Architecture

Architecture

Design systems from scratch: map flows, then draw components. For retrofitting existing code, use codebase-hygiene instead.

Intake Routing

Functionality-First Design Process

Phase 1: Map Functionality Flows

Map every user flow end-to-end before designing any component:

Flow: [name]
1. [step] → [what the system does] → [what the user sees]
2. [step] → [what the system does] → [what the user sees]
Error paths:
- [error] → [system response] → [user sees]

Every flow must have its error paths mapped. Unmapped error paths become unmapped components.

Phase 2: Map to Architecture

Translate flows into components:

• Each flow step maps to one or more components
• Each error path maps to a component's error handling
• Data crossings between components become interfaces

Phase 3: Design Components

For each component:

• Interface: what it receives and returns (the contract)
• Responsibility: what it does (one sentence)
• Dependencies: what it needs (other components, external services)
• State: what it remembers (if anything)
• Error handling: what can go wrong and what it does about it

Before finalizing any component boundary, apply the Deletion Test and Two-Adapter Rule as defined in cc10x:codebase-design. A component that fails the deletion test (complexity vanishes if deleted) or fails the two-adapter rule (it is a port with only one adapter — an ordinary caller or test exercising the interface is not an adapter) is not a real boundary yet — fold it into its caller or defer the split until a second concrete need appears.

Architecture Views

System Context (C4 Level 1)

Box diagram: your system + external systems it talks to. One paragraph per external system: what it provides, what you depend on.

Container View (C4 Level 2)

Internal boxes: web app, API, database, queue, worker. Arrows show data flow. One paragraph per container: technology choice, responsibility.

Component View (C4 Level 3)

Inside each container: the modules/classes. Arrows show call relationships. This is what the builder will implement.

LSP-Powered Architecture Analysis

Use LSP to understand existing architecture before designing new:

• Go to Definition on key functions to trace the call graph
• Find References to understand blast radius of existing interfaces
• Go to Type Definition to understand data models
• Incoming/Outgoing Calls to map the dependency graph

API Design (Functionality-Aligned)

Design APIs from the flow, not from the data model:

1. What does the user need to do? (action, not resource)
2. What's the minimal interface that enables it? (fewest endpoints/parameters)
3. What's the error contract? (every error case from the flow mapping)
4. What's the type contract? (input/output types, not just shapes)

// Good: functionality-aligned
POST /orders/{id}/cancel  →  { status, cancelledAt }

// Bad: data-model-aligned
PUT /orders/{id}  →  { ..., status: "cancelled", ... }

Integration Patterns

For each integration:

Dependency Classification

Wrapped dependencies can be swapped. Consumed dependencies cannot. Track which is which — it determines your coupling risk.

Observability Design

For each component:

• Logging: what to log (not "everything" — specific events)
• Metrics: what to track (business-relevant, not infra noise)
• Tracing: what to trace (cross-component flows, not every function call)
• Alerting: when to alert (user-visible impact, not internal noise)

Architecture Vocabulary

The deep-module vocabulary (module, interface, depth, seam, adapter, leverage, locality, deletion test, two-adapter rule) is defined in cc10x:codebase-design. Use those terms exactly.

Three extra terms specific to greenfield architecture (not in codebase-design):

• Concealed complexity — work done behind a simple interface. The goal of deep modules.
• Temporal coupling — caller must know the order of operations. Design defect — remove or document explicitly.
• Leaky abstraction — interface exposes internal details callers must know. Design defect — fix the interface.

Before finalizing any component boundary, apply the Deletion Test and Two-Adapter Rule as defined in cc10x:codebase-design. A component that fails the deletion test (complexity vanishes if deleted) or fails the two-adapter rule (it is a port with only one adapter — an ordinary caller or test exercising the interface is not an adapter) is not a real boundary yet — fold it into its caller or defer the split until a second concrete need appears.

Design It Twice

When a module's interface is non-trivial, design it twice:

1. First design: the obvious approach. Write it out fully.
2. Second design: a different approach (not a refinement of the first).

Compare both. The first design is usually shallow — it mirrors the implementation. The second design reveals what the interface should hide. Use the better one, or a hybrid.

Why: One-pass interfaces optimize for the implementer. Two-pass interfaces optimize for the caller.

Decision Framework

For architectural decisions with material trade-offs:

### Decision: [Title]
**Context:** [why this decision is needed]
**Options:** [2-3 alternatives with trade-offs]
**Decision:** [what was chosen]
**Rejected:** [what was not chosen and why]
**Consequences:** [what this enables and prevents]
**Reversibility:** [reversible or irreversible — irreversible decisions need more evidence]