Counters

Counters track how many times events occur or increment/decrement numeric values. The language provides three counter operations: count, increment, and decrement.

Count

The count operation increments a property by 1 each time an event occurs:

count {Property}

Example

from PageViewed
  count ViewCount
  LastViewedAt = $eventContext.occurred

Each PageViewed event increases ViewCount by 1.

Increment

The increment operation increases a property by 1:

increment {Property}

Example

from UserLoggedIn
  increment LoginCount
  LastLogin = $eventContext.occurred

Decrement

The decrement operation decreases a property by 1:

decrement {Property}

Example

from ItemConsumed
  decrement RemainingStock
  LastConsumed = $eventContext.occurred

Difference Between Count and Increment

While count and increment both add 1, they have semantic differences:

• count: Counts occurrences of a specific event (conceptually counting events)
• increment: Increases a value (conceptually modifying state)

In practice, they behave identically but express different intent.

Multiple Counters

You can use multiple counter operations in the same event:

from OrderPlaced
  increment TotalOrders
  count OrderCount
  add TotalRevenue by total

Examples

User Activity Tracking

projection User => UserReadModel
  from UserRegistered
    Name = name
    Email = email

  from UserLoggedIn
    count LoginCount
    LastLogin = $eventContext.occurred

  from UserProfileViewed
    increment ProfileViewCount

Inventory Management

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

  from ItemAdded
    increment StockLevel

  from ItemSold
    decrement StockLevel
    count SalesCount

  from ItemReturned
    increment StockLevel
    decrement SalesCount

Blog Post Engagement

projection BlogPost => BlogPostReadModel
  from PostPublished
    Title = title
    Content = content

  from PostViewed
    count ViewCount

  from PostLiked
    increment LikeCount

  from PostUnliked
    decrement LikeCount

  from CommentAdded
    count CommentCount

Account Balance (Alternative to Arithmetic)

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

  from DepositMade
    increment TransactionCount
    add Balance by amount

  from WithdrawalMade
    increment TransactionCount
    subtract Balance by amount

Forum User Reputation

projection ForumUser => ForumUserReadModel
  from UserJoined
    Username = username

  from QuestionPosted
    count QuestionsPosted

  from AnswerPosted
    count AnswersPosted

  from UpvoteReceived
    increment Reputation

  from DownvoteReceived
    decrement Reputation

Task Completion Tracking

projection Project => ProjectReadModel
  from ProjectCreated
    Name = name

  from TaskAdded
    increment TotalTasks

  from TaskCompleted
    increment CompletedTasks
    decrement RemainingTasks

  from TaskAdded
    increment RemainingTasks

Property Requirements

The target property must be a numeric type:

• int
• long
• decimal
• double
• float

Attempting to use counters on non-numeric properties will result in a validation error.

Initial Values

Counter properties should have an initial value. Declare counter targets such as LoginCount or ProfileViews as numeric properties on the read model, starting at 0, so the first counter operation produces 1.

Best Practices

1. Initialize to Zero: Ensure counter properties start at 0 in the read model
2. Use Count for Events: Use count when tracking event occurrences
3. Use Increment/Decrement for State: Use increment/decrement for state changes
4. Combine with Timestamps: Pair counters with timestamps for richer tracking
5. Validate Decrements: Ensure decrements won’t result in negative values if that’s not valid for your domain
6. Consider Arithmetic: For more complex numeric operations, use Arithmetic operations

See Also

• Arithmetic - Add and subtract operations with variable amounts
• Property Mapping - Setting properties to specific values