Arithmetic Operations

Arithmetic operations allow you to add or subtract values from numeric properties using values from events. Unlike counters that always change by 1, arithmetic operations use variable amounts.

Add

The add operation increases a property by a specified amount:

add {Property} by {expression}

Example

from PaymentReceived
  add Balance by amount
  LastPayment = $eventContext.occurred

Subtract

The subtract operation decreases a property by a specified amount:

subtract {Property} by {expression}

Example

from WithdrawalMade
  subtract Balance by amount
  LastWithdrawal = $eventContext.occurred

Expression Values

The expression after by can be:

• Event property: amount, value, cost.total
• Literal: 100, 10.50
• Template: `${baseAmount}`

Multiple Operations

Combine multiple operations in one event:

from OrderPlaced
  add TotalRevenue by total
  add OrderCount by 1
  increment TotalOrders

Examples

Account Balance

projection Account => AccountReadModel
  from AccountOpened
    AccountNumber = accountNumber
    Balance = initialBalance

  from DepositMade
    add Balance by amount
    LastDeposit = $eventContext.occurred

  from WithdrawalMade
    subtract Balance by amount
    LastWithdrawal = $eventContext.occurred

  from InterestApplied
    add Balance by interestAmount

Loyalty Points

projection Customer => CustomerReadModel
  from CustomerRegistered
    Name = name
    Points = 0

  from PurchaseMade
    add Points by pointsEarned

  from PointsRedeemed
    subtract Points by pointsUsed
    LastRedemption = $eventContext.occurred

Inventory with Variable Quantities

projection Product => ProductReadModel
  from ProductCreated
    Name = name
    StockLevel = initialStock

  from StockAdded
    add StockLevel by quantity
    LastRestocked = $eventContext.occurred

  from StockSold
    subtract StockLevel by quantity

  from StockAdjusted
    add StockLevel by adjustmentAmount

Budget Tracking

projection Budget => BudgetReadModel
  from BudgetCreated
    Category = category
    AllocatedAmount = amount
    RemainingAmount = amount

  from ExpenseRecorded
    subtract RemainingAmount by cost
    add TotalSpent by cost

  from BudgetIncreased
    add AllocatedAmount by additionalAmount
    add RemainingAmount by additionalAmount

Order Totals

projection Order => OrderReadModel
  from OrderPlaced
    OrderNumber = orderNumber
    Subtotal = 0
    Tax = 0
    Total = 0

  from LineItemAdded
    add Subtotal by itemTotal
    add Total by itemTotal

  from TaxCalculated
    add Tax by taxAmount
    add Total by taxAmount

  from DiscountApplied
    subtract Subtotal by discountAmount
    subtract Total by discountAmount

Gaming Score

projection PlayerScore => PlayerScoreReadModel
  from PlayerJoined
    PlayerName = name
    Score = 0

  from PointsEarned
    add Score by points
    LastScored = $eventContext.occurred

  from PointsLost
    subtract Score by points

  from BonusAwarded
    add Score by bonusPoints
    add BonusTotal by bonusPoints

Nested Properties

You can use nested properties from events:

from TransactionProcessed
  add Balance by transaction.amount
  add TotalTransactions by 1

With Counters

Combine arithmetic with counters for comprehensive tracking:

from SaleCompleted
  add TotalRevenue by saleAmount
  add TaxCollected by taxAmount
  increment SalesCount
  count CompletedTransactions

Property Requirements

The target property must be a numeric type:

• int
• long
• decimal
• double
• float

The expression value must be compatible with the target type.

Error Handling

Chronicle handles arithmetic operations safely:

• Type mismatches result in compilation errors
• Ensure the event property used exists and is numeric
• Consider domain rules (e.g., balance can’t go negative) in your application logic

Best Practices

1. Initialize Values: Ensure numeric properties have initial values
2. Use Appropriate Types: Use decimal for money, int for counts
3. Track Both Directions: Pair add with corresponding subtract for bidirectional operations
4. Audit Trail: Combine with timestamps and counters for complete tracking
5. Validation: Consider adding validation in your domain to prevent invalid states
6. Precision: Be aware of floating-point precision issues with double and float

Difference from Counters

Operation	Amount	Use Case
count / increment / decrement	Always ±1	Event counting, simple state changes
add / subtract	Variable from event	Balances, quantities, scores

See Also

• Counters - Fixed increment/decrement operations
• Property Mapping - Setting properties to specific values
• Expressions - Understanding expression syntax