Do While
The Do While task executes a sequence of tasks as long as a given condition is true. The list of tasks is executed first before the condition is checked, even for the first iteration, just like a regular do .. while statement in programming.
When a Do While loop is executed, each task in the loop will have its taskReferenceName concatenated with __i, with i as the iteration number starting at 1. If one of the loop task fails, the Do While task status will be set as FAILED and upon retry the iteration number will restart from 1.
Task parameters
Configure these parameters for the Do While task.
| Parameter | Description | Required/ Optional |
|---|---|---|
| evaluatorType | The type of evaluator used. Supported types:
| Required. |
| loopCondition | The condition that is evaluated by the Do While task after every iteration. The expression format depends on the evaluator type:
| Required. |
| loopOver | The list of tasks to be executed as long as the condition is true. | Required. |
| inputParameters. keepLastN | The number of required iterations. On enabling this option, this value is set to 2 by default. You can also choose not to include this parameter if there are no limits on the number of iterations. | Optional. |
Task configuration
This is the task configuration for a Do While task.
{
"name": "do_while",
"taskReferenceName": "do_while_ref",
"inputParameters": {},
"type": "DO_WHILE",
"loopCondition": "", // Condition
"evaluatorType": "value-param",
"loopOver": [ // List of tasks to be executed in the loop
{//taskDefinition},
{//taskDefinition}
]
}
Task output
The Do While task will return the following parameters.
| Parameter | Description |
|---|---|
| iteration | The number of iterations. If the Do While task is in progress, iteration will show the current iteration number. If the Do While task is completed, iteration will show the final number of iterations. |
| i | The iteration number, mapped to the task references names and their output. |
In addition to the two parameters above, the output payload may also contain any state stored in loopCondition. For example, storage will exist as an output parameter if the loopCondition is if ($.LoopTask['iteration'] <= 10) {$.LoopTask.storage = 3; true } else {false}.
Example output
{
"1": {
"taskA_ref": {
"response": {
"headers": {},
"body": {},
"statusCode": 200
}
},
"taskB_ref": {
//taskBOutput
}
},
"2": {
"taskA_ref": {
"response": {
"headers": {},
"body": {},
"statusCode": 200
}
},
"taskB_ref": {
//taskBOutput
}
},
"iteration": 2
}
Accessing the Do While task output
Each time a task in the do while loop is completed, the output is saved and indexed by the iteration value. This makes it possible for the condition to check the output of a specific task iteration using $.TaskName[’iteration’]['loopTask'], replacing TaskName with the actual Do While task reference name and loopTask with the loop task reference name.
Adding a Do While task in UI
To add a Do While task:
- In your workflow, select the (+) icon and add a Do While task.
- In Script params, add the parameter that will be evaluated in the expression.
- In Loop condition, select the evaluator type and enter the loop condition.
- Value-Param—Ensure that the expression value matches the parameter key you have defined in Script params.
- ECMASCRIPT—Enter a JavaScript script.
- In your workflow, select the (+) icon to add tasks to the do while loop.
- (Optional) Set the number of required iterations by unchecking No Limits. There is no limit on the number of iterations by default.
Examples
Here are some examples for using the Do While task.
Example Do While task
{
"name": "Loop Task",
"taskReferenceName": "LoopTask",
"type": "DO_WHILE",
"inputParameters": {
"value": "${workflow.input.value}"
},
"loopCondition": "if ( ($.LoopTask['iteration'] < $.value ) || ( $.first_task['response']['body'] > 10)) { false; } else { true; }",
"loopOver": [
{
"name": "first task",
"taskReferenceName": "first_task",
"inputParameters": {
"http_request": {
"uri": "http://localhost:8082",
"method": "POST"
}
},
"type": "HTTP"
},
{
"name": "second task",
"taskReferenceName": "second_task",
"inputParameters": {
"http_request": {
"uri": "http://localhost:8082",
"method": "POST"
}
},
"type": "HTTP"
}
]
}
When executed successfully, the above Do While task will produce the following execution JSON, assuming three iterations occurred:
{
"taskType": "DO_WHILE",
"outputData": {
"iteration": 3,
"1": {
"first_task": {
"response": {},
"headers": {
"Content-Type": "application/json"
}
},
"second_task": {
"response": {},
"headers": {
"Content-Type": "application/json"
}
}
},
"2": {
"first_task": {
"response": {},
"headers": {
"Content-Type": "application/json"
}
},
"second_task": {
"response": {},
"headers": {
"Content-Type": "application/json"
}
}
},
"3": {
"first_task": {
"response": {},
"headers": {
"Content-Type": "application/json"
}
},
"second_task": {
"response": {},
"headers": {
"Content-Type": "application/json"
}
}
}
}
}
Using the iteration key in a loop task
Sometimes, you may want to use the Do While iteration value/counter inside your loop tasks. In this example, an API call is made to a GitHub repository to get all stargazers and each iteration increases the pagination.
The Do While task’s taskReferenceName is "get_all_stars_loop_ref". To evaluate the current iteration, the parameter $.get_all_stars_loop_ref['iteration'] is used in the loopCondition. In the HTTP task embedded in the loop, ${get_all_stars_loop_ref.output.iteration} is used to define which page of results the API should return.
{
"name": "get_all_stars",
"taskReferenceName": "get_all_stars_loop_ref",
"inputParameters": {
"stargazers": "4000"
},
"type": "DO_WHILE",
"loopCondition": "if ($.get_all_stars_loop_ref['iteration'] < Math.ceil($.stargazers/100)) { true; } else { false; }",
"loopOver": [
{
"name": "100_stargazers",
"taskReferenceName": "hundred_stargazers_ref",
"inputParameters": {
"counter": "${get_all_stars_loop_ref.output.iteration}",
"http_request": {
"uri": "https://api.github.com/repos/conductor-oss/conductor/stargazers?page=${get_all_stars_loop_ref.output.iteration}&per_page=100",
"method": "GET",
"headers": {
"Authorization": "token ${workflow.input.gh_token}",
"Accept": "application/vnd.github.v3.star+json"
}
}
},
"type": "HTTP",
}
]
}
Limitations
There are several limitations for the Do While task:
- Branching—Within a Do While task, branching using Switch, Fork/Join, Dynamic Fork tasks are supported. However, the Do While task may not execute as expected if the branching crosses outside its scope, since the loop tasks will be executed within the scope.
- Nested loops—Nested Do While tasks are not supported. To achieve a similar functionality as a nested do while loop, you can use a Sub Workflow task inside the Do While task.
- Isolation group execution—Isolation group execution is not supported. However, domain is supported for loop tasks inside the Do While task.
Errors
If an exception occurs during evaluation of the loopCondition, the task is set to FAILED_WITH_TERMINAL_ERROR.