# Asynchronous activity completion
There are certain scenarios when completing an activity upon completion of its function is not possible or desirable. For example, you might have an application that requires user input in order to complete the activity. You could implement the activity with a polling mechanism, but a simpler and less resource-intensive implementation is to asynchronously complete a Cadence activity.
There two parts to implementing an asynchronously completed activity:
- The activity provides the information necessary for completion from an external system and notifies the Cadence service that it is waiting for that outside callback.
- The external service calls the Cadence service to complete the activity.
The following example demonstrates the first part:
// Retrieve the activity information needed to asynchronously complete the activity. activityInfo := cadence.GetActivityInfo(ctx) taskToken := activityInfo.TaskToken // Send the taskToken to the external service that will complete the activity. ... // Return from the activity a function indicating that Cadence should wait for an async completion // message. return "", activity.ErrResultPending
The following code demonstrates how to complete the activity successfully:
// Instantiate a Cadence service client. // The same client can be used to complete or fail any number of activities. cadence.Client client = cadence.NewClient(...) // Complete the activity. client.CompleteActivity(taskToken, result, nil)
To fail the activity, you would do the following:
// Fail the activity. client.CompleteActivity(taskToken, nil, err)
Following are the parameters of the
taskToken: The value of the binary
TaskTokenfield of the
ActivityInfostruct retrieved inside the activity.
result: The return value to record for the activity. The type of this value must match the type of the return value declared by the activity function.
err: The error code to return if the activity terminates with an error.
error is not null, the value of the
result field is ignored.