architecture

Architecture

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

Intake Routing

Request type	Use
New system/major feature (greenfield)	This skill
Existing code with shallow modules	codebase-hygiene
Multi-component integration	This skill
Single-component refactor	planning + building

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 (defined below under Architecture Vocabulary). A component that fails the deletion test (complexity vanishes if deleted) or fails the two-adapter rule (only one caller/adapter exists) 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:

Field	Value
System	[name]
Protocol	[HTTP/gRPC/CLI/message queue]
Direction	[we call them / they call us / both]
Contract	[request/response schema or event schema]
Failure mode	[what happens when it's down]
Retry policy	[retries, backoff, circuit breaker]

Dependency Classification

Class	Meaning	Example
Owned	We control the code and deploy it	Internal service
Wrapped	We depend on it but wrap it in our interface	Third-party SDK behind adapter
Consumed	We depend on it directly, no wrapper	External API called directly
Infra	Platform-level dependency	Database, message queue

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 (Precise Language)

Use these terms exactly — don't substitute. They make trade-offs explicit and carry precise meaning from Ousterhout's "A Philosophy of Software Design":

Term	Meaning	Signal
Module	A unit of code with an interface and a hidden implementation	—
Interface	The surface a module exposes — the contract, not the signature	—
Deep module	Small interface, lots hidden inside (high leverage: complexity hidden ÷ interface size)	Good — complexity is contained
Shallow module	Interface as complex as implementation	Bad — wrapper adds complexity without hiding any
Concealed complexity	Work done behind a simple interface	The goal of deep modules
Leaky abstraction	Interface exposes internal details callers must know	Design defect — fix the interface
Temporal coupling	Caller must know the order of operations	Design defect — remove or document explicitly
Seam	A place where behavior can be substituted (test boundary, adapter point)	—
Adapter	Code that bridges your interface to an external system	—
Locality	Whether related logic lives together — good locality means changes touch few files	—

The Deletion Test

For every module or abstraction, ask: "If I deleted this module and inlined its code at every call site, where does the complexity go?"

• It vanishes → the module was a pass-through / shallow. It adds indirection without hiding complexity. Delete it or deepen its interface.
• It reappears across N call sites → the module is deep. It earns its existence by hiding complexity that would otherwise be duplicated.

This is a falsifiable test, not a matter of taste — apply it before accepting any new module boundary.

The Two-Adapter Rule

• One adapter means a hypothetical seam — something might need to vary. This is premature abstraction.
• Two concrete adapters means a real seam — something does vary. This is evidence-based design.

When wrapping a third-party dependency, the first adapter is allowed (it isolates the external API). A second adapter stacked on top of the first is a smell — it means the first adapter's interface is wrong; fix the first adapter instead of stacking abstractions. Don't introduce a new seam until you have two concrete adapters that need it.

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]

Note

Deep-module vocabulary, the Deletion Test, and the Two-Adapter Rule are defined once above under Architecture Vocabulary (Precise Language), and Phase 3 (Design Components) applies them directly. There is no second copy.