Servis Sınırları — Bounded Context ve Database-per-Service

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// One listing for comparison; in the codebase each lives in its own service.
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// KYC (onboarding): who is this person, and did we verify it?
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record KycProfile(CustomerNo no, String nationalId, String fullName, List<Document> documents,
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                  VerificationStatus status) {}
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// Lending: can this person repay?
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record Borrower(CustomerNo no, BigDecimal monthlyIncome, BigDecimal totalExposure,
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                RiskRating riskRating, boolean kycVerified) {}
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// Cards: what may this person spend, and what do they owe?
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record Cardholder(CustomerNo no, String embossedName, BigDecimal creditLimit, LocalDate statementDay) {}

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@Entity
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@Table(name = "borrowers")
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class BorrowerRecord {
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    @Id
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    private String customerNo;
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    private BigDecimal monthlyIncome;       // owned by lending: declared in the application
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    private BigDecimal totalExposure;       // owned by lending: sum of its own loans
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    // Copied from KYC events. Lending never writes these and never asks KYC synchronously.
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    private boolean kycVerified;
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    @Enumerated(EnumType.STRING)
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    private RiskRating riskRating;
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    private long kycVersion;                // the last KYC event applied
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    void applyKyc(boolean verified, RiskRating rating, long version) {
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        if (version <= kycVersion) return;  // an older or duplicate event: ignore
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        this.kycVerified = verified;
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        this.riskRating = rating;
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        this.kycVersion = version;
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    }
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    Borrower view() {
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        return new Borrower(new CustomerNo(customerNo), monthlyIncome, totalExposure, riskRating, kycVerified);
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    }
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}

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@Component
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class KycEventsListener {
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    private final BorrowerRepository borrowers;
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    KycEventsListener(BorrowerRepository borrowers) {
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        this.borrowers = borrowers;
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    }
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    @KafkaListener(topics = "kyc.customer-status", groupId = "lending")
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    @Transactional
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    void on(CustomerStatusChanged event) {
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        borrowers.findOrCreate(event.customerNo())
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                .applyKyc(event.verified(), event.riskRating(), event.version());
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    }
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}

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@Service
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class ApproveLoan {
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    private final BorrowerRepository borrowers;
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    private final LoanRepository loans;
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    private final CreditPolicy policy;
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    private final Outbox outbox;
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    ApproveLoan(BorrowerRepository borrowers, LoanRepository loans, CreditPolicy policy, Outbox outbox) {
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        this.borrowers = borrowers;
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        this.loans = loans;
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        this.policy = policy;
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        this.outbox = outbox;
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    }
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    @Transactional
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    public Decision decide(LoanApplication application) {
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        Borrower borrower = borrowers.get(application.customerNo()).view();   // local, no network
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        if (!borrower.kycVerified()) return Decision.pending("KYC_NOT_VERIFIED");
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        Decision decision = policy.evaluate(borrower, application.amount(), application.termMonths());
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        if (decision.approved()) {
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            Loan loan = loans.save(Loan.approved(application, decision.rate()));
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            // Disbursement belongs to core banking: tell it, do not call it.
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            outbox.add(new LoanApproved(loan.id(), application.customerNo(), application.amount()));
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        }
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        return decision;
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    }
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}

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-- Lending's own database. No foreign key to KYC or cards: other contexts are
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-- reached through their events and APIs, never through their tables.
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CREATE TABLE borrowers (
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    customer_no     VARCHAR(20)    PRIMARY KEY,
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    monthly_income  NUMERIC(15, 2),
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    total_exposure  NUMERIC(15, 2) NOT NULL DEFAULT 0,
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    kyc_verified    BOOLEAN        NOT NULL DEFAULT FALSE,
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    risk_rating     VARCHAR(10),
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    kyc_version     BIGINT         NOT NULL DEFAULT 0
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);
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CREATE TABLE loans (
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    id           BIGINT         PRIMARY KEY,
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    customer_no  VARCHAR(20)    NOT NULL REFERENCES borrowers (customer_no),
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    principal    NUMERIC(15, 2) NOT NULL,
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    annual_rate  NUMERIC(6, 4)  NOT NULL,
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    term_months  INT            NOT NULL
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);