modular-architecture-documentation-decision

# Decision (+6)

## Decision


Implement a modular architecture with three independent modules (Invoice Automation, Tax Preparation, Tax Filing) rather than a monolithic business automation system, enabling gradual feature adoption aligned with user needs.

## Context


Small engineering firms need flexibility to adopt automation features incrementally:
- Not all firms need all features simultaneously
- Budget constraints limit initial investment
- Different features have different priorities (invoice automation urgent, tax filing seasonal)
- Need to prove value before full system adoption

## Modular Approach


**Module Structure:**
1. **Invoice Automation:** `invoice-gen` - Immediate 80% time savings, highest ROI
2. **Tax Preparation:** `tax-prep` - Eliminate manual categorization, quarterly benefit
3. **Tax Filing:** `tax-file` - Annual compliance automation, seasonal use

**Shared Components:**
- Configuration management (YAML)
- PDF utilities (pypdf)
- Logging framework (loguru)
- Testing infrastructure (pytest)

*See sub-skills for full details.*

## Alternatives Considered


**1. Monolithic All-in-One System**
- Pros: Single unified interface, simpler initial architecture
- Cons: All-or-nothing adoption, $25K+ perceived investment, harder to justify ROI
- **Why Rejected:** Firms want proof-of-value before full investment

**2. Microservices**
- Pros: Maximum independence, cloud deployment ready
- Cons: Over-engineering for local Python scripts, network complexity
- **Why Rejected:** Unnecessary complexity for desktop automation


*See sub-skills for full details.*

## Consequences


**Positive:**
- **Gradual Adoption:** Firms adopt invoice automation first ($1,950/year value), then expand
- **Risk Mitigation:** Prove value module-by-module before full investment
- **Independent Development:** Each module can evolve independently
- **Clear Testing:** 80% coverage per module, 95% for critical invoice generation
- **Flexible Pricing:** Could charge per module if productized

**Negative:**
- **More Setup:** Three module directories vs. one
- **Shared Component Discipline:** Must minimize shared code to avoid coupling

*See sub-skills for full details.*

## Implementation


**pyproject.toml:**
```toml
[project.scripts]
invoice-gen = "aceengineer_admin.invoice:main"
tax-prep = "aceengineer_admin.tax_prep:main"
tax-file = "aceengineer_admin.tax_file:main"
```

**Directory Structure:**
```

*See sub-skills for full details.*

## References


- tech-stack.md: Modular Architecture section
- roadmap.md: Phase 1 (Invoice), Phase 2 (Tax Prep), Phase 3 (Filing)
- mission.md: Implementation Approach with phased rollout
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