Code Structure skill

Service Layer Architecture

Overview

This collection prefers a two-layer separation: actions orchestrate domain rules, while services implement reusable operations. Use it where the project has no established convention. Preserve existing domain services, repositories, transaction boundaries, and framework patterns when they already fit the task.

This prevents duplicated code, inconsistent behavior, and bugs fixed in one path but not others.

When to Use

• Multiple callers need the same low-level operation (sandbox creation, email sending, payment processing)
• You're copy-pasting operational logic between action files
• A bug fix in one workflow doesn't propagate to others doing the same thing
• Adding a new feature that shares mechanics with existing flows

Avoid extraction that adds indirection without a concrete benefit. Repetition is a useful signal, but a single caller can justify a boundary for testing, provider isolation, transactions, or complex logic.

Core Pattern

Orchestration Layer (Actions)          Service Layer (Shared Mechanics)
├── owns business rules                ├── owns reusable operations
├── owns state transitions             ├── owns provider/SDK interactions
├── owns auth/ownership checks         ├── owns command execution details
├── owns failure classification        ├── owns health checks / readiness
├── owns retries / user-facing errors  └── returns structured results
└── calls service functions

Rule of thumb:

• "What this product flow means" → keep in actions
• "How to do this operation reliably" → move to service layer

Quick Reference

Design Principle	Do	Don't
API shape	Composable capability blocks	One giant "do everything" method
Inputs/outputs	Explicit dependencies and meaningful results	Hidden global state or undocumented side effects
Migration	Extract one block, replace one caller, verify, then migrate rest	Refactor everything at once
Domain logic	Put policy and transactions in the project's established owner	Scatter the same policy across callers
Extraction trigger	Repeated mechanics or a concrete boundary benefit	Speculative abstractions with no current benefit

Designing Service Functions

Design as capability blocks, not monoliths:

// Good: composable, each caller chooses what to use
createManagedSandbox(...)
prepareRepo(...)
detectPackageManager(...)
installDependencies(...)
runBuildCommand(...)
startSandboxRuntime(...)

Each function should:

• Accept all required data as explicit parameters
• Return structured outputs (e.g., { ready, previewUrl, proxyPort })
• Inject database or repository access when the service owns persistence under the project's architecture. State writes and transaction boundaries explicitly.
• Make failure explicit through typed results or documented exceptions

This lets callers choose strict vs relaxed behavior per flow.

Migration Checklist

When extracting shared logic:

1. Identify the existing owners of policy, persistence, and transactions
2. Mark repeated operations or boundaries with a concrete current benefit
3. Extract a cohesive operation consistent with those owners
4. Replace one caller → verify → replace remaining callers
5. Preserve auth, status transitions, error handling, and transaction behavior
6. Run verification: typecheck, lint, confirm all flows still work

Anti-Patterns

Anti-Pattern	Problem
God service	One huge function hides all control flow
Leaky service	Hidden state changes or unclear transaction ownership
Inconsistent API	Each function uses different argument styles and error semantics
Over-abstraction	Indirection with no reuse, testability, or boundary benefit

Example: Email Service (Simple)

// emailService.ts — shared mechanics
export async function sendWelcomeEmail(params: { to: string; name: string }) {
  const html = `<h1>Welcome ${params.name}</h1>`;
  await emailProvider.send(params.to, "Welcome", html);
}

// userSignup.ts — orchestration (owns WHEN to send)
if (user.marketingOptIn) {
  await sendWelcomeEmail({ to: user.email, name: user.name });
}

// adminInvite.ts — orchestration (different business rule, same mechanic)
await sendWelcomeEmail({ to: invitee.email, name: invitee.name });

Mental Model

Existing architecture? → Follow its boundaries
No convention? → Prefer actions for orchestration and services for operations
Extraction? → Name its concrete benefit before adding another layer

Keep responsibilities clear, dependencies explicit, and side effects visible.