Design Document

This template defines the format for design documents produced during architecture and planning sessions. See Complex Project Planning for the workflow that produces this document.

A design document captures the technical architecture of a feature or component — structural diagrams, behavioral specifications, decision traceability, and testing strategy. It is a living document updated iteratively as design decisions are made and recorded in the accompanying decision log.

When to Create a Design Document

Create a design document when:

A new component, module, or significant feature is being designed.
The implementation spans multiple classes, files, or repositories.
You are following the Complex Project Planning workflow — a design document is mandatory in that case.

For simpler changes where the specification itself describes the implementation, specification.md per the Project Planning Workflow is sufficient.

Template

---
author: <!-- REPLACE: your name -->
title: "Design: <!-- REPLACE: feature or component name -->"
created: <!-- REPLACE: YYYY-MM-DD -->
---

# Design: <!-- REPLACE: Feature/Component Name -->

## Overview

<!-- REPLACE: 2-3 paragraphs describing the purpose, scope, and key design choices.
Reference the decision log for detailed rationale rather than repeating it here. -->

## Decision Summary

| # | Decision | Chosen Option |
|---|----------|---------------|
| DQ-001 | <!-- REPLACE: question summary --> | <!-- REPLACE: option chosen --> |
| DQ-002 | | |

> Full rationale in [Decision Log](./decision-log.md).

---

## Structural Design

### Class Diagram

```plantuml
@startuml
<!-- REPLACE: class diagram source -->
@enduml
```

<!-- REPLACE: Brief narrative explaining the key classes, their responsibilities,
and relationships. Reference DQ numbers for design choices that shaped the structure.
Example: "Per DQ-002, the repository layer is separated from the service layer to
enable independent testing." -->

### Key Classes and Interfaces

<!-- For each significant class or interface, add a subsection: -->

#### `<!-- REPLACE: ClassName -->`

- **Package**: `<!-- REPLACE: fully.qualified.package -->`
- **Responsibility**: <!-- REPLACE: single sentence -->
- **Key fields/properties**: <!-- REPLACE: list -->
- **Key methods**: <!-- REPLACE: list with signatures -->
- **Design decision**: <!-- REPLACE: DQ-00N -->

---

## Behavioral Design

### Sequence Diagrams

<!-- One sequence diagram per key interaction flow. -->

```plantuml
@startuml
hide footbox
title <!-- REPLACE: Flow Name -->
<!-- REPLACE: sequence diagram source -->
@enduml
```

<!-- REPLACE: Narrative explaining the flow, entry points, and error paths.
Lead with the happy path; note error paths in the narrative. -->

### API Contract

<!-- Include this section only if the design includes a REST endpoint or external interface. -->

- **Method**: `<!-- REPLACE: GET / POST / PUT / DELETE -->`
- **Path**: `<!-- REPLACE: /path -->`
- **Authentication**: <!-- REPLACE: mechanism (e.g., Bearer token, API key) -->
- **Request schema**: <!-- REPLACE: inline schema or reference to a schema file -->
- **Response schema**: <!-- REPLACE: inline schema or reference -->
- **Error responses**:
  - `400` — <!-- REPLACE: condition -->
  - `404` — <!-- REPLACE: condition -->
  - `500` — <!-- REPLACE: condition -->

---

## Implementation Scope

### Files to Create

| File | Package/Path | Purpose |
|------|-------------|---------|
| `<!-- REPLACE: FileName -->` | `<!-- REPLACE: package or path -->` | <!-- REPLACE: purpose --> |

### Files to Modify

| File | Change Description |
|------|-------------------|
| `<!-- REPLACE: existing file -->` | <!-- REPLACE: what changes and why --> |

### Out of Scope

<!-- REPLACE: Explicitly list what this design does NOT cover. This prevents scope
creep during implementation and sets clear boundaries.
Example:
- Authentication and authorization (handled by the existing security layer)
- Pagination (out of scope for v1; tracked in DQ-007 as Deferred)
- Bulk mutation endpoints (separate feature) -->

---

## Testing Strategy

### Unit Tests

| Test | Target | Validates |
|------|--------|-----------|
| `<!-- REPLACE: test description -->` | `<!-- REPLACE: class/function -->` | <!-- REPLACE: what behavior --> |

### Integration Tests

| Test | Setup | Validates |
|------|-------|-----------|
| `<!-- REPLACE: test description -->` | <!-- REPLACE: dependencies/fixtures --> | <!-- REPLACE: end-to-end behavior --> |

### API Tests

<!-- Include only if the design includes a REST endpoint. -->

| Test | Method | Path | Expected |
|------|--------|------|----------|
| `<!-- REPLACE: test description -->` | `GET` | `<!-- REPLACE: /path -->` | <!-- REPLACE: response/status --> |

---

## References

- [Decision Log](./decision-log.md)
- [Project Plan](./project-plan.md)
- <!-- REPLACE: Additional references to existing code, documentation, or specs -->

---

__________
Copyright: (c) Arda Systems 2025-2026, All rights reserved

Conventions

Diagrams

Use PlantUML for class diagrams and sequence diagrams. Embed diagrams directly in the document using fenced plantuml code blocks — see the PlantUML Guide for syntax, conventions, and common pitfalls.

Sequence diagrams should focus on the happy path first, with error paths described in the accompanying narrative rather than in the diagram.

Decision Traceability

Every significant design choice must reference a DQ number from the Decision Log template.
The Decision Summary table at the top provides a quick reference to all decisions affecting this design.
Inline references in section text (e.g., “Per DQ-003, we use…”) provide contextual traceability within the document.

This traceability makes it possible to answer “why does the design look this way?” long after the design session has ended.

Iterative Updates

Design documents are updated after each decision log round:

Reviewers make decisions in the decision log.
The design document is updated to reflect each decision (diagrams, class descriptions, scope).
The Decision Summary table is updated.
The updated document is reviewed for new ambiguities, which are fed back into the decision log as a new round.

Scope Management

The Out of Scope section is mandatory. It prevents scope creep during implementation by making exclusions explicit. An implementing engineer who hits an ambiguous case should be able to consult this section to determine whether the case is in or out of scope — rather than guessing or expanding the work unilaterally.

Integration with Other Documents

Document	Relationship
`decision-log.md`	Design choices reference DQ numbers; the Decision Summary table mirrors the Decision Log’s resolution
`project-plan.md`	The design document provides the architectural context; the project plan references design sections when describing tasks
`specification.md`	For simpler changes, the specification subsumes the design document; for complex work, the design document is separate