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# 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:

```ts
// 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)

```ts
// 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.**

From kausthubh-coder/skills, MIT.

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