Counters

Counters track how many times events occur or increment/decrement numeric values. Chronicle 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:

public class UserReadModel
{
    public string Name { get; set; }
    public int LoginCount { get; set; } = 0;  // Initialize to 0
    public int ProfileViews { get; set; } = 0;
}

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