# Setting task priority in Amazon SWF
<a name="programming-priority"></a>

By default, tasks on a task list are delivered based upon their *arrival time*: tasks that are scheduled first are generally run first, as far as possible. By setting an optional *task priority*, you can give priority to certain tasks: Amazon SWF will attempt to deliver higher-priority tasks on a task list before those with lower priority.

**Note**  
Tasks that are scheduled first generally run first, but this is not guaranteed.

You can set task priorities for both workflows and activities. A workflow's task priority doesn't affect the priority of any activity tasks it schedules, nor does it affect any child workflows it starts. The default priority for an activity or workflow is set (either by you or by Amazon SWF) during registration, and the registered task priority is always used unless it is overridden while scheduling the activity or starting a workflow execution.

Task priority values can range from "-2147483648" to "2147483647", with higher numbers indicating higher priority. If you don't set the task priority for an activity or workflow, it will be assigned a priority of zero ("0").

**Topics**
+ [Setting Task Priority for Workflows](#task-priority-workflows)
+ [Setting Task Priority for Activities](#task-priority-activities)
+ [Actions that Return Task Priority Information](#task-priority-responses)

## Setting Task Priority for Workflows
<a name="task-priority-workflows"></a>

You can set the task priority for a workflow when you register it or start it. The task priority that is set when the workflow type is registered is used as the default for any workflow executions of that type, unless it is overridden when starting the workflow execution.

To register a workflow type with a default task priority, set the *defaultTaskPriority* option when using the [RegisterWorkflowType](https://docs.aws.amazon.com/amazonswf/latest/apireference/API_RegisterWorkflowType.html) action:

```
{
  "domain": "867530901",
  "name": "expeditedOrderWorkflow",
  "version": "1.0",
  "description": "Expedited customer orders workflow",
  "defaultTaskStartToCloseTimeout": "600",
  "defaultExecutionStartToCloseTimeout": "3600",
  "defaultTaskList": {"name": "mainTaskList"},
  "defaultTaskPriority": "10",
  "defaultChildPolicy": "TERMINATE"
}
```

You can override a workflow type's registered task priority when you start a workflow execution with [StartWorkflowExecution](https://docs.aws.amazon.com/amazonswf/latest/apireference/API_StartWorkflowExecution.html):

```
{
  "childPolicy": "TERMINATE",
  "domain": "867530901",
  "executionStartToCloseTimeout": "1800",
  "input": "arbitrary-string-that-is-meaningful-to-the-workflow",
  "tagList": ["music purchase", "digital", "ricoh-the-dog"],
  "taskList": {"name": "specialTaskList"},
  "taskPriority": "-20",
  "taskStartToCloseTimeout": "600",
  "workflowId": "20110927-T-1",
  "workflowType": {"name": "customerOrderWorkflow", "version": "1.0"},
}
```

You can also override the registered task priority when starting a child workflow or when continuing a workflow as new, such as when responding to a decision with [RespondDecisionTaskCompleted](https://docs.aws.amazon.com/amazonswf/latest/apireference/API_RespondDecisionTaskCompleted.html).

To set a child workflow's task priority, provide the value in `startChildWorkflowExecutionDecisionAttributes`:

```
{
  "taskToken": "AAAAKgAAAAEAAAAAAAAAA...",
  "decisions": [
    {
      "decisionType": "StartChildWorkflowExecution",
      "startChildWorkflowExecutionDecisionAttributes": {
        "childPolicy": "TERMINATE",
        "control": "digital music",
        "executionStartToCloseTimeout": "900",
        "input": "201412-Smith-011x",
        "taskList": {"name": "specialTaskList"},
        "taskPriority": "5",
        "taskStartToCloseTimeout": "600",
        "workflowId": "verification-workflow",
        "workflowType": {
          "name": "MyChildWorkflow",
          "version": "1.0"
        }
      }
    }
  ]
}
```

When continuing a workflow as new, set the task priority in `continueAsNewWorkflowExecutionDecisionAttributes`:

```
{
  "taskToken": "AAAAKgAAAAEAAAAAAAAAA...",
  "decisions": [
    {
      "decisionType": "ContinueAsNewWorkflowExecution",
      "continueAsNewWorkflowExecutionDecisionAttributes": {
        "childPolicy": "TERMINATE",
        "executionStartToCloseTimeout": "1800",
        "input": "5634-0056-4367-0923,12/12,437",
        "taskList": {"name": "specialTaskList"},
        "taskStartToCloseTimeout": "600",
        "taskPriority": "100",
        "workflowTypeVersion": "1.0"
      }
    }
  ]
}
```

## Setting Task Priority for Activities
<a name="task-priority-activities"></a>

You can set the task priority for an activity either when registering it or when scheduling it. The task priority that is set when registering an activity type is used as the default priority when the activity is run, unless it is overridden when scheduling the activity.

To set task priority when registering an activity type, set the *defaultTaskPriority* option when using the [RegisterActivityType](https://docs.aws.amazon.com/amazonswf/latest/apireference/API_RegisterActivityType.html) action:

```
{
  "defaultTaskHeartbeatTimeout": "120",
  "defaultTaskList": {"name": "mainTaskList"},
  "defaultTaskPriority": "10",
  "defaultTaskScheduleToCloseTimeout": "900",
  "defaultTaskScheduleToStartTimeout": "300",
  "defaultTaskStartToCloseTimeout": "600",
  "description": "Verify the customer credit card",
  "domain": "867530901",
  "name": "activityVerify",
  "version": "1.0"
}
```

To schedule a task with a task priority, use the *taskPriority* option when scheduling the activity with the [RespondDecisionTaskCompleted](https://docs.aws.amazon.com/amazonswf/latest/apireference/API_RespondDecisionTaskCompleted.html) action:

```
{
  "taskToken": "AAAAKgAAAAEAAAAAAAAAA...",
  "decisions": [
    {
      "decisionType": "ScheduleActivityTask",
      "scheduleActivityTaskDecisionAttributes": {
        "activityId": "verify-account",
        "activityType": {
            "name": "activityVerify",
            "version": "1.0"
        },
        "control": "digital music",
        "input": "abab-101",
        "taskList": {"name": "mainTaskList"},
        "taskPriority": "15"
      }
    }
  ]
}
```

## Actions that Return Task Priority Information
<a name="task-priority-responses"></a>

You can get information about the set task priority (or set default task priority) from the following Amazon SWF actions:
+ [DescribeActivityType](https://docs.aws.amazon.com/amazonswf/latest/apireference/API_DescribeActivityType.html) returns the *defaultTaskPriority* of the activity type in the `configuration` section of the response.
+ [DescribeWorkflowExecution](https://docs.aws.amazon.com/amazonswf/latest/apireference/API_DescribeWorkflowExecution.html) returns the *taskPriority* of the workflow execution in the `executionConfiguration` section of the response.
+ [DescribeWorkflowType](https://docs.aws.amazon.com/amazonswf/latest/apireference/API_DescribeWorkflowType.html) returns the *defaultTaskPriority* of the workflow type in the `configuration` section of the response.
+ [GetWorkflowExecutionHistory](https://docs.aws.amazon.com/amazonswf/latest/apireference/API_GetWorkflowExecutionHistory.html) and [PollForDecisionTask](https://docs.aws.amazon.com/amazonswf/latest/apireference/API_PollForDecisionTask.html) provide task priority information in the `activityTaskScheduledEventAttributes`, `decisionTaskScheduledEventAttributes`, `workflowExecutionContinuedAsNewEventAttributes`, and `workflowExecutionStartedEventAttributes` sections of the response.