com.amazonaws.services.applicationautoscaling

Interface AWSApplicationAutoScaling

All Known Subinterfaces:

AWSApplicationAutoScalingAsync

All Known Implementing Classes:

AbstractAWSApplicationAutoScaling, AbstractAWSApplicationAutoScalingAsync, AWSApplicationAutoScalingAsyncClient, AWSApplicationAutoScalingClient

@Generated(value="com.amazonaws:aws-java-sdk-code-generator")
public interface AWSApplicationAutoScaling

Interface for accessing Application Auto Scaling.

Note: Do not directly implement this interface, new methods are added to it regularly. Extend from AbstractAWSApplicationAutoScaling instead.

With Application Auto Scaling, you can automatically scale your AWS resources. The experience is similar to that of Auto Scaling. You can use Application Auto Scaling to accomplish the following tasks:

• Define scaling policies to automatically scale your AWS resources

• Scale your resources in response to CloudWatch alarms

• View the history of your scaling events

Application Auto Scaling can scale the following AWS resources:

• Amazon ECS services. For more information, see Service Auto Scaling in the Amazon EC2 Container Service Developer Guide.

• Amazon EC2 Spot fleets. For more information, see Automatic Scaling for Spot Fleet in the Amazon EC2 User Guide.

• Amazon EMR clusters. For more information, see Using Automatic Scaling in Amazon EMR in the Amazon EMR Management Guide.

• AppStream 2.0 fleets. For more information, see Fleet Auto Scaling for Amazon AppStream 2.0 in the Amazon AppStream 2.0 Developer Guide.

• Provisioned read and write capacity for Amazon DynamoDB tables and global secondary indexes. For more information, see Managing Throughput Capacity Automatically with DynamoDB Auto Scaling in the Amazon DynamoDB Developer Guide.

For a list of supported regions, see AWS Regions and Endpoints: Application Auto Scaling in the AWS General Reference.

Field Summary

Fields

Modifier and Type	Field and Description
static String	ENDPOINT_PREFIX The region metadata service name for computing region endpoints.

Method Summary

Modifier and Type	Method and Description
DeleteScalingPolicyResult	deleteScalingPolicy(DeleteScalingPolicyRequest deleteScalingPolicyRequest) Deletes the specified Application Auto Scaling scaling policy.
DeleteScheduledActionResult	deleteScheduledAction(DeleteScheduledActionRequest deleteScheduledActionRequest) Deletes the specified Application Auto Scaling scheduled action.
DeregisterScalableTargetResult	deregisterScalableTarget(DeregisterScalableTargetRequest deregisterScalableTargetRequest) Deregisters a scalable target.
DescribeScalableTargetsResult	describeScalableTargets(DescribeScalableTargetsRequest describeScalableTargetsRequest) Provides descriptive information about the scalable targets in the specified namespace.
DescribeScalingActivitiesResult	describeScalingActivities(DescribeScalingActivitiesRequest describeScalingActivitiesRequest) Provides descriptive information about the scaling activities in the specified namespace from the previous six weeks.
DescribeScalingPoliciesResult	describeScalingPolicies(DescribeScalingPoliciesRequest describeScalingPoliciesRequest) Describes the scaling policies for the specified service namespace.
DescribeScheduledActionsResult	describeScheduledActions(DescribeScheduledActionsRequest describeScheduledActionsRequest) Describes the scheduled actions for the specified service namespace.
ResponseMetadata	getCachedResponseMetadata(AmazonWebServiceRequest request) Returns additional metadata for a previously executed successful request, typically used for debugging issues where a service isn't acting as expected.
PutScalingPolicyResult	putScalingPolicy(PutScalingPolicyRequest putScalingPolicyRequest) Creates or updates a policy for an Application Auto Scaling scalable target.
PutScheduledActionResult	putScheduledAction(PutScheduledActionRequest putScheduledActionRequest) Creates or updates a scheduled action for an Application Auto Scaling scalable target.
RegisterScalableTargetResult	registerScalableTarget(RegisterScalableTargetRequest registerScalableTargetRequest) Registers or updates a scalable target.
void	setEndpoint(String endpoint) Deprecated. use AwsClientBuilder#setEndpointConfiguration(AwsClientBuilder.EndpointConfiguration) for example: builder.setEndpointConfiguration(new EndpointConfiguration(endpoint, signingRegion));
void	setRegion(Region region) Deprecated. use AwsClientBuilder#setRegion(String)
void	shutdown() Shuts down this client object, releasing any resources that might be held open.

Field Detail

ENDPOINT_PREFIX

static final String ENDPOINT_PREFIX

The region metadata service name for computing region endpoints. You can use this value to retrieve metadata (such as supported regions) of the service.

See Also:

RegionUtils.getRegionsForService(String), Constant Field Values

Method Detail

setEndpoint

@Deprecated
void setEndpoint(String endpoint)

Deprecated. use AwsClientBuilder#setEndpointConfiguration(AwsClientBuilder.EndpointConfiguration) for example: builder.setEndpointConfiguration(new EndpointConfiguration(endpoint, signingRegion));

Overrides the default endpoint for this client ("https://autoscaling.us-east-1.amazonaws.com"). Callers can use this method to control which AWS region they want to work with.

Callers can pass in just the endpoint (ex: "autoscaling.us-east-1.amazonaws.com") or a full URL, including the protocol (ex: "https://autoscaling.us-east-1.amazonaws.com"). If the protocol is not specified here, the default protocol from this client's ClientConfiguration will be used, which by default is HTTPS.

For more information on using AWS regions with the AWS SDK for Java, and a complete list of all available endpoints for all AWS services, see: http://developer.amazonwebservices.com/connect/entry.jspa?externalID=3912

This method is not threadsafe. An endpoint should be configured when the client is created and before any service requests are made. Changing it afterwards creates inevitable race conditions for any service requests in transit or retrying.

Parameters:

endpoint - The endpoint (ex: "autoscaling.us-east-1.amazonaws.com") or a full URL, including the protocol (ex: "https://autoscaling.us-east-1.amazonaws.com") of the region specific AWS endpoint this client will communicate with.

setRegion

@Deprecated
void setRegion(Region region)

Deprecated. use AwsClientBuilder#setRegion(String)

An alternative to setEndpoint(String), sets the regional endpoint for this client's service calls. Callers can use this method to control which AWS region they want to work with.

By default, all service endpoints in all regions use the https protocol. To use http instead, specify it in the ClientConfiguration supplied at construction.

This method is not threadsafe. A region should be configured when the client is created and before any service requests are made. Changing it afterwards creates inevitable race conditions for any service requests in transit or retrying.

Parameters:

region - The region this client will communicate with. See Region.getRegion(com.amazonaws.regions.Regions) for accessing a given region. Must not be null and must be a region where the service is available.

See Also:

Region.getRegion(com.amazonaws.regions.Regions), Region.createClient(Class, com.amazonaws.auth.AWSCredentialsProvider, ClientConfiguration), Region.isServiceSupported(String)

deleteScalingPolicy

DeleteScalingPolicyResult deleteScalingPolicy(DeleteScalingPolicyRequest deleteScalingPolicyRequest)

Deletes the specified Application Auto Scaling scaling policy.

Deleting a policy deletes the underlying alarm action, but does not delete the CloudWatch alarm associated with the scaling policy, even if it no longer has an associated action.

To create a scaling policy or update an existing one, see PutScalingPolicy.

Parameters:

deleteScalingPolicyRequest -

Returns:

Result of the DeleteScalingPolicy operation returned by the service.

Throws:

ValidationException - An exception was thrown for a validation issue. Review the available parameters for the API request.

ObjectNotFoundException - The specified object could not be found. For any Put or Register API operation, which depends on the existence of a scalable target, this exception is thrown if the scalable target with the specified service namespace, resource ID, and scalable dimension does not exist. For any Delete or Deregister API operation, this exception is thrown if the resource that is to be deleted or deregistered cannot be found.

ConcurrentUpdateException - Concurrent updates caused an exception, for example, if you request an update to an Application Auto Scaling resource that already has a pending update.

InternalServiceException - The service encountered an internal error.

See Also:

AWS API Documentation

Samples:

• To delete a scaling policy

  This example deletes a scaling policy for the Amazon ECS service called web-app, which is running in the default cluster.
  

  AWSApplicationAutoScaling client = new AWSApplicationAutoScalingClient();
  DeleteScalingPolicyRequest request = new DeleteScalingPolicyRequest()
          .withPolicyName("web-app-cpu-lt-25")
          .withResourceId("service/default/web-app")
          .withScalableDimension("ecs:service:DesiredCount")
          .withServiceNamespace("ecs");
  DeleteScalingPolicyResult response = client
          .deleteScalingPolicy(request);

deleteScheduledAction

DeleteScheduledActionResult deleteScheduledAction(DeleteScheduledActionRequest deleteScheduledActionRequest)

Deletes the specified Application Auto Scaling scheduled action.

Parameters:

deleteScheduledActionRequest -

Returns:

Result of the DeleteScheduledAction operation returned by the service.

Throws:

ValidationException - An exception was thrown for a validation issue. Review the available parameters for the API request.

ObjectNotFoundException - The specified object could not be found. For any Put or Register API operation, which depends on the existence of a scalable target, this exception is thrown if the scalable target with the specified service namespace, resource ID, and scalable dimension does not exist. For any Delete or Deregister API operation, this exception is thrown if the resource that is to be deleted or deregistered cannot be found.

ConcurrentUpdateException - Concurrent updates caused an exception, for example, if you request an update to an Application Auto Scaling resource that already has a pending update.

InternalServiceException - The service encountered an internal error.

See Also:

AWS API Documentation

deregisterScalableTarget

DeregisterScalableTargetResult deregisterScalableTarget(DeregisterScalableTargetRequest deregisterScalableTargetRequest)

Deregisters a scalable target.

Deregistering a scalable target deletes the scaling policies that are associated with it.

To create a scalable target or update an existing one, see RegisterScalableTarget.

Parameters:

deregisterScalableTargetRequest -

Returns:

Result of the DeregisterScalableTarget operation returned by the service.

Throws:

ValidationException - An exception was thrown for a validation issue. Review the available parameters for the API request.

ObjectNotFoundException - The specified object could not be found. For any Put or Register API operation, which depends on the existence of a scalable target, this exception is thrown if the scalable target with the specified service namespace, resource ID, and scalable dimension does not exist. For any Delete or Deregister API operation, this exception is thrown if the resource that is to be deleted or deregistered cannot be found.

ConcurrentUpdateException - Concurrent updates caused an exception, for example, if you request an update to an Application Auto Scaling resource that already has a pending update.

InternalServiceException - The service encountered an internal error.

See Also:

AWS API Documentation

Samples:

• To deregister a scalable target

  This example deregisters a scalable target for an Amazon ECS service called web-app that is running in the default cluster.
  

  AWSApplicationAutoScaling client = new AWSApplicationAutoScalingClient();
  DeregisterScalableTargetRequest request = new DeregisterScalableTargetRequest()
          .withResourceId("service/default/web-app")
          .withScalableDimension("ecs:service:DesiredCount")
          .withServiceNamespace("ecs");
  DeregisterScalableTargetResult response = client
          .deregisterScalableTarget(request);

describeScalableTargets

DescribeScalableTargetsResult describeScalableTargets(DescribeScalableTargetsRequest describeScalableTargetsRequest)

Provides descriptive information about the scalable targets in the specified namespace.

You can filter the results using the ResourceIds and ScalableDimension parameters.

To create a scalable target or update an existing one, see RegisterScalableTarget. If you are no longer using a scalable target, you can deregister it using DeregisterScalableTarget.

Parameters:

describeScalableTargetsRequest -

Returns:

Result of the DescribeScalableTargets operation returned by the service.

Throws:

ValidationException - An exception was thrown for a validation issue. Review the available parameters for the API request.

InvalidNextTokenException - The next token supplied was invalid.

ConcurrentUpdateException - Concurrent updates caused an exception, for example, if you request an update to an Application Auto Scaling resource that already has a pending update.

InternalServiceException - The service encountered an internal error.

See Also:

AWS API Documentation

Samples:

• To describe scalable targets

  This example describes the scalable targets for the ecs service namespace.
  

  AWSApplicationAutoScaling client = new AWSApplicationAutoScalingClient();
  DescribeScalableTargetsRequest request = new DescribeScalableTargetsRequest()
          .withServiceNamespace("ecs");
  DescribeScalableTargetsResult response = client
          .describeScalableTargets(request);

describeScalingActivities

DescribeScalingActivitiesResult describeScalingActivities(DescribeScalingActivitiesRequest describeScalingActivitiesRequest)

Provides descriptive information about the scaling activities in the specified namespace from the previous six weeks.

You can filter the results using the ResourceId and ScalableDimension parameters.

Scaling activities are triggered by CloudWatch alarms that are associated with scaling policies. To view the scaling policies for a service namespace, see DescribeScalingPolicies. To create a scaling policy or update an existing one, see PutScalingPolicy.

Parameters:

describeScalingActivitiesRequest -

Returns:

Result of the DescribeScalingActivities operation returned by the service.

Throws:

ValidationException - An exception was thrown for a validation issue. Review the available parameters for the API request.

InvalidNextTokenException - The next token supplied was invalid.

ConcurrentUpdateException - Concurrent updates caused an exception, for example, if you request an update to an Application Auto Scaling resource that already has a pending update.

InternalServiceException - The service encountered an internal error.

See Also:

AWS API Documentation

Samples:

• To describe scaling activities for a scalable target

  This example describes the scaling activities for an Amazon ECS service called web-app that is running in the default cluster.
  

  AWSApplicationAutoScaling client = new AWSApplicationAutoScalingClient();
  DescribeScalingActivitiesRequest request = new DescribeScalingActivitiesRequest()
          .withResourceId("service/default/web-app")
          .withScalableDimension("ecs:service:DesiredCount")
          .withServiceNamespace("ecs");
  DescribeScalingActivitiesResult response = client
          .describeScalingActivities(request);

describeScalingPolicies

DescribeScalingPoliciesResult describeScalingPolicies(DescribeScalingPoliciesRequest describeScalingPoliciesRequest)

Describes the scaling policies for the specified service namespace.

You can filter the results using the ResourceId, ScalableDimension, and PolicyNames parameters.

To create a scaling policy or update an existing one, see PutScalingPolicy. If you are no longer using a scaling policy, you can delete it using DeleteScalingPolicy.

Parameters:

describeScalingPoliciesRequest -

Returns:

Result of the DescribeScalingPolicies operation returned by the service.

Throws:

ValidationException - An exception was thrown for a validation issue. Review the available parameters for the API request.

FailedResourceAccessException - Failed access to resources caused an exception. This exception is thrown when Application Auto Scaling is unable to retrieve the alarms associated with a scaling policy due to a client error, for example, if the role ARN specified for a scalable target does not have permission to call the CloudWatch DescribeAlarms API operation on behalf of your account.

InvalidNextTokenException - The next token supplied was invalid.

ConcurrentUpdateException - Concurrent updates caused an exception, for example, if you request an update to an Application Auto Scaling resource that already has a pending update.

InternalServiceException - The service encountered an internal error.

See Also:

AWS API Documentation

Samples:

• To describe scaling policies

  This example describes the scaling policies for the ecs service namespace.
  

  AWSApplicationAutoScaling client = new AWSApplicationAutoScalingClient();
  DescribeScalingPoliciesRequest request = new DescribeScalingPoliciesRequest()
          .withServiceNamespace("ecs");
  DescribeScalingPoliciesResult response = client
          .describeScalingPolicies(request);

describeScheduledActions

DescribeScheduledActionsResult describeScheduledActions(DescribeScheduledActionsRequest describeScheduledActionsRequest)

Describes the scheduled actions for the specified service namespace.

You can filter the results using the ResourceId, ScalableDimension, and ScheduledActionNames parameters.

To create a scheduled action or update an existing one, see PutScheduledAction. If you are no longer using a scheduled action, you can delete it using DeleteScheduledAction.

Parameters:

describeScheduledActionsRequest -

Returns:

Result of the DescribeScheduledActions operation returned by the service.

Throws:

ValidationException - An exception was thrown for a validation issue. Review the available parameters for the API request.

InvalidNextTokenException - The next token supplied was invalid.

ConcurrentUpdateException - Concurrent updates caused an exception, for example, if you request an update to an Application Auto Scaling resource that already has a pending update.

InternalServiceException - The service encountered an internal error.

See Also:

AWS API Documentation

putScalingPolicy

PutScalingPolicyResult putScalingPolicy(PutScalingPolicyRequest putScalingPolicyRequest)

Creates or updates a policy for an Application Auto Scaling scalable target.

Each scalable target is identified by a service namespace, resource ID, and scalable dimension. A scaling policy applies to the scalable target identified by those three attributes. You cannot create a scaling policy without first registering a scalable target using RegisterScalableTarget.

To update a policy, specify its policy name and the parameters that you want to change. Any parameters that you don't specify are not changed by this update request.

You can view the scaling policies for a service namespace using DescribeScalingPolicies. If you are no longer using a scaling policy, you can delete it using DeleteScalingPolicy.

Parameters:

putScalingPolicyRequest -

Returns:

Result of the PutScalingPolicy operation returned by the service.

Throws:

ValidationException - An exception was thrown for a validation issue. Review the available parameters for the API request.

LimitExceededException - Your account exceeded a limit. This exception is thrown when a per-account resource limit is exceeded. For more information, see Application Auto Scaling Limits.

ObjectNotFoundException - The specified object could not be found. For any Put or Register API operation, which depends on the existence of a scalable target, this exception is thrown if the scalable target with the specified service namespace, resource ID, and scalable dimension does not exist. For any Delete or Deregister API operation, this exception is thrown if the resource that is to be deleted or deregistered cannot be found.

ConcurrentUpdateException - Concurrent updates caused an exception, for example, if you request an update to an Application Auto Scaling resource that already has a pending update.

FailedResourceAccessException - Failed access to resources caused an exception. This exception is thrown when Application Auto Scaling is unable to retrieve the alarms associated with a scaling policy due to a client error, for example, if the role ARN specified for a scalable target does not have permission to call the CloudWatch DescribeAlarms API operation on behalf of your account.

InternalServiceException - The service encountered an internal error.

See Also:

AWS API Documentation

Samples:

• To apply a scaling policy to an Amazon EC2 Spot fleet

  This example applies a scaling policy to an Amazon EC2 Spot fleet. The policy increases the target capacity of the spot fleet by 200%, with a cool down period of 180 seconds.",
  

  AWSApplicationAutoScaling client = new AWSApplicationAutoScalingClient();
  PutScalingPolicyRequest request = new PutScalingPolicyRequest()
          .withPolicyName("web-app-cpu-gt-75")
          .withPolicyType("StepScaling")
          .withResourceId("service/default/web-app")
          .withScalableDimension("ecs:service:DesiredCount")
          .withServiceNamespace("ecs")
          .withStepScalingPolicyConfiguration(
                  new StepScalingPolicyConfiguration()
                          .withAdjustmentType("PercentChangeInCapacity")
                          .withCooldown(60)
                          .withStepAdjustments(
                                  new StepAdjustment()
                                          .withMetricIntervalLowerBound(
                                                  0d)
                                          .withScalingAdjustment(200)));
  PutScalingPolicyResult response = client.putScalingPolicy(request);
  AWSApplicationAutoScaling client = new AWSApplicationAutoScalingClient();
  PutScalingPolicyRequest request = new PutScalingPolicyRequest()
          .withPolicyName("fleet-cpu-gt-75")
          .withPolicyType("StepScaling")
          .withResourceId(
                  "spot-fleet-request/sfr-45e69d8a-be48-4539-bbf3-3464e99c50c3")
          .withScalableDimension("ec2:spot-fleet-request:TargetCapacity")
          .withServiceNamespace("ec2")
          .withStepScalingPolicyConfiguration(
                  new StepScalingPolicyConfiguration()
                          .withAdjustmentType("PercentChangeInCapacity")
                          .withCooldown(180)
                          .withStepAdjustments(
                                  new StepAdjustment()
                                          .withMetricIntervalLowerBound(
                                                  0d)
                                          .withScalingAdjustment(200)));
  PutScalingPolicyResult response = client.putScalingPolicy(request);

putScheduledAction

PutScheduledActionResult putScheduledAction(PutScheduledActionRequest putScheduledActionRequest)

Creates or updates a scheduled action for an Application Auto Scaling scalable target.

Each scalable target is identified by a service namespace, resource ID, and scalable dimension. A scheduled action applies to the scalable target identified by those three attributes. You cannot create a scheduled action without first registering a scalable target using RegisterScalableTarget.

To update an action, specify its name and the parameters that you want to change. If you don't specify start and end times, the old values are deleted. Any other parameters that you don't specify are not changed by this update request.

You can view the scheduled actions using DescribeScheduledActions. If you are no longer using a scheduled action, you can delete it using DeleteScheduledAction.

Parameters:

putScheduledActionRequest -

Returns:

Result of the PutScheduledAction operation returned by the service.

Throws:

ValidationException - An exception was thrown for a validation issue. Review the available parameters for the API request.

LimitExceededException - Your account exceeded a limit. This exception is thrown when a per-account resource limit is exceeded. For more information, see Application Auto Scaling Limits.

ObjectNotFoundException - The specified object could not be found. For any Put or Register API operation, which depends on the existence of a scalable target, this exception is thrown if the scalable target with the specified service namespace, resource ID, and scalable dimension does not exist. For any Delete or Deregister API operation, this exception is thrown if the resource that is to be deleted or deregistered cannot be found.

ConcurrentUpdateException - Concurrent updates caused an exception, for example, if you request an update to an Application Auto Scaling resource that already has a pending update.

InternalServiceException - The service encountered an internal error.

See Also:

AWS API Documentation

registerScalableTarget

RegisterScalableTargetResult registerScalableTarget(RegisterScalableTargetRequest registerScalableTargetRequest)

Registers or updates a scalable target. A scalable target is a resource that Application Auto Scaling can scale out or scale in. After you have registered a scalable target, you can use this operation to update the minimum and maximum values for your scalable dimension.

After you register a scalable target, you can create and apply scaling policies using PutScalingPolicy. You can view the scaling policies for a service namespace using DescribeScalableTargets. If you are no longer using a scalable target, you can deregister it using DeregisterScalableTarget.

Parameters:

registerScalableTargetRequest -

Returns:

Result of the RegisterScalableTarget operation returned by the service.

Throws:

ValidationException - An exception was thrown for a validation issue. Review the available parameters for the API request.

LimitExceededException - Your account exceeded a limit. This exception is thrown when a per-account resource limit is exceeded. For more information, see Application Auto Scaling Limits.

ConcurrentUpdateException - Concurrent updates caused an exception, for example, if you request an update to an Application Auto Scaling resource that already has a pending update.

InternalServiceException - The service encountered an internal error.

See Also:

AWS API Documentation

Samples:

• To register an EC2 Spot fleet as a scalable target

  This example registers a scalable target from an Amazon EC2 Spot fleet with a minimum target capacity of 1 and a maximum of 10.
  

  AWSApplicationAutoScaling client = new AWSApplicationAutoScalingClient();
  RegisterScalableTargetRequest request = new RegisterScalableTargetRequest()
          .withMaxCapacity(10)
          .withMinCapacity(1)
          .withResourceId("service/default/web-app")
          .withRoleARN(
                  "arn:aws:iam::012345678910:role/ApplicationAutoscalingECSRole")
          .withScalableDimension("ecs:service:DesiredCount")
          .withServiceNamespace("ecs");
  RegisterScalableTargetResult response = client
          .registerScalableTarget(request);AWSApplicationAutoScaling client = new AWSApplicationAutoScalingClient();
  RegisterScalableTargetRequest request = new RegisterScalableTargetRequest()
          .withMaxCapacity(10)
          .withMinCapacity(1)
          .withResourceId(
                  "spot-fleet-request/sfr-45e69d8a-be48-4539-bbf3-3464e99c50c3")
          .withRoleARN(
                  "arn:aws:iam::012345678910:role/ApplicationAutoscalingSpotRole")
          .withScalableDimension("ec2:spot-fleet-request:TargetCapacity")
          .withServiceNamespace("ec2");
  RegisterScalableTargetResult response = client
          .registerScalableTarget(request);

shutdown

void shutdown()

Shuts down this client object, releasing any resources that might be held open. This is an optional method, and callers are not expected to call it, but can if they want to explicitly release any open resources. Once a client has been shutdown, it should not be used to make any more requests.

getCachedResponseMetadata

ResponseMetadata getCachedResponseMetadata(AmazonWebServiceRequest request)

Returns additional metadata for a previously executed successful request, typically used for debugging issues where a service isn't acting as expected. This data isn't considered part of the result data returned by an operation, so it's available through this separate, diagnostic interface.

Response metadata is only cached for a limited period of time, so if you need to access this extra diagnostic information for an executed request, you should use this method to retrieve it as soon as possible after executing a request.

Parameters:

request - The originally executed request.

Returns:

The response metadata for the specified request, or null if none is available.