Using guards to add conditions to workflow state transitions

You can define a guard within a task definition to control a state transition. A guard is a Boolean expression that operates on field values described using LUSID’s filtering syntax. A guard must evaluate to ‘true’ in order for the state transition to succeed. 

For example, let's imagine you want to model the following simple workflow in a task definition:

You might want three possible state transitions, with three possible triggers, two of which have the following conditions:

• The transition from Pending to Approved should only succeed if a value for the assignee field is provided.

• The transition from Pending to Denied should only succeed if values for the assignee and reasonForDecision fields are provided.

To enforce these conditions, you can add a guard to each of these state transitions. To do this, call the CreateTaskDefinition or UpdateTaskDefinition API and, within transitions, pass in a guard value using LUSID's filtering syntax. This portion of the API request might look like this:

"transitions": [
  {
    "fromState": "Pending",
    "toState": "Approved",
    "trigger": "grant",
    "guard": "fields['assignee'] exists"
  },
  {
    "fromState": "Pending",
    "toState": "Denied",
    "trigger": "reject",
    "guard": "fields['reasonForDecision'] exists and fields['assignee'] exists"
  }

If a state transition is attempted but the guard conditions are not met, the request produces an error.

Unique to the Workflow Service, you can also reference a task's CurrentUser, UserCreated and UserUpdated in guard conditions for state transitions. This means you can, for example, prevent a user from approving a data approval task they created themselves. See the examples below for more information.