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# 通过 Amazon Braket SDK 跟踪量子任务
<a name="braket-monitor-tasks-sdk"></a>

命令 `device.run(…​)` 定义具有唯一量子任务 ID 的量子任务。您可以使用 `task.state()` 查询和跟踪状态，如以下示例所示。

 **注意**：`task = device.run()` 是一种异步操作，这意味着当系统在后台处理量子任务时，您可以继续工作。

 **检索结果** 

当您调用 `task.result()` 时，SDK 开始轮询 Amazon Braket，查看量子任务是否完成。SDK 使用您在 `.run()` 中定义的轮询参数。量子任务完成后，SDK 会从 S3 存储桶中检索结果并将其作为 `QuantumTaskResult` 对象返回。

```
# create a circuit, specify the device and run the circuit
circ = Circuit().rx(0, 0.15).ry(1, 0.2).cnot(0,2)
device = AwsDevice("arn:aws:braket:::device/quantum-simulator/amazon/sv1")
task = device.run(circ, s3_location, shots=1000)

# get ID and status of submitted task
task_id = task.id
status = task.state()
print('ID of task:', task_id)
print('Status of task:', status)
# wait for job to complete
while status != 'COMPLETED':
    status = task.state()
    print('Status:', status)
```

```
ID of task:
arn:aws:braket:us-west-2:123412341234:quantum-task/b68ae94b-1547-4d1d-aa92-1500b82c300d
Status of task: QUEUED
Status: QUEUED
Status: QUEUED
Status: QUEUED
Status: QUEUED
Status: QUEUED
Status: QUEUED
Status: QUEUED
Status: RUNNING
Status: RUNNING
Status: COMPLETED
```

 **取消量子任务** 

要取消量子任务，请调用 `cancel()` 方法，如以下示例所示。

```
# cancel quantum task
task.cancel()
status = task.state()
print('Status of task:', status)
```

```
Status of task: CANCELLING
```

 **检查元数据** 

您可以检查已完成的量子任务的元数据，如以下示例所示。

```
# get the metadata of the quantum task
metadata = task.metadata()
# example of metadata
shots = metadata['shots']
date = metadata['ResponseMetadata']['HTTPHeaders']['date']
# print example metadata
print("{} shots taken on {}.".format(shots, date))

# print name of the s3 bucket where the result is saved
results_bucket = metadata['outputS3Bucket']
print('Bucket where results are stored:', results_bucket)
# print the s3 object key (folder name)
results_object_key = metadata['outputS3Directory']
print('S3 object key:', results_object_key)

# the entire look-up string of the saved result data
look_up = 's3://'+results_bucket+'/'+results_object_key
print('S3 URI:', look_up)
```

```
1000 shots taken on Wed, 05 Aug 2020 14:44:22 GMT.
Bucket where results are stored: amazon-braket-123412341234
S3 object key: simulation-output/b68ae94b-1547-4d1d-aa92-1500b82c300d
S3 URI: s3://amazon-braket-123412341234/simulation-output/b68ae94b-1547-4d1d-aa92-1500b82c300d
```

 **检索量子任务或结果** 

如果您的内核在您提交量子任务后死亡，或者您关闭了 Notebook 或电脑，您可以用其唯一的 ARN（量子任务 ID）重建 `task` 对象。然后，您可以调用 `task.result()`，从存储结果的 S3 存储桶中获取结果。

```
from braket.aws import AwsSession, AwsQuantumTask

# restore task with unique arn
task_load = AwsQuantumTask(arn=task_id)
# retrieve the result of the task
result = task_load.result()
```