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Event Context

Event context provides access to metadata about the event itself, such as when it occurred, its sequence number, and correlation information.

Syntax

Section titled “Syntax”

Access event context properties using the $eventContext. prefix:

$eventContext.{property}

Available Properties

Section titled “Available Properties”

occurred

Section titled “occurred”

The timestamp when the event occurred:

from UserRegistered
  Name = name
  CreatedAt = $eventContext.occurred

sequenceNumber

Section titled “sequenceNumber”

The sequence number of the event in its stream:

from EventProcessed
  LastSequenceNumber = $eventContext.sequenceNumber

correlationId

Section titled “correlationId”

The correlation ID associated with the event:

from OrderPlaced
  OrderId = orderId
  CorrelationId = $eventContext.correlationId

eventSourceId

Section titled “eventSourceId”

The event source ID (also available as shorthand $eventSourceId):

from UserAssignedToGroup
  GroupId = $eventContext.eventSourceId
  # or
  GroupId = $eventSourceId

Event Source ID Shorthand

Section titled “Event Source ID Shorthand”

The $eventSourceId is a shorthand for $eventContext.eventSourceId:

from UserCreated key $eventSourceId
  Name = name

Common Patterns

Section titled “Common Patterns”

Audit Fields

Section titled “Audit Fields”

Track when things happen:

from RecordCreated
  Name = name
  CreatedAt = $eventContext.occurred


from RecordUpdated
  Name = name
  UpdatedAt = $eventContext.occurred

Correlation Tracking

Section titled “Correlation Tracking”

Link related operations:

from OrderPlaced
  OrderNumber = orderNumber
  CorrelationId = $eventContext.correlationId
  PlacedAt = $eventContext.occurred

Sequence Tracking

Section titled “Sequence Tracking”

Track event order:

from StateChanged
  CurrentState = state
  SequenceNumber = $eventContext.sequenceNumber
  ChangedAt = $eventContext.occurred

Parent-Child Relationships

Section titled “Parent-Child Relationships”

Use event source ID for relationships:

children members id userId
  from UserAddedToGroup key userId
    parent $eventContext.eventSourceId
    Role = role
    AddedAt = $eventContext.occurred

In Composite Keys

Section titled “In Composite Keys”

Event context can be used in composite keys:

from EventProcessed
  key ProcessingKey {
    EventId = eventId
    SequenceNumber = $eventContext.sequenceNumber
    CorrelationId = $eventContext.correlationId
  }
  ProcessedAt = $eventContext.occurred

In Templates

Section titled “In Templates”

Event context works in string templates:

from OrderShipped
  TrackingInfo = `Shipped at ${$eventContext.occurred}`
  Reference = `${orderNumber}-${$eventContext.sequenceNumber}`

Examples

Section titled “Examples”

User Activity Log

Section titled “User Activity Log”
projection UserActivity => UserActivityReadModel
  from UserLoggedIn
    LastLogin = $eventContext.occurred
    LastLoginSequence = $eventContext.sequenceNumber
    count LoginCount


  from UserAction
    LastAction = actionType
    LastActionTime = $eventContext.occurred
    LastCorrelation = $eventContext.correlationId

Versioned Document

Section titled “Versioned Document”
projection Document => DocumentReadModel
  from DocumentCreated
    Title = title
    Content = content
    Version = $eventContext.sequenceNumber
    CreatedAt = $eventContext.occurred


  from DocumentUpdated
    Content = content
    Version = $eventContext.sequenceNumber
    UpdatedAt = $eventContext.occurred

Group Membership with Audit

Section titled “Group Membership with Audit”
projection Group => GroupReadModel
  from GroupCreated
    Name = name
    CreatedAt = $eventContext.occurred
    CreatedBy = $eventContext.eventSourceId


  children members id userId
    from UserAddedToGroup key userId
      parent groupId
      Name = userName
      Role = role
      AddedAt = $eventContext.occurred
      AddedSequence = $eventContext.sequenceNumber

Order Processing

Section titled “Order Processing”
projection Order => OrderReadModel
  from OrderPlaced
    OrderNumber = orderNumber
    CustomerId = customerId
    Total = total
    PlacedAt = $eventContext.occurred
    TrackingId = $eventContext.correlationId


  from OrderShipped
    ShippedAt = $eventContext.occurred
    ShipmentSequence = $eventContext.sequenceNumber
    Status = "Shipped"

Best Practices

Section titled “Best Practices”
  1. Audit Timestamps: Use occurred for created/updated timestamps
  2. Correlation: Use correlationId to link related operations
  3. Versioning: Use sequenceNumber for version tracking
  4. Relationships: Use eventSourceId for parent-child relationships
  5. Debugging: Include correlation IDs and timestamps for troubleshooting
  6. Immutability: Event context values are immutable and reliable