Architecture Design

Position in workflow: v0.5 Technical Stack Selection → v0.6 Architecture Design → v0.6 Technical Specification

Architecture defines how your system components connect. This skill transforms stack selections into a coherent system design with explicit boundaries and integration patterns.

Consumes

This skill requires prior work from v0.3-v0.5:

• TECH-* technology decisions (from v0.5 Technical Stack Selection) — Tech choices become components; Build items become internal services, Buy/Integrate items become external connections
• RISK-* risk entries (from v0.5 Risk Discovery Interview) — High-priority RISK-* entries must have architectural mitigations; drives design decisions on failover, security, scaling
• FEA-* feature entries (from v0.3 Features Value Planning) — Features determine what components must be built; complex features may require distributed patterns
• ARC-* existing architecture decisions (from prior products if brownfield) — Inherited patterns constrain new designs (monolith with microservice, library reuse, auth pattern, etc.)

This skill assumes v0.5 Technical Stack Selection is complete with TECH- entries providing technology foundations.

Produces

This skill creates/updates:

• ARC-* entries (architecture decisions, status-based) — Decisions for structure, integration, security, performance, data, DevOps with rationale, alternatives considered, and consequences. No confidence scores; decisions have Status: Proposed/Accepted/Superseded
• System boundary diagram — Visual representation showing trust boundaries, components, and external integrations
• RISK-to-Architecture mapping — Validation showing every high-priority RISK-* has corresponding ARC-* mitigation or explicit acceptance

All ARC- entries should include:

• Category: Structure/Integration/Security/Performance/Data/DevOps
• Context: What prompted this decision (derived from TECH-/RISK-/FEA- inputs)
• Decision: What was chosen
• Rationale: Why (not just what)
• Alternatives Rejected: Options considered and why not chosen
• Consequences: What this enables and constrains
• Related IDs: TECH-XXX, RISK-XXX, FEA-XXX references

Example ARC- entry (Structure):

ARC-001: Monolith with Module Boundaries
Category: Structure
Context: Team of 2 developers; unclear domain boundaries at v0.6; TECH-001 (Next.js) supports monolith pattern
Decision: Single Next.js application with domain-based module folders (auth/, reports/, data-sources/), clear module interfaces

Rationale:
  - TECH-001 (Next.js) designed for monoliths
  - Avoids ops complexity of microservices
  - Can extract services later when scaling needs emerge
  - Enables fast iteration for MVP

Alternatives Rejected:
  - Microservices: Premature (team too small); adds ops burden
  - Serverless: Harder to share code; cold start latency concerns (impacts UJ-001 response time)
  - Layered monolith: Less clear boundaries; module pattern better for future extraction

Consequences:
  - Enables: Fast iteration, simple deployment, shared state across domains
  - Constrains: Single scaling unit (can't scale auth independently); must be disciplined about module boundaries

Related IDs: TECH-001 (Next.js), RISK-005 (scaling concerns), FEA-001..FEA-020 (all features in one deployment)
Status: Accepted

Example ARC- entry (Security, addressing RISK-):

ARC-005: JWT with HTTP-Only Cookies
Category: Security
Context: Need session management for authenticated users; RISK-008 (security compliance) and TECH-003 (Clerk auth) guide this
Decision: JWTs stored in HTTP-only cookies, 1-hour expiry, refresh via /refresh endpoint

Rationale:
  - HTTP-only prevents XSS token theft (mitigates RISK-008 surface area)
  - Short expiry limits damage window if token stolen
  - Refresh flow handles long sessions gracefully
  - TECH-003 (Clerk) handles token lifecycle, we just enforce storage pattern

Alternatives Rejected:
  - localStorage: Vulnerable to XSS (RISK-008 violation)
  - Long-lived tokens: Increases risk exposure time (RISK-008)
  - Server sessions: Scaling complexity; would require Redis (not in TECH-)

Consequences:
  - Enables: Stateless auth, horizontal scaling
  - Constrains: Must handle refresh flow in frontend; logout requires token invalidation

Related IDs: TECH-003 (Clerk handles token generation), RISK-008 (security compliance), BR-010 (auth requirements)
Status: Accepted

Architecture Decision Categories

Category	What It Covers	Example Decisions
Structure	Component organization, boundaries	Monolith vs microservices, module structure
Integration	External service connections	API gateway pattern, webhook handlers
Security	Auth, authorization, data protection	JWT strategy, role-based access
Performance	Scaling, caching, optimization	CDN strategy, database indexing
Data	Storage, flow, consistency	Event sourcing, CQRS, replication
DevOps	Deployment, monitoring, CI/CD	Container orchestration, observability

Design Process

1. Pull TECH- decisions — What technologies are we building with?
2. Pull RISK- constraints — What must the architecture account for?
3. Pull FEA- features — What must the system do?
4. Define system boundaries — What's in/out of scope?
5. Map component relationships — How do parts connect?
6. Document integration patterns — How do Buy/Integrate items connect?
7. Create ARC- entries — Record decisions with rationale

System Boundary Definition

Before designing components, define what's inside and outside your system:

Inside (Build):

• Core business logic
• Differentiating features
• Custom workflows

Outside (Buy/Integrate):

• Authentication provider
• Payment processor
• Email service
• Analytics

Boundary Questions:

• Where does data enter the system?
• Where does data leave the system?
• What trust boundaries exist?
• What must be fast vs. can be eventual?

Component Relationship Patterns

For Build Components

Pattern	When to Use	Example
Monolith	MVP, small team, unclear boundaries	Single Next.js app
Modular Monolith	Growing codebase, clear domains	Modules with defined interfaces
Microservices	Clear boundaries, scaling needs	Separate auth, billing, core services

Rule for MVP: Start monolith, extract services when you have evidence of need.

For Buy/Integrate Components

Pattern	When to Use	Example
Direct Integration	Simple, trusted service	Call Stripe API directly
Adapter Layer	Want to swap providers later	Abstract over auth provider
Event Bridge	Async, decoupled	Webhooks → event queue → handlers

Integration Architecture Patterns

Pattern: Vendor Abstraction

When you Buy a service but want flexibility to switch:

┌─────────────────────────────────────┐
│           Your Application          │
├─────────────────────────────────────┤
│       Payment Abstraction Layer     │
│   interface PaymentProvider {       │
│     charge(amount, token): Result   │
│   }                                 │
├─────────────────────────────────────┤
│  StripeAdapter  │  PaddleAdapter    │
└─────────────────┴───────────────────┘

When to use: High switching cost, multiple viable providers, strategic flexibility needed.

Pattern: Webhook Handler

When integrating with external events:

External Service → Webhook Endpoint → Event Queue → Handler
                        ↓
                   Signature Verify
                        ↓
                   Idempotency Check
                        ↓
                   Enqueue for processing

When to use: External services push events (Stripe, GitHub, etc.).

ARC- Output Template

ARC-XXX: [Decision Title]
Category: [Structure | Integration | Security | Performance | Data | DevOps]
Context: [What prompted this decision]
Decision: [What we decided]
Rationale: [Why this choice]

Alternatives Rejected:
  - [Option A]: [Why not]
  - [Option B]: [Why not]

Consequences:
  - Enables: [What this makes possible]
  - Constrains: [What this limits]

Related IDs: [TECH-XXX, RISK-XXX, FEA-XXX]
Status: [Proposed | Accepted | Superseded]

Example ARC- entry:

ARC-001: Monolith with Module Boundaries
Category: Structure
Context: Need to choose application structure for MVP launch
Decision: Single Next.js application with domain-based module folders

Rationale:
  - Team of 2 developers, single deployment simplifies ops
  - Unclear domain boundaries at this stage
  - Can extract services later when patterns emerge

Alternatives Rejected:
  - Microservices: Premature; adds ops complexity without proven need
  - Serverless functions: Harder to share code, cold start concerns

Consequences:
  - Enables: Fast iteration, simple deployment, shared state
  - Constrains: Single scaling unit, must be disciplined about module boundaries

Related IDs: TECH-001 (Next.js), RISK-005 (scaling concerns)
Status: Accepted

Example ARC- entry (Security):

ARC-005: JWT with HTTP-Only Cookies
Category: Security
Context: Need session management strategy for authenticated users
Decision: JWTs stored in HTTP-only cookies, 1-hour expiry, refresh via /refresh endpoint

Rationale:
  - HTTP-only prevents XSS access to tokens
  - Short expiry limits damage from stolen tokens
  - Refresh flow handles long sessions gracefully

Alternatives Rejected:
  - localStorage: Vulnerable to XSS
  - Long-lived tokens: Security risk if compromised
  - Server-side sessions: Scaling complexity, Redis dependency

Consequences:
  - Enables: Stateless auth, horizontal scaling
  - Constrains: Must handle refresh flow in frontend, logout requires invalidation strategy

Related IDs: TECH-001 (Clerk handles this), RISK-008 (security compliance)
Status: Accepted

System Diagram Elements

When creating architecture diagrams, include:

Element	Symbol	Purpose
Service/Component	Box	Internal services, modules
External System	Cloud/cylinder	Third-party services, DBs
Trust Boundary	Dashed line	Security perimeters
Data Flow	Arrow	How data moves
Integration Point	Diamond	Where systems connect

Example Diagram Structure

┌─────────────────────────────────────────────────────────┐
│                    TRUST BOUNDARY                        │
│  ┌─────────────┐     ┌─────────────┐    ┌────────────┐  │
│  │   Frontend  │────▶│   API       │───▶│  Database  │  │
│  │   (Next.js) │     │  (tRPC)     │    │  (Supabase)│  │
│  └─────────────┘     └──────┬──────┘    └────────────┘  │
│                             │                            │
└─────────────────────────────┼────────────────────────────┘
                              │
              ┌───────────────┼───────────────┐
              ▼               ▼               ▼
        ┌──────────┐   ┌──────────┐   ┌──────────┐
        │  Stripe  │   │  Clerk   │   │  Resend  │
        │ (payments)│  │  (auth)  │   │ (email)  │
        └──────────┘   └──────────┘   └──────────┘
                    EXTERNAL SERVICES

RISK- to Architecture Mapping

Every high-priority risk should have an architectural response:

Risk	Architecture Response
RISK-001: API dependency outage	ARC-010: Add retry + circuit breaker
RISK-003: Data breach	ARC-005: Encryption at rest + transit
RISK-007: Scaling bottleneck	ARC-012: Cache layer, read replicas

Anti-Patterns to Avoid

Anti-Pattern	Signal	Fix
Architecture astronaut	Over-engineering for 1000x scale	Design for 10x current needs
Missing boundaries	Everything can call everything	Define clear interfaces
Ignoring RISK-	Architecture doesn't address risks	Map each High RISK- to ARC-
Vendor lock-in	No abstraction over critical services	Add adapter layer for switching
Diagram without decisions	Pretty pictures, no ARC- records	Every box needs documented rationale
Premature microservices	5 services for MVP	Start monolith, extract later

Quality Gates

Before proceeding to Technical Specification:

• All TECH- Build items have component placement
• All TECH- Buy/Integrate items have integration pattern
• High-priority RISK- entries have architectural mitigation
• Trust boundaries clearly defined
• Data flow documented
• ARC- entries created for major decisions

Downstream Connections

ARC- entries feed into:

Consumer	What It Uses	Example
Technical Specification	ARC- informs API design	ARC-001 (monolith) → unified API surface
v0.7 Build Execution	ARC- defines EPIC scope	ARC-003 (auth module) → EPIC-02
Infrastructure Setup	ARC- drives deployment	ARC-010 (edge caching) → CDN config
Security Review	Security ARC- entries	ARC-005 → pen test scope

Detailed References

• Architecture pattern examples: See

  references/examples.md

• ARC- entry template: See

  assets/arc.md

• Diagram templates: See

  references/diagrams.md