehensive migration tooling and support

Documentation Requirements

• Architecture Decision Record (ADR-016)
• API documentation update
• Architecture documentation update
• Migration guide (templates and examples)
• Developer guide updates

Stakeholder Impact

Library Users

Impact: Positive - Predictable evolution with clear migration support

Library Contributors

Impact: Neutral - Additional process overhead but clearer development guidelines

Ecosystem Integration

Impact: Positive - Improved stability for framework and tooling integration

Related Decisions

• Connected to configuration management for settings evolution
• Related to SPI provider discovery for provider API evolution
• Dependencies on usability requirements for migration experience

References

• Semantic Versioning Specification
• Eclipse API Evolution Guidelines
• Google API Design Guide

Timeline

Target Decision Date: Q1 2025
Target Implementation Date: Q1 2025
Target Release: Version 1.0 (establish framework)

Additional Context

API Stability Annotations

/**
 * Marks APIs with their stability level and evolution guarantees
 */
@Target({ElementType.TYPE, ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface ApiStability {
    Level value();
    
    enum Level {
        /**
         * Stable API with backward compatibility guarantee.
         * Changes follow semantic versioning rules.
         */
        STABLE,
        
        /**
         * Evolving API that may change in minor versions.
         * Deprecated with migration path before removal.
         */
        EVOLVING,
        
        /**
         * Experimental API that may change frequently.
         * No backward compatibility guarantee.
         */
        EXPERIMENTAL,
        
        /**
         * Deprecated API scheduled for removal.
         * Migration path provided.
         */
        @Deprecated
        DEPRECATED
    }
}

// Usage examples
@ApiStability(STABLE)
public interface RangeReader {
    @ApiStability(STABLE)
    ByteBuffer read(long start, int length) throws IOException;
    
    @ApiStability(EVOLVING)
    CompletableFuture<ByteBuffer> readAsync(long start, int length);
    
    @ApiStability(EXPERIMENTAL)
    void prefetchHint(long start, int length);
    
    @ApiStability(DEPRECATED)
    @Deprecated(since = "1.1", forRemoval = true)
    byte[] readBytes(long start, int length);
}

Semantic Versioning Guidelines

Version Format: MAJOR.MINOR.PATCH

MAJOR version increment:
- Breaking changes to stable APIs
- Removal of deprecated APIs after minimum 6-month deprecation period
- Architectural changes requiring migration

MINOR version increment:
- New stable APIs
- New evolving APIs
- Deprecation of existing APIs (with migration path)
- Non-breaking enhancements

PATCH version increment:
- Bug fixes without API changes
- Security updates
- Documentation improvements
- Internal optimizations

Deprecation Process

// Step 1: Mark as deprecated with replacement guidance
@Deprecated(since = "1.2", forRemoval = true)
@ApiStability(DEPRECATED)
public void oldMethod() {
    // Implementation calls new method
    newMethod();
    
    // Log deprecation warning (rate-limited)
    DeprecationLogger.logUsage("oldMethod", "1.2", "Use newMethod() instead");
}

// Step 2: Provide migration path
/**
 * @deprecated Use {@link #newMethod()} instead.
 * This method will be removed in version 2.0.
 * 
 * Migration example:
 * <pre>{@code
 * // Old code
 * reader.oldMethod();
 * 
 * // New code
 * reader.newMethod();
 * }</pre>
 */
@Deprecated(since = "1.2", forRemoval = true)
public void oldMethod() { ... }

// Step 3: Remove in next major version (after minimum 6 months)

Compatibility Testing Strategy

// Automated compatibility validation
@Test
void testApiCompatibility() {
    ApiCompatibilityChecker checker = new ApiCompatibilityChecker();
    
    // Load previous version API signature
    ApiSignature previousVersion = ApiSignature.load("1.0.0");
    ApiSignature currentVersion = ApiSignature.current();
    
    // Check for breaking changes
    CompatibilityReport report = checker.compare(previousVersion, currentVersion);
    
    // Ensure no breaking changes in minor/patch versions
    if (isMinorOrPatchVersion()) {
        assertThat(report.getBreakingChanges()).isEmpty();
    }
    
    // Ensure deprecated APIs have migration paths
    for (DeprecatedApi api : report.getDeprecatedApis()) {
        assertThat(api.getMigrationGuide()).isNotEmpty();
        assertThat(api.getRemovalVersion()).isAfter(getCurrentVersion().plus(6, MONTHS));
    }
}

Migration Support Infrastructure

// Automated migration detection and guidance
public class MigrationAssistant {
    public MigrationReport analyzeCode(Path projectPath) {
        CodeAnalyzer analyzer = new CodeAnalyzer();
        List<ApiUsage> usages = analyzer.findApiUsages(projectPath);
        
        MigrationReport report = new MigrationReport();
        for (ApiUsage usage : usages) {
            if (usage.isDeprecated()) {
                MigrationSuggestion suggestion = createMigrationSuggestion(usage);
                report.addSuggestion(suggestion);
            }
        }
        
        return report;
    }
    
    private MigrationSuggestion createMigrationSuggestion(ApiUsage usage) {
        return MigrationSuggestion.builder()
            .deprecatedApi(usage.getApiSignature())
            .replacementApi(usage.getReplacementSignature())
            .migrationExample(usage.getMigrationExample())
            .automatedRefactoring(usage.getRefactoringScript())
            .build();
    }
}

This architectural decision establishes a foundation for responsible API evolution that balances stability with innovation, providing clear guidelines for long-term library development and user adoption confidence.