# Evolution Document: Plugin Architecture

**Project ID**: plugin-architecture
**Completed**: 2026-02-03
**Methodology**: nWave ATDD with Outside-In TDD
**Complexity**: Major Architectural Refactoring
**Version**: 1.7.0

---

## Executive Summary

Successfully transformed the nWave installation system from a monolithic hardcoded approach to a modular plugin-based architecture. This enables addition of new features (like DES) without modifying core installer code, supports selective installation/uninstallation, and provides automatic dependency resolution via topological sort.

**Business Impact**:
- **Modular installation** - plugins can be installed/uninstalled independently
- **Extensibility** - new features added without modifying core installer
- **Reliability** - rollback mechanism protects against installation failures
- **Test coverage**: 87.85% (>80% threshold)
- **Mutation testing**: 100% kill rate

---

## Feature Achievement Summary

### Problem Solved

**Before**: Monolithic `install_nwave.py`:
- Hardcoded installation logic for all components
- DES tightly coupled to core installer
- Cannot install/uninstall components independently
- Cannot add new components without modifying installer
- No dependency resolution between components

**After**: Plugin-based architecture:
- 5 independent plugins: agents, commands, templates, utilities, des
- Automatic dependency resolution via Kahn's topological sort
- Selective installation via `--exclude` flag
- Selective uninstallation via `--uninstall` flag
- Automatic rollback on plugin failure
- Behavioral equivalence with monolithic installer validated

### Quality Metrics

| Metric | Target | Achieved | Status |
|--------|--------|----------|--------|
| Test Coverage | >80% | 87.85% | PASS |
| Mutation Kill Rate | 80% | 100% | PASS |
| Steps Completed | 17 | 17 | PASS |
| Quality Gates | 6 | 6 | PASS |

### Technical Achievements

**Core Infrastructure**:
- `base.py`: InstallationPlugin abstract base class with install/verify interface
- `registry.py`: PluginRegistry with topological sort dependency resolution
- `InstallContext`: Dataclass for shared state across plugins

**Plugin Implementations**:
1. **AgentsPlugin** (priority 10) - copies agent files to ~/.claude/agents/nw/
2. **CommandsPlugin** (priority 20) - copies command files to ~/.claude/commands/nw/
3. **TemplatesPlugin** (priority 30) - copies template files to ~/.claude/templates/
4. **UtilitiesPlugin** (priority 40) - copies utility scripts to ~/.claude/scripts/
5. **DESPlugin** (priority 50, depends on templates + utilities) - installs DES module, scripts, templates

**Key Features**:
- Circular dependency detection with descriptive error messages
- Fallback verification logic when InstallationVerifier unavailable
- Rollback mechanism using BackupManager on plugin failure
- Behavioral equivalence validation ensuring identical results to baseline

---

## Implementation Journey

### 17-Step Outside-In TDD Approach

#### Phase 01: Walking Skeleton + Wrapper Plugins

| Step | Component | Outcome | Commit |
|------|-----------|---------|--------|
| 01-01 | Walking Skeleton - AgentsPlugin E2E | PASS | cc48737 |
| 01-02 | CommandsPlugin Wrapper | PASS | 794b05c |
| 01-03 | TemplatesPlugin Wrapper | PASS | 87d794c |
| 01-04 | UtilitiesPlugin Wrapper | PASS | 5715d58 |
| 01-05 | Multi-Plugin Dependency Resolution | PASS | bfa111e |

#### Phase 02: Switchover to Plugin System

| Step | Component | Outcome | Commit |
|------|-----------|---------|--------|
| 02-01 | Modify install_framework() | PASS | 9118936 |
| 02-02 | Behavioral Equivalence Validation | PASS | 4d0cc84 |
| 02-03 | Rollback Mechanism | PASS | 60860ec |

#### Phase 03: DES Plugin Integration

| Step | Component | Outcome | Commit |
|------|-----------|---------|--------|
| 03-01 | DES Prerequisites | PASS | 10d173c |
| 03-02 | DESPlugin Implementation | PASS | 452e2af |
| 03-03 | DES Import Validation | PASS | 71fc750 |
| 03-04 | Graceful Failure Handling | PASS | 3243035 |

#### Phase 04: Testing and Documentation

| Step | Component | Outcome | Commit |
|------|-----------|---------|--------|
| 04-01 | Test Coverage >80% | PASS | 6ce2756 |
| 04-02 | Selective Install/Uninstall | PASS | cbd6c0b |
| 04-03 | Upgrade Scenario Testing | PASS | 50e0118 |

#### Phase 05: Deployment

| Step | Component | Outcome | Commit |
|------|-----------|---------|--------|
| 05-01 | Version Bump to 1.7.0 | PASS | 48d38fc |
| 05-02 | E2E User Journey | PASS | 032cde5 |

**Total Commits**: 17

---

## Key Architectural Decisions

### Decision 1: Circular Import Prevention

**Resolution**: Extract module-level functions from install_nwave.py
- Class methods become thin wrappers calling extracted functions
- Plugins import functions, not classes, avoiding circular dependencies
- Testable in isolation without class instantiation

### Decision 2: Plugin Verification Strategy

**Resolution**: Fallback verification pattern
- Primary: Use InstallationVerifier if available
- Fallback: Minimal file existence check if verifier unavailable
- Each plugin has independent verification logic

### Decision 3: Dependency Resolution

**Resolution**: Kahn's topological sort algorithm
- Builds dependency graph from plugin declarations
- Calculates in-degree for each plugin
- Processes plugins with zero in-degree first
- Detects circular dependencies with clear error messages

### Decision 4: Version Strategy

**Resolution**: Incremental semantic versioning
- 1.2.0 (monolithic) -> 1.7.0 (plugin-based production release)
- Marketing version jump to 1.7.0 for clarity

---

## Key Deliverables

### Plugin Infrastructure

| File | Purpose |
|------|---------|
| scripts/install/plugins/__init__.py | Package initialization |
| scripts/install/plugins/base.py | InstallationPlugin ABC, InstallContext, PluginResult |
| scripts/install/plugins/registry.py | PluginRegistry with dependency resolution |

### Plugin Implementations

| File | Plugin | Priority | Dependencies |
|------|--------|----------|--------------|
| scripts/install/plugins/agents_plugin.py | agents | 10 | none |
| scripts/install/plugins/commands_plugin.py | commands | 20 | none |
| scripts/install/plugins/templates_plugin.py | templates | 30 | none |
| scripts/install/plugins/utilities_plugin.py | utilities | 40 | templates |
| scripts/install/plugins/des_plugin.py | des | 50 | templates, utilities |

### Test Suite

| Directory | Coverage |
|-----------|----------|
| tests/nwave/plugin-architecture/unit/ | Unit tests for all plugins |
| tests/nwave/plugin-architecture/integration/ | Behavioral equivalence, upgrade scenarios |
| tests/nwave/plugin-architecture/acceptance/ | E2E acceptance tests |
| tests/nwave/plugin-architecture/e2e/ | User journey validation |

---

## Usage Examples

```bash
# Full installation (all 5 plugins)
python install_nwave.py

# Install core only (exclude DES)
python install_nwave.py --exclude des

# Add DES later
python install_nwave.py --plugin des

# Uninstall DES
python install_nwave.py --uninstall des

# Dry-run preview
python install_nwave.py --dry-run
```

---

## Artifacts Archived

### Feature Documentation

- docs/feature/plugin-architecture/roadmap.yaml - Implementation roadmap (17 steps, 5 phases)
- docs/feature/plugin-architecture/architecture-decisions.md - Gap analysis and decisions
- docs/feature/plugin-architecture/execution-log.yaml - Step completion tracking
- docs/feature/plugin-architecture/mutation/mutation-report.md - Mutation testing results

### Implementation Files

- scripts/install/plugins/ - All plugin implementations (8 files)
- scripts/install/install_nwave.py - Modified to use PluginRegistry

### Test Files

- tests/nwave/plugin-architecture/ - Complete test suite

---

## Lessons Learned

### What Worked Well

1. **Walking Skeleton First**: Implementing AgentsPlugin E2E before other plugins proved the architecture worked before investing in all wrappers

2. **Behavioral Equivalence Validation**: Explicit test comparing baseline vs plugin installation caught integration issues early

3. **Explicit Prerequisites**: Creating DES scripts and templates BEFORE DESPlugin implementation avoided blocking issues

### Process Improvements

1. **Version Strategy**: Early decision on incremental versioning avoided confusion during deployment phase

2. **Rollback Planning**: Documenting rollback procedure before switchover provided safety net for critical changes

3. **Dependency Resolution Testing**: Comprehensive tests for topological sort caught edge cases in circular dependency detection

---

## Quality Gates Passed

| Gate | Phase | Criteria | Status |
|------|-------|----------|--------|
| Walking Skeleton Complete | 01-01 | AgentsPlugin E2E test passes | PASS |
| All Wrapper Plugins Functional | 01-05 | All 4 plugins install via registry | PASS |
| Switchover Behavioral Equivalence | 02-02 | Plugin installation identical to baseline | PASS |
| DES Plugin Operational | 03-03 | DES module importable, scripts executable | PASS |
| Test Coverage Threshold | 04-01 | >80% coverage for plugin system | PASS (87.85%) |
| Production Ready | 05-02 | E2E user journey completes | PASS |

---

## Next Steps (Future Enhancements)

1. **Dynamic Plugin Discovery** - Auto-discover plugins without explicit registration
2. **Plugin Versioning** - Independent version tracking per plugin
3. **Third-Party Plugin Support** - External plugin installation from registry
4. **Additional Plugins** - Mutation testing, VS Code integration, GitHub Actions

---

---

## Retrospective

### Clean Execution

All 17 implementation steps completed without issues:
- **Phase failures**: 0
- **Skipped phases**: 0
- **Mutation testing warnings**: 0
- **Manual interventions required**: 0
- **Review rejections**: 0

The DELIVER wave executed cleanly with all quality gates passing on the first attempt.

### Key Success Factors

1. **Prior DISTILL wave**: Well-defined acceptance criteria from distill phase provided clear implementation targets
2. **Walking skeleton approach**: Proving architecture first (AgentsPlugin E2E) validated the design early
3. **Incremental complexity**: Moving from simple wrappers to complex DES integration minimized integration risk
4. **Comprehensive test coverage**: 87.85% coverage and 100% mutation kill rate ensured high-quality implementation

---

**Document Status**: COMPLETE
**Feature Status**: PRODUCTION READY
**Version Released**: 1.7.0
