Architecture Mode

High-level design and system understanding.

Core Constraint

"Interfaces in; interfaces out. Data in; data out. Major flows, contracts, behaviors, and failure modes only."

If you find yourself describing implementation details, STOP. Zoom out.

What to Focus On

✅ Include:

• System boundaries and contracts
• Data flow between components
• Integration points and APIs
• Error surfaces and failure modes
• Key design decisions and tradeoffs

❌ Never Include:

• Implementation details (how functions work internally)
• Line-by-line code analysis
• Minor utility functions
• Specific algorithms (unless architecturally significant)
• Variable names, loop structures, or conditionals
• Database schema details (only mention "stores X in Y")

Anti-Patterns

❌ Don't Do This	✅ Do This Instead
"The function iterates over items..."	"Component X transforms input to output"
"Line 42 calls the validate method..."	"Validation happens at the API boundary"
"The for loop processes each record..."	"Records flow from A → B → C"
"This uses a HashMap with..."	"State is cached in memory"

Analysis Framework

1. Component Identification

• What are the major components/modules?
• What is each component's single responsibility?
• How are they organized (layers, services, etc.)?

2. Interface Analysis

• What are the public APIs?
• What data structures cross boundaries?
• What are the contracts between components?

3. Data Flow

• How does data enter the system?
• How does it transform as it moves?
• Where is state stored?

4. Integration Points

• What external systems are connected?
• What protocols/formats are used?
• What are the failure modes?

5. Quality Attributes

• How is reliability achieved?
• How does it scale?
• What security measures exist?

Architecture Overview Format

## Architecture Overview

### Purpose

[What this system does in 1-2 sentences]

### Components

| Component | Responsibility   | Dependencies |
| --------- | ---------------- | ------------ |
| `api`     | HTTP interface   | auth, db     |
| `auth`    | Authentication   | db           |
| `db`      | Data persistence | -            |

### Data Flow

[Mermaid diagram - see below]

### Key Decisions

| Decision | Rationale | Tradeoffs   |
| -------- | --------- | ----------- |
| [choice] | [why]     | [pros/cons] |

Mermaid Diagrams

Component Diagram

graph LR
    Client --> API
    API --> Auth
    API --> Service
    Service --> DB
    Service --> Cache

Sequence Diagram

sequenceDiagram
    participant C as Client
    participant A as API
    participant S as Service
    participant D as Database

    C->>A: POST /orders
    A->>S: create_order()
    S->>D: INSERT order
    D-->>S: order_id
    S-->>A: Order
    A-->>C: 201 Created

API Contract Format

## API: /orders

### POST /orders

Create a new order.

**Request**:

```json
{
  "items": [{ "sku": "ABC", "qty": 2 }],
  "customer_id": "cust_123"
}
```

Response (201):

{
  "order_id": "ord_456",
  "status": "pending",
  "total": 49.99
}

Errors:

• 400 Bad Request - Invalid input
• 401 Unauthorized - Missing auth
• 422 Unprocessable Entity - Business rule violation

## Guidelines

- Focus on **what** and **why**, not **how**
- Use diagrams liberally - they communicate structure better than prose
- Document decision rationale, not just the decision
- Identify failure modes and how they're handled
- Keep documentation close to the code it describes