# Example: Banking Domain

This example demonstrates DDD modeling for a banking domain, covering all phases of the workflow.

## Phase 1: Domain Discovery

### Domain Description
A digital banking platform that allows customers to open accounts, make transfers, and earn loyalty rewards.

### Subdomain Classification

| Subdomain | Type | DDD Investment |
|-----------|------|----------------|
| **Accounts** | Core | Full DDD - competitive advantage through account features |
| **Transfers** | Core | Full DDD - business-critical payment processing |
| **Loyalty** | Core | Full DDD - customer retention differentiator |
| **Fees** | Supporting | Simplified DDD - necessary but not differentiating |
| **Notifications** | Generic | CRUD - standard email/SMS sending |

---

## Phase 2: Bounded Contexts

### Context Map

```
                    ┌────────────────────────────────────────────────┐
                    │                BANKING DOMAIN                  │
                    │                                                │
                    │  ┌──────────────┐      ┌──────────────┐       │
                    │  │   ACCOUNTS   │─────>│  TRANSFERS   │       │
                    │  │              │ CS   │              │       │
                    │  │  - Account   │      │  - Transfer  │       │
                    │  │  - Balance   │      │  - Payment   │       │
                    │  │  - Holder    │      │  - Status    │       │
                    │  └──────────────┘      └──────────────┘       │
                    │         │                     │                │
                    │         │ CF                  │ P              │
                    │         v                     v                │
                    │  ┌──────────────┐      ┌──────────────┐       │
                    │  │     FEES     │      │   LOYALTY    │       │
                    │  │ (Supporting) │      │    (Core)    │       │
                    │  │              │      │              │       │
                    │  │  - Fee       │      │  - Program   │       │
                    │  │  - Schedule  │      │  - Tier      │       │
                    │  └──────────────┘      └──────────────┘       │
                    │                                                │
                    └────────────────────────────────────────────────┘

Legend:
  CS = Customer-Supplier (Accounts supplies, Transfers consumes)
  CF = Conformist (Fees conforms to Accounts model)
  P  = Partnership (Transfers and Loyalty evolve together)
```

### Ubiquitous Language

#### Accounts Context
| Term | Definition |
|------|------------|
| Account | A financial account owned by one or more customers |
| Balance | Current amount of money in the account (in minor units) |
| Holder | A person with access to the account (Owner, Operator, Viewer) |
| Freeze | Temporarily block all debit operations on an account |
| Standard Account | Account that cannot have negative balance |
| Credit Account | Account with approved credit limit |

#### Transfers Context
| Term | Definition |
|------|------------|
| Transfer | Movement of money between two accounts |
| Internal Transfer | Transfer within the same bank |
| Source Account | Account debited in a transfer |
| Destination Account | Account credited in a transfer |
| Pending | Transfer initiated but not yet completed |
| Completed | Transfer successfully processed |
| Failed | Transfer that could not be completed |

#### Loyalty Context
| Term | Definition |
|------|------------|
| Program | Loyalty program a customer is enrolled in |
| Tier | Level within the program (Bronze, Silver, Gold, Platinum) |
| Points | Accumulated loyalty points |
| Earn Rate | Points earned per currency unit spent |
| Redemption | Converting points to rewards |

---

## Phase 3: Tactical Modeling

### Accounts Bounded Context

#### Aggregate: Account

```
Account Aggregate
├── Account (Aggregate Root)
│   ├── AccountID (Value Object)
│   ├── Balance (Value Object: Money)
│   ├── Status (Value Object: Active|Frozen|Closed)
│   ├── AccountType (Value Object: Standard|Credit)
│   ├── CreditLimit (Value Object: Money)
│   └── Holders[] (Entity)
│       ├── HolderID (Value Object)
│       ├── UserID (Value Object - reference to User aggregate)
│       └── Role (Value Object: Owner|Operator|Viewer)
│
└── Invariants:
    - Balance >= 0 for Standard accounts
    - Balance >= -CreditLimit for Credit accounts
    - At least one Holder with Owner role
    - Cannot debit Frozen account
    - Cannot modify Closed account
```

#### Value Objects

```go
// AccountID - unique identifier
type AccountID struct { value uuid.UUID }

// Money - monetary amount with currency
type Money struct {
    amount   int64   // cents
    currency string  // ISO 4217
}

// Status - account status
type Status struct { value string }
var (
    StatusActive = Status{"active"}
    StatusFrozen = Status{"frozen"}
    StatusClosed = Status{"closed"}
)

// AccountType
type AccountType struct { value string }
var (
    AccountTypeStandard = AccountType{"standard"}
    AccountTypeCredit   = AccountType{"credit"}
)

// Role - holder role
type Role struct { value string }
var (
    RoleOwner    = Role{"owner"}
    RoleOperator = Role{"operator"}
    RoleViewer   = Role{"viewer"}
)
```

### Transfers Bounded Context

#### Aggregate: Transfer

```
Transfer Aggregate
├── Transfer (Aggregate Root)
│   ├── TransferID (Value Object)
│   ├── SourceAccountID (Value Object - reference only!)
│   ├── DestinationAccountID (Value Object - reference only!)
│   ├── Amount (Value Object: Money)
│   ├── Status (Value Object: Pending|Completed|Failed)
│   ├── FailureReason (Value Object, optional)
│   ├── InitiatedAt (timestamp)
│   └── CompletedAt (timestamp, optional)
│
└── Invariants:
    - Amount must be positive
    - Source and Destination must be different
    - Status transitions: Pending → Completed|Failed only
    - Cannot modify after Completed or Failed
```

### Loyalty Bounded Context

#### Aggregate: LoyaltyProgram

```
LoyaltyProgram Aggregate
├── LoyaltyProgram (Aggregate Root)
│   ├── ProgramID (Value Object)
│   ├── CustomerID (Value Object - reference only!)
│   ├── Tier (Value Object: Bronze|Silver|Gold|Platinum)
│   ├── Points (Value Object)
│   ├── EarnRate (Value Object)
│   └── PointsHistory[] (Value Object)
│       ├── Date
│       ├── Amount
│       ├── Type (Earned|Redeemed|Expired)
│       └── Description
│
└── Invariants:
    - Points >= 0
    - Tier determined by points thresholds
    - Cannot redeem more points than available
```

---

## Phase 4: Invariants

### Account Aggregate Invariants

```go
// Account aggregate invariants:
//
// Invariant: Balance cannot be negative for standard accounts
// Enforced in: Withdraw(), constructor
//
// Invariant: Balance cannot exceed credit limit for credit accounts
// Enforced in: Withdraw(), constructor
//
// Invariant: Account must have at least one holder with OWNER role
// Enforced in: RemoveHolder(), constructor
//
// Invariant: Frozen account cannot process debit operations
// Enforced in: Withdraw()
//
// Invariant: Closed account cannot be modified
// Enforced in: all mutation methods
```

### Transfer Aggregate Invariants

```go
// Transfer aggregate invariants:
//
// Invariant: Transfer amount must be positive
// Enforced in: constructor
//
// Invariant: Source and destination accounts must be different
// Enforced in: constructor
//
// Invariant: Status can only transition Pending → Completed or Pending → Failed
// Enforced in: Complete(), Fail()
//
// Invariant: Completed or Failed transfer cannot be modified
// Enforced in: all mutation methods
```

### LoyaltyProgram Aggregate Invariants

```go
// LoyaltyProgram aggregate invariants:
//
// Invariant: Points balance cannot be negative
// Enforced in: RedeemPoints()
//
// Invariant: Tier is determined by points thresholds
// Enforced in: EarnPoints() (automatic upgrade), calculateTier()
//
// Invariant: Cannot redeem more points than available
// Enforced in: RedeemPoints()
```

---

## Phase 5: Generated Code

### Account Aggregate

```go
package accounts

import (
    "errors"
    "time"
)

var (
    ErrInsufficientFunds     = errors.New("insufficient funds")
    ErrAccountFrozen         = errors.New("account is frozen")
    ErrAccountClosed         = errors.New("account is closed")
    ErrCannotRemoveLastOwner = errors.New("cannot remove last owner")
    ErrNegativeAmount        = errors.New("amount must be positive")
    ErrCreditLimitExceeded   = errors.New("credit limit exceeded")
)

// AccountAggregate represents a bank account.
//
// Invariants:
// - Balance >= 0 for standard accounts
// - Balance >= -CreditLimit for credit accounts
// - At least one holder with OWNER role
// - Frozen account cannot process debit operations
// - Closed account cannot be modified
type AccountAggregate struct {
    id          AccountID
    balance     Money
    holders     []Holder
    status      Status
    accountType AccountType
    creditLimit Money

    createdAt time.Time
    updatedAt time.Time
    events    []DomainEvent
}

// NewAccount creates a new standard account.
func NewAccount(id AccountID, initialHolder Holder) (*AccountAggregate, error) {
    if initialHolder.Role() != RoleOwner {
        return nil, ErrCannotRemoveLastOwner
    }

    return &AccountAggregate{
        id:          id,
        balance:     MustMoney(0, "USD"),
        holders:     []Holder{initialHolder},
        status:      StatusActive,
        accountType: AccountTypeStandard,
        creditLimit: MustMoney(0, "USD"),
        createdAt:   time.Now(),
        updatedAt:   time.Now(),
        events:      make([]DomainEvent, 0),
    }, nil
}

// NewCreditAccount creates a new credit account with limit.
func NewCreditAccount(id AccountID, initialHolder Holder, creditLimit Money) (*AccountAggregate, error) {
    account, err := NewAccount(id, initialHolder)
    if err != nil {
        return nil, err
    }
    account.accountType = AccountTypeCredit
    account.creditLimit = creditLimit
    return account, nil
}

func (a *AccountAggregate) ID() AccountID         { return a.id }
func (a *AccountAggregate) Balance() Money        { return a.balance }
func (a *AccountAggregate) Status() Status        { return a.status }
func (a *AccountAggregate) AccountType() AccountType { return a.accountType }
func (a *AccountAggregate) CreditLimit() Money    { return a.creditLimit }
func (a *AccountAggregate) CreatedAt() time.Time  { return a.createdAt }
func (a *AccountAggregate) UpdatedAt() time.Time  { return a.updatedAt }

// Deposit adds money to the account.
func (a *AccountAggregate) Deposit(amount Money) error {
    if a.status == StatusClosed {
        return ErrAccountClosed
    }
    if amount.IsNegativeOrZero() {
        return ErrNegativeAmount
    }

    newBalance, err := a.balance.Add(amount)
    if err != nil {
        return err
    }

    a.balance = newBalance
    a.updatedAt = time.Now()
    a.raise(DepositedEvent{AccountID: a.id, Amount: amount, NewBalance: a.balance})
    return nil
}

// Withdraw removes money from the account.
func (a *AccountAggregate) Withdraw(amount Money) error {
    if a.status == StatusClosed {
        return ErrAccountClosed
    }
    if a.status == StatusFrozen {
        return ErrAccountFrozen
    }
    if amount.IsNegativeOrZero() {
        return ErrNegativeAmount
    }

    newBalance, err := a.balance.Subtract(amount)
    if err != nil {
        return err
    }

    // Check balance constraints based on account type
    if a.accountType == AccountTypeStandard && newBalance.IsNegative() {
        return ErrInsufficientFunds
    }
    if a.accountType == AccountTypeCredit && newBalance.IsNegative() {
        if newBalance.Abs().GreaterThan(a.creditLimit) {
            return ErrCreditLimitExceeded
        }
    }

    a.balance = newBalance
    a.updatedAt = time.Now()
    a.raise(WithdrawnEvent{AccountID: a.id, Amount: amount, NewBalance: a.balance})
    return nil
}

// Freeze blocks debit operations on the account.
func (a *AccountAggregate) Freeze() error {
    if a.status == StatusClosed {
        return ErrAccountClosed
    }
    a.status = StatusFrozen
    a.updatedAt = time.Now()
    a.raise(AccountFrozenEvent{AccountID: a.id})
    return nil
}

// Unfreeze allows debit operations again.
func (a *AccountAggregate) Unfreeze() error {
    if a.status == StatusClosed {
        return ErrAccountClosed
    }
    a.status = StatusActive
    a.updatedAt = time.Now()
    a.raise(AccountUnfrozenEvent{AccountID: a.id})
    return nil
}

// AddHolder adds a new holder to the account.
func (a *AccountAggregate) AddHolder(holder Holder) error {
    if a.status == StatusClosed {
        return ErrAccountClosed
    }
    a.holders = append(a.holders, holder)
    a.updatedAt = time.Now()
    return nil
}

// RemoveHolder removes a holder from the account.
func (a *AccountAggregate) RemoveHolder(holderID HolderID) error {
    if a.status == StatusClosed {
        return ErrAccountClosed
    }

    // Check invariant: must keep at least one owner
    remainingOwners := 0
    for _, h := range a.holders {
        if h.ID() != holderID && h.Role() == RoleOwner {
            remainingOwners++
        }
    }
    if remainingOwners == 0 {
        return ErrCannotRemoveLastOwner
    }

    // Remove holder
    newHolders := make([]Holder, 0, len(a.holders)-1)
    for _, h := range a.holders {
        if h.ID() != holderID {
            newHolders = append(newHolders, h)
        }
    }
    a.holders = newHolders
    a.updatedAt = time.Now()
    return nil
}

// Holders returns a copy of the holders list.
func (a *AccountAggregate) Holders() []Holder {
    result := make([]Holder, len(a.holders))
    copy(result, a.holders)
    return result
}

// Events returns and clears pending domain events.
func (a *AccountAggregate) Events() []DomainEvent {
    events := a.events
    a.events = make([]DomainEvent, 0)
    return events
}

func (a *AccountAggregate) raise(event DomainEvent) {
    a.events = append(a.events, event)
}
```

### Money Value Object

```go
package accounts

import (
    "errors"
    "fmt"
)

var (
    ErrMoneyCurrencyMismatch = errors.New("cannot operate on different currencies")
    ErrMoneyCurrencyRequired = errors.New("currency is required")
    ErrMoneyCurrencyInvalid  = errors.New("currency must be 3-letter ISO code")
)

// Money represents a monetary amount with currency.
// Immutable value object.
type Money struct {
    amount   int64  // cents/minor units
    currency string // ISO 4217
}

// NewMoney creates a new Money value object.
func NewMoney(amount int64, currency string) (Money, error) {
    if currency == "" {
        return Money{}, ErrMoneyCurrencyRequired
    }
    if len(currency) != 3 {
        return Money{}, ErrMoneyCurrencyInvalid
    }
    return Money{amount: amount, currency: currency}, nil
}

// MustMoney creates Money or panics. Use in tests only.
func MustMoney(amount int64, currency string) Money {
    m, err := NewMoney(amount, currency)
    if err != nil {
        panic(err)
    }
    return m
}

// USD creates Money in US Dollars.
func USD(cents int64) Money {
    return MustMoney(cents, "USD")
}

func (m Money) Amount() int64    { return m.amount }
func (m Money) Currency() string { return m.currency }

// Add returns a new Money with the sum.
func (m Money) Add(other Money) (Money, error) {
    if m.currency != other.currency {
        return Money{}, ErrMoneyCurrencyMismatch
    }
    return Money{amount: m.amount + other.amount, currency: m.currency}, nil
}

// Subtract returns a new Money with the difference.
func (m Money) Subtract(other Money) (Money, error) {
    if m.currency != other.currency {
        return Money{}, ErrMoneyCurrencyMismatch
    }
    return Money{amount: m.amount - other.amount, currency: m.currency}, nil
}

// Multiply returns Money multiplied by factor.
func (m Money) Multiply(factor int) Money {
    return Money{amount: m.amount * int64(factor), currency: m.currency}
}

func (m Money) IsNegative() bool       { return m.amount < 0 }
func (m Money) IsNegativeOrZero() bool { return m.amount <= 0 }
func (m Money) IsPositive() bool       { return m.amount > 0 }
func (m Money) IsZero() bool           { return m.amount == 0 }

func (m Money) Abs() Money {
    if m.amount < 0 {
        return Money{amount: -m.amount, currency: m.currency}
    }
    return m
}

func (m Money) GreaterThan(other Money) bool { return m.amount > other.amount }
func (m Money) LessThan(other Money) bool    { return m.amount < other.amount }

func (m Money) Equals(other Money) bool {
    return m.amount == other.amount && m.currency == other.currency
}

func (m Money) String() string {
    sign := ""
    amount := m.amount
    if amount < 0 {
        sign = "-"
        amount = -amount
    }
    dollars := amount / 100
    cents := amount % 100
    return fmt.Sprintf("%s%d.%02d %s", sign, dollars, cents, m.currency)
}
```

### Repository Interface

```go
package accounts

import (
    "context"
    "errors"
)

var ErrAccountNotFound = errors.New("account not found")

// AccountRepository defines persistence operations for Account aggregates.
type AccountRepository interface {
    Save(ctx context.Context, account *AccountAggregate) error
    FindByID(ctx context.Context, id AccountID) (*AccountAggregate, error)
    FindByHolderUserID(ctx context.Context, userID UserID) ([]*AccountAggregate, error)
    Exists(ctx context.Context, id AccountID) (bool, error)
    Delete(ctx context.Context, id AccountID) error
}
```

---

## Phase 6: Validation Checklist

### Account Aggregate ✅

- [x] Aggregate Root is the only entry point
- [x] Child entities (Holder) accessed via aggregate methods
- [x] All changes through aggregate methods (Deposit, Withdraw, etc.)
- [x] Invariants documented in comments
- [x] Invariants checked in constructor
- [x] Invariants checked in all mutation methods
- [x] No direct references to other Aggregates (UserID is ID only)
- [x] One Aggregate = one transaction boundary

### Value Objects ✅

- [x] Money is immutable (no setters)
- [x] Money validates in constructor
- [x] Money operations return new instances
- [x] Equals compares all fields
- [x] MustXxx variants for tests

### Repository ✅

- [x] Interface in domain layer
- [x] Methods operate on aggregates
- [x] Context parameter for all methods
- [x] Domain-specific errors (ErrAccountNotFound)

### Clean Architecture ✅

- [x] Domain layer has no external dependencies
- [x] Repository interface in domain, implementation in infrastructure
- [x] Domain events for cross-aggregate communication