Expressions

We use a simple expression engine called expr.

You can use some built in variables in your expressions. We extended the functionality by some useful functions.

Variables

• metadata: Event metadata. Possible fields:
  • metadata.id: The id of the event (string)
  • metadata.tenantId: The tenant id of the event (string)
  • metadata.stream: The stream name of the event (string)
  • metadata.type: The type name of the event (string)
  • metadata.correlationId: The tenant correlation id of the event (string)
  • metadata.causationId: The causation id of the event (string)
  • metadata.reason: The reason string of the event (string)
  • metadata.topic: The topic of the event (string)
  • metadata.raisedAt: The time when this event was raised, represented as unix timestamp (milliseconds) (int64)
• payload
  • all fields that the event payload contains
  • if your payload field is an object (not a reference to an other projection), you can access all object fields, too: payload.user.name would access the name field of the user object in the user field of the event payload
• projection
  • all fields that the event projection contains
  • the projection variable contains the state of the projection before the event is applied to the projection and will therefore be empty for the first event that generates a new projection data entry
  • if your projection field is an object (not a reference to an other projection), you can access all object fields, too: payload.user.name would access the name field of the user object in the user field of the projection
• now: current unix timestamp (int value)
• second: number of milliseconds in a second
• minute: number of milliseconds in a minute
• hour: number of milliseconds in an hour
• day: number of milliseconds in a day
• week: number of milliseconds in a week
• month: number of milliseconds in a 30 day month
• year: number of milliseconds in a 365 day year

Functions

• append(array, value): appends the value to the array
• intSum(arrayOfInts): calculates the sum of all elements in the arrayOfInts (requires actual int values in the array, you can make use of intSum(map(arrayOfNumbers, {int(#)})) to convert all values to integers)

Conditions

You can use ternaries to evaluate results based on conditions:

user.isOnline ? user.name : "offline";

This expression would return the username in case the user is online and return "offline" for all users that are not online.

Language Definition

Please have a look at the language definition of the expression engine that we use to evaluate expressions.