Target Architecture Design

Target Architecture Design

Design a pragmatic, scalable, and secure target architecture grounded in the current state assessment and business requirements. Produce a High-Level Design (HLD) with Architecture Decision Records (ADRs) for every significant choice.

Overview

Good architecture is the minimum structural decisions needed to achieve the required quality attributes. This skill produces a target HLD that is specific enough to guide implementation but flexible enough to adapt as understanding deepens.

Architecture Design Process

Step 1: Define Architectural Drivers

Before designing, clarify the forces shaping the architecture:

• Functional requirements: the most architecturally significant features
• Quality attributes: performance, scalability, security, availability targets
• Constraints: technology mandates, regulatory requirements, budget, team skills
• Migration constraints: what must stay, what can change, what must be preserved

Step 2: Select Architectural Pattern

Based on drivers and constraints, select the target pattern:

• Determine scale requirements (single team vs. multi-team, low vs. high traffic)
• Match pattern to team size and maturity (microservices require operational maturity)
• Consider migration path from current state (big-bang vs. strangler fig vs. phased)

Document the decision in an ADR.

Step 3: Design the System Structure (C4 Model)

Context (Level 1): The system and its external actors and systems.

Containers (Level 2): The major deployable units (web app, API, worker, database, queue).

Components (Level 3, for critical containers): The key internal building blocks.

Describe each diagram in text using Mermaid syntax where helpful:

graph TD
  User -->|HTTPS| WebApp
  WebApp -->|REST| API
  API -->|SQL| Database
  API -->|AMQP| Queue
  Queue --> Worker

Step 4: Technology Stack Decisions

For each layer, select technology and produce an ADR:

Layer	Technology	Rationale	Alternatives Considered
Frontend	[tech]	[why]	[what else was evaluated]
Backend API	[tech]	[why]	[...]
Database	[tech]	[why]	[...]
Cache	[tech]	[why]	[...]
Queue/Event	[tech]	[why]	[...]
Infrastructure	[tech]	[why]	[...]

Step 5: Architecture Decision Records (ADRs)

For every significant decision, produce an ADR:

# ADR-[N]: [Short Decision Title]

**Status**: Proposed | Accepted | Deprecated

**Context**: [What forces led to this decision? What constraints apply?]

**Decision**: [What was decided?]

**Consequences**:
- Positive: [benefits of this choice]
- Negative: [trade-offs accepted]

**Alternatives Considered**:
- [Option A]: [why rejected]
- [Option B]: [why rejected]

Step 6: Migration Strategy

Describe the path from current to target state:

• Strangler fig: new capability built alongside old, traffic gradually migrated
• Branch by abstraction: abstract interfaces introduced, then implementation swapped
• Big bang: complete rewrite and cutover (justify if recommending this)
• Phased decomposition: extract services one at a time starting with the highest value

Output Format

Produce:

1. HLD document with context, container, and component diagrams (Mermaid)
2. Technology stack table with rationale
3. One ADR per major decision
4. Migration strategy section

Additional Resources

• references/architectural-patterns.md — Pattern catalogue with trade-offs
• references/adr-library.md — Example ADRs for common decisions
• assets/hld-template.md — Blank HLD template