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Tasks

This page discusses the following:

What is a Task?​

A Task is the context that a Worker needs to progress with a specific Workflow Execution, Activity Execution, or a Nexus Task Execution.

There are three types of Tasks:

What is a Workflow Task?​

A Workflow Task is a Task that contains the context needed to make progress with a Workflow Execution.

  • Every time a new external event that might affect a Workflow state is recorded, a Workflow Task that contains the event is added to a Task Queue and then picked up by a Workflow Worker.
  • After the new event is handled, the Workflow Task is completed with a list of Commands.
  • Handling of a Workflow Task is usually very fast and is not related to the duration of operations that the Workflow invokes.

What is a Workflow Task Execution?​

A Workflow Task Execution occurs when a Worker picks up a Workflow Task and uses it to make progress on the execution of a Workflow Definition (also known as a Workflow function).

What is an Activity Task?​

An Activity Task contains the context needed to proceed with an Activity Task Execution. Activity Tasks largely represent the Activity Task Scheduled Event, which contains the data needed to execute an Activity Function.

If Heartbeat data is being passed, an Activity Task will also contain the latest Heartbeat details.

What is an Activity Task Execution?​

An Activity Task Execution occurs when a Worker uses the context provided from the Activity Task and executes the Activity Definition (also known as the Activity Function).

The ActivityTaskScheduled Event corresponds to when the Temporal Service puts the Activity Task into the Task Queue.

The ActivityTaskStarted Event corresponds to when the Worker picks up the Activity Task from the Task Queue.

Either ActivityTaskCompleted or one of the other Closed Activity Task Events corresponds to when the Worker has yielded back to the Temporal Service.

The API to schedule an Activity Execution provides an "effectively once" experience, even though there may be several Activity Task Executions that take place to successfully complete an Activity.

Once an Activity Task finishes execution, the Worker responds to the Temporal Service with a specific Event:

  • ActivityTaskCanceled
  • ActivityTaskCompleted
  • ActivityTaskFailed
  • ActivityTaskTerminated
  • ActivityTaskTimedOut

What is a Nexus Task?​

A Nexus Task represents a single Nexus request to start or cancel a Nexus Operation. The Nexus Task includes details such as the Nexus Service and Nexus Operation names, and other information required to process the Nexus request. The Temporal Worker triggers the registered Operation handler based on the Nexus task information.

What is a Nexus Task Execution?​

A Nexus Task Execution occurs when a Worker uses the context provided from the Nexus Task and executes an action associated with a Nexus Operation which commonly includes starting a Nexus Operation using it's Nexus Operation handler plus many additional actions that may be performed on a Nexus Operation.

The NexusOperationScheduled Event corresponds to when the Temporal Service records the Workflow's intent to schedule an operation.

The NexusOperationStarted Event corresponds to when the Worker picks up the Nexus Task from the Task Queue, starts an asynchronous Nexus Operation, and returns an Operation token to the caller indicating the asynchronous Nexus Operation has started.

Either NexusOperationCompleted or one of the other Closed Nexus Operation Events corresponds to when the Nexus Operation has reached a final state due to successfully completing the operation or unsuccessfully completing the operation in the case of a failure, timeout, or cancellation.

A Nexus Operation Execution appears to the caller Workflow as a single RPC, while under the hood the Temporal Service may issue several Nexus Tasks to attempt to start the Operation. Hence, a Nexus Operation Handler implementation should be idempotent. The WorkflowRunOperation provided by the SDK leverages Workflow ID based deduplication to ensures idempotency and provide an "effectively once" experience.

A Nexus Task Execution completes when a Worker responds to the Temporal Service with either a RespondNexusTaskCompleted or RespondNexusTaskFailed call, or when the Task times out.

The Temporal Service interprets the outcome and determines whether to retry the Task or record the progress in a History Event:

  • NexusTaskCompleted
  • NexusTaskFailed