Orthogonal Code

Orthogonal Code

Heuristics for building and evaluating component independence. Use alongside system-design for depth/complexity concerns.

Core Principle

Orthogonality: Two components are orthogonal if changing one does not require changing the other.

Non-orthogonal systems exhibit a multiplier effect—a single change ripples across many modules. Orthogonal systems localize change: each module can be developed, tested, deployed, and replaced independently.

The Helicopter Test: Pick any implementation detail (a database schema column, a serialization format, an API endpoint). Change it. How many other files must also change? If the answer is more than 1–2, coupling is too high.

Relationship to system-design: That skill covers what makes a good module (deep interfaces, information hiding, complexity management). This skill covers how modules relate to each other—minimizing the connective tissue between them.

Red Flags

Thirteen coupling smells. For each, ask the yes/no test question. A "yes" answer indicates a coupling problem.

RF1 — Shotgun Surgery

Does a single logical change require edits in 3+ files?

A feature or bug fix that touches many modules means those modules share knowledge they shouldn't.

// Bad: adding a new user role requires changes in
// UserModel, AuthMiddleware, RoleValidator, AdminPanel, UserSerializer, RoleSeeder

RF2 — Train Wreck

Does any expression chain 3+ accessor calls?

Long chains reveal that a module knows the internal structure of distant objects (Law of Demeter violation).

// Bad
const zip = order.getCustomer().getAddress().getZipCode();

RF3 — Boolean Blindness

Does a function accept a boolean that controls its behavior mode?

A boolean parameter means the function does two different things. Callers must know internal branching logic.

// Bad
function createUser(name: string, isAdmin: boolean) {
  if (isAdmin) { /* entirely different path */ }
}

RF4 — Intimate Knowledge

Does a module directly build or mutate another module's internal types?

When module A constructs or modifies types that belong to module B's internals, A breaks if B's internals change.

// Bad: OrderService directly constructs PaymentGateway's internal request type
const req = new PaymentGatewayInternalRequest({ txnId: "...", ledgerCode: 42 });

RF5 — Shared Mutable State

Is there mutable state that 2+ modules both read and write?

Shared mutable state is the strongest form of coupling. Changes to how one module uses the state silently break the other.

// Bad: both OrderService and InventoryService read/write the same `stock` map
const stock: Map<string, number> = new Map(); // shared between modules

RF6 — Positional Coupling

If you reorder function calls, does the system silently break?

Implicit ordering dependencies mean callers must know internal sequencing.

// Bad: must call in exact order with no enforcement
initialize();
loadConfig();   // fails silently if called before initialize()
startServer();  // corrupts data if called before loadConfig()

RF7 — Fat Interface

Does a consumer use less than half the methods on an interface it depends on?

Depending on unused methods means the consumer is coupled to changes it doesn't care about (Interface Segregation violation).

// Bad: NotificationService only needs sendEmail() but depends on all of UserService
interface UserService {
  getUser(): User; updateUser(): void; deleteUser(): void;
  getPreferences(): Prefs; sendEmail(msg: string): void; // only method used
}

RF8 — Concrete Dependency

Does a module contain new ConcreteClass() for a dependency it could receive?

Hard-coding a concrete dependency prevents swapping implementations and forces tests to use the real thing.

// Bad
class OrderService {
  private db = new PostgresDatabase(); // can't swap, can't test without Postgres
}

RF9 — Data Clump

Do 3+ parameters appear together in 2+ function signatures?

Repeated parameter groups indicate a missing abstraction. All callers are coupled to the same positional knowledge.

// Bad: (street, city, zip) repeated everywhere
function validateAddress(street: string, city: string, zip: string) { }
function formatAddress(street: string, city: string, zip: string) { }

RF10 — Leaky Encoding

Would changing storage format require changing the public interface?

If your public API exposes storage details (column names, serialization format), every consumer is coupled to your storage layer.

// Bad: public method exposes that storage is JSON with specific keys
function getUserData(): { json_blob: string; updated_at_epoch_ms: number } { }

RF11 — Connascence of Meaning

Are there magic strings/numbers that must match between modules with no shared type?

When two modules agree on a value by convention rather than a shared type, renaming or changing the value silently breaks the other.

// Bad: both modules use "PREMIUM" string independently
// user-service.ts
if (user.tier === "PREMIUM") { ... }
// billing-service.ts
if (tierCode === "PREMIUM") { ... }

RF12 — Divergent Change

Can you describe the module without using "and"?

If a module has multiple reasons to change, it violates Single Responsibility. "This module handles orders and sends emails and updates inventory."

RF13 — Parasitic Test

Does the unit test require real infrastructure or another module's concrete implementation?

If a "unit" test needs a database, network, or another module's real code, the module under test isn't isolated.

// Bad: "unit" test requires running database
beforeAll(async () => { await database.connect(); await database.seed(); });

Green Flags

Thirteen orthogonality indicators. A "yes" answer confirms good decoupling.

GF1 — Single-Sentence Purpose

Can you state the module's purpose in one sentence, no "and"?

A focused module has one reason to exist and one reason to change.

// Good: "UserRepository persists and retrieves user entities."
class UserRepository { /* only persistence concerns */ }

GF2 — Swappable Implementation

Can you swap the implementation by only changing wiring/config?

If replacing Postgres with SQLite requires editing only the DI container or config file, the abstraction boundary is clean.

// Good: swap by changing one line of wiring
container.bind(Database).to(SqliteDatabase); // was PostgresDatabase

GF3 — Test in Isolation

Can you unit test with only mocks/stubs for dependencies?

If tests need nothing beyond the module itself and lightweight test doubles, the module's boundaries are well-defined.

// Good
const repo = new UserRepository(mockDatabase);
expect(repo.findById("1")).resolves.toEqual(mockUser);

GF4 — Tell, Don't Ask

Do callers send commands rather than query state and branch?

Telling objects what to do keeps decision logic inside the module that owns the data.

// Good: caller tells, doesn't ask-then-decide
order.applyDiscount(coupon); // order decides how to apply

GF5 — Narrow Interface

Does every public method have at least one external consumer?

No dead public surface area. Every exposed method justifies its existence.

GF6 — Data In, Data Out

Given same args, same result, no side effects?

Pure functions are inherently orthogonal—they depend on nothing beyond their inputs.

// Good
function calculateTax(amount: number, rate: number): number {
  return amount * rate;
}

GF7 — Composed via Interfaces

Are all inter-module dependencies typed as interfaces/protocols/abstract types?

Depending on abstractions rather than concretions means modules can evolve independently.

// Good
class OrderService {
  constructor(private readonly payments: PaymentGateway) {} // interface, not concrete
}

GF8 — Event-Driven Decoupling

Can the producer operate without knowing which consumers exist?

Event emitters don't import their listeners. Adding/removing consumers requires zero changes to the producer.

// Good: OrderService emits event, doesn't know who listens
eventBus.emit("order.placed", { orderId });

GF9 — Stable Boundary Types

Are public types defined at the boundary, not imported from internals?

Boundary types (DTOs, API contracts) should live at the module's edge, not be re-exported internals.

// Good: public type is a boundary DTO, not an internal entity
export interface OrderSummary { id: string; total: number; status: string }

GF10 — Configuration Over Convention

Are behavioral variations driven by config, not code kept in sync?

When behavior changes come from configuration, adding variants doesn't require coordinated code changes.

// Good: new notification channel = new config entry, no code change
const channels = config.get<NotificationChannel[]>("notifications.channels");

GF11 — Pipeable / Composable

Can you chain output into the next module without glue code?

UNIX philosophy: each module transforms data and passes it along a standard interface.

// Good
const result = await pipeline(
  readOrders,
  filterActive,
  calculateTotals,
  formatReport
)(input);

GF12 — Connascence of Name Only

Is coupling limited to shared names (no shared algorithms/timing/meaning)?

The weakest form of connascence—modules only agree on names of interfaces and methods. See references/connascence-taxonomy.md for the full hierarchy.

GF13 — Symmetric Change Cost

Adding a new variant requires changes in exactly one place?

When the cost of adding a new case (new payment method, new notification channel) is a single file change, the design is properly open/closed.

// Good: new payment method = one new class implementing PaymentGateway
class StripePayment implements PaymentGateway { /* ... */ }

Evaluation Protocol

Use this protocol to assess a module, component, or PR.

Step 1: Identify Modules

List the modules/classes/files under evaluation. Define their stated purpose.

Step 2: Map Dependencies

For each module, list what it imports, constructs, or calls. Draw the dependency graph (mentally or literally).

Step 3: Apply Red Flags

Walk through RF1–RF13. For each, answer the yes/no question. Record which flags trigger.

Step 4: Apply Green Flags

Walk through GF1–GF13. For each, answer the yes/no question. Record which flags are present.

Step 5: Score

Count red flags triggered and green flags present. Map to the scale:

Step 6: Recommend

For each triggered red flag, propose a specific refactoring. Prioritize by impact: fix shared mutable state (RF5) and shotgun surgery (RF1) before cosmetic issues like data clumps (RF9).

See references/evaluation-examples.md for two fully worked examples.

Writing Mode

When writing new code, run through this checklist before committing:

1. State the purpose — Write a single-sentence description (GF1). If you need "and," split the module.
2. Inject dependencies — Accept dependencies via constructor/parameter, never hard-code concretions (RF8 → GF7).
3. Define boundary types — Create DTOs/interfaces at the module's public edge. Don't export internals (GF9, RF10).
4. Prefer pure transforms — Where possible, write functions that are data-in/data-out (GF6). Push side effects to the edges.
5. Emit, don't import consumers — If other modules need to react, emit events or return results. Don't call them directly (GF8).
6. Verify the Helicopter Test — Before merging, mentally change one implementation detail. If more than 1–2 files need updating, revisit your boundaries.

For information hiding and interface depth concerns, defer to system-design.

Review Mode

When reviewing code or PRs for coupling:

Quick Scan (top 5 red flags to check first)

1. RF1 — Shotgun Surgery: Does this change touch many files for one logical change?
2. RF5 — Shared Mutable State: Is any state written by multiple modules?
3. RF2 — Train Wreck: Any long accessor chains?
4. RF8 — Concrete Dependency: Any new ConcreteClass() for swappable dependencies?
5. RF13 — Parasitic Test: Do tests require real infrastructure?

Full Review Checklist

1. Scan imports — Does the module import from many unrelated packages? High fan-in suggests a module doing too much (RF12).
2. Check constructors/factories — Are dependencies injected or hard-coded? (RF8)
3. Trace a hypothetical change — Pick one implementation detail and trace what would break. Count files affected (RF1, Helicopter Test).
4. Examine test setup — If beforeAll or beforeEach is lengthy, the module may have hidden dependencies (RF13).
5. Look for matching magic values — Search for string/number literals that appear in multiple modules (RF11).
6. Validate interface width — Does the consumer use most of what it depends on? (RF7)

For depth/complexity red flags (shallow modules, pass-through methods, information leakage), invoke system-design.

References

• Connascence Taxonomy — Full hierarchy of coupling strength from Name to Identity, with refactoring strategies. Load when doing deep coupling analysis.
• Evaluation Examples — Two worked examples applying the full evaluation protocol: a "Tangled" OrderService and a "Strong" notification pipeline.