System Design

System Design

Guide architectural decisions and produce structured design artifacts.

When to Use

• Designing a new system or major feature from scratch
• Evaluating trade-offs between architectural approaches
• Creating Architecture Decision Records (ADRs)
• Reviewing existing architecture for scalability, reliability, or maintainability concerns
• Decomposing a monolith or planning a migration

Workflow

Step 1: Clarify Requirements

Before designing, gather:

1. Functional requirements - What must the system do?
2. Non-functional requirements - Performance, availability, security, cost constraints
3. Scope boundaries - What is explicitly out of scope?
4. Existing constraints - Current tech stack, team skills, timeline

Ask the user to clarify any ambiguous requirements before proceeding.

Step 2: Identify Components

Break the system into components:

• Data stores - What data exists? How is it accessed?
• Services / modules - What are the logical units of work?
• Interfaces - How do components communicate? (API, events, shared DB, files)
• External dependencies - Third-party services, APIs, infrastructure

Step 3: Evaluate Alternatives

For each significant decision, document at least 2 options:

Recommend one option with clear reasoning.

Step 4: Produce Design Artifact

Output a structured design document:

# Design: <Title>

## Context
[Problem statement and motivation]

## Requirements
- Functional: ...
- Non-functional: ...

## Architecture
[Component diagram or description]

## Key Decisions
| Decision | Choice | Rationale |
|----------|--------|-----------|
| ... | ... | ... |

## Data Model
[Schema or entity relationships]

## API Surface
[Key endpoints or interfaces]

## Risks & Mitigations
| Risk | Impact | Mitigation |
|------|--------|------------|
| ... | ... | ... |

## Open Questions
- ...

Step 5: Review Checklist

Before finalizing, verify:

• Single responsibility per component
• No circular dependencies between modules
• Failure modes identified and handled
• Data consistency model is explicit (strong vs eventual)
• Security boundaries are defined
• Observability points (logging, metrics, tracing) are planned

Principles

1. Start simple - Add complexity only when requirements demand it
2. Make trade-offs explicit - Every choice has a cost; document it
3. Design for change - Interfaces should be stable; implementations should be replaceable
4. Separate concerns - Data, logic, and presentation should not be entangled
5. Fail gracefully - Design for partial failures, not just the happy path