waynesutton/code-structure

Service Layer Architecture

Overview

Two-layer separation: Actions orchestrate domain rules (the "why/when"), while a service layer centralizes reusable operational mechanics (the "how").

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

Don't use when: Logic is truly domain-specific and used by only one caller.

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 params, structured returns | Hidden global state, reaching into DB | | Migration | Extract one block, replace one caller, verify, then migrate rest | Refactor everything at once | | Domain logic | Keep auth, policy, error classification in actions | Let service mutate domain state directly | | Extraction trigger | Logic repeated across 2+ callers | Logic used once (over-abstraction) |

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 })
• Never reach into database/state directly
• Make failure explicit (structured results, not swallowed errors)

This lets callers choose strict vs relaxed behavior per flow.

Migration Checklist

When extracting shared logic:

1. Write the flow in action code first (clear behavior)
2. Mark repeated operational chunks across callers
3. Extract only repeated, non-domain chunks to service
4. Replace one caller → verify → replace remaining callers
5. Keep domain policy in actions (auth, status transitions, error classification)
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 | Service mutates database tables directly | | Inconsistent API | Each function uses different argument styles and error semantics | | Over-abstraction | Extracting logic used by only one caller |

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

New feature? → Write in action first → See repeated ops? → Extract to service
                                      → No repetition?  → Keep in action

Your architecture in one sentence: Actions orchestrate domain rules, while the service layer centralizes reusable operational mechanics with a composable, explicit-input API.