Controller/Service/Selector Patterns for LWC

• Controller — thin, @AuraEnabled-only. Its job is the LWC-facing contract: receiving parameters, calling into the Service layer, wrapping the result in a DTO (Module 7). No real business logic lives here.
• Service — the actual business logic and orchestration. A plain Apex class with no @AuraEnabled annotations at all — it doesn't know or care that LWC exists, which is exactly what makes it independently unit-testable and reusable.
• Selector — isolates the actual SOQL query construction. Query logic can change (new filters, different fields) without touching the business logic that consumes its results.

Each layer can be tested and changed independently — a query change in the Selector doesn't risk breaking business logic in the Service, and vice versa. More importantly, the same Service/Selector logic can be reused by more than one Controller — for instance, one exposed to LWC and a completely separate one used by a scheduled batch job — without duplicating the actual business rules in two places.

// Selector — isolates query construction
public inherited sharing class AccountSelector {
    public static List<Account> findByNameLike(String searchTerm) {
        return Database.query(
            'SELECT Id, Name, (SELECT Id FROM Opportunities WHERE IsClosed = false) ' +
            'FROM Account WHERE Name LIKE :searchTerm',
            AccessLevel.USER_MODE
        );
    }
}

// Service — the actual business logic, no @AuraEnabled at all
public inherited sharing class AccountService {
    public static List<AccountResult> search(String searchTerm) {
        List<AccountResult> results = new List<AccountResult>();
        for (Account acc : AccountSelector.findByNameLike('%' + searchTerm + '%')) {
            results.add(new AccountResult(acc.Id, acc.Name, acc.Opportunities.size()));
        }
        return results;
    }
}

// Controller — thin, the only class @AuraEnabled-annotated
public with sharing class AccountController {
    @AuraEnabled(cacheable=true)
    public static List<AccountResult> search(String searchTerm) {
        return AccountService.search(searchTerm);
    }
}

This is Module 7's Account search capstone, now properly layered — AccountController is a thin, one-line pass-through; AccountService holds the real logic; AccountSelector owns the query. Each piece could be tested (Module 14) and changed independently.