Interface AWSLogs

All Known Subinterfaces:
AWSLogsAsync
All Known Implementing Classes:
AbstractAWSLogs, AbstractAWSLogsAsync, AWSLogsAsyncClient, AWSLogsClient

public interface AWSLogs
Interface for accessing Amazon CloudWatch Logs.

Amazon CloudWatch Logs API Reference

You can use Amazon CloudWatch Logs to monitor, store, and access your log files from Amazon Elastic Compute Cloud (Amazon EC2) instances, Amazon CloudTrail, or other sources. You can then retrieve the associated log data from CloudWatch Logs using the Amazon CloudWatch console, the CloudWatch Logs commands in the AWS CLI, the CloudWatch Logs API, or the CloudWatch Logs SDK.

You can use CloudWatch Logs to:

  • Monitor Logs from Amazon EC2 Instances in Real-time: You can use CloudWatch Logs to monitor applications and systems using log data. For example, CloudWatch Logs can track the number of errors that occur in your application logs and send you a notification whenever the rate of errors exceeds a threshold you specify. CloudWatch Logs uses your log data for monitoring; so, no code changes are required. For example, you can monitor application logs for specific literal terms (such as "NullReferenceException") or count the number of occurrences of a literal term at a particular position in log data (such as "404" status codes in an Apache access log). When the term you are searching for is found, CloudWatch Logs reports the data to a Amazon CloudWatch metric that you specify.

  • Monitor Amazon CloudTrail Logged Events: You can create alarms in Amazon CloudWatch and receive notifications of particular API activity as captured by CloudTrail and use the notification to perform troubleshooting.

  • Archive Log Data: You can use CloudWatch Logs to store your log data in highly durable storage. You can change the log retention setting so that any log events older than this setting are automatically deleted. The CloudWatch Logs agent makes it easy to quickly send both rotated and non-rotated log data off of a host and into the log service. You can then access the raw log data when you need it.

  • Method Details

    • setEndpoint

      void setEndpoint(String endpoint)
      Overrides the default endpoint for this client ("https://logs.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: "logs.us-east-1.amazonaws.com") or a full URL, including the protocol (ex: "https://logs.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: "logs.us-east-1.amazonaws.com") or a full URL, including the protocol (ex: "https://logs.us-east-1.amazonaws.com") of the region specific AWS endpoint this client will communicate with.
    • setRegion

      void setRegion(Region region)
      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:
    • cancelExportTask

      CancelExportTaskResult cancelExportTask(CancelExportTaskRequest cancelExportTaskRequest)

      Cancels an export task if it is in PENDING or RUNNING state.

      Parameters:
      cancelExportTaskRequest -
      Returns:
      Result of the CancelExportTask operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      InvalidOperationException - Returned if the operation is not valid on the specified resource
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • createExportTask

      CreateExportTaskResult createExportTask(CreateExportTaskRequest createExportTaskRequest)

      Creates an ExportTask which allows you to efficiently export data from a Log Group to your Amazon S3 bucket.

      This is an asynchronous call. If all the required information is provided, this API will initiate an export task and respond with the task Id. Once started, DescribeExportTasks can be used to get the status of an export task. You can only have one active ( RUNNING or PENDING) export task at a time, per account.

      You can export logs from multiple log groups or multiple time ranges to the same Amazon S3 bucket. To separate out log data for each export task, you can specify a prefix that will be used as the Amazon S3 key prefix for all exported objects.

      Parameters:
      createExportTaskRequest -
      Returns:
      Result of the CreateExportTask operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      LimitExceededException - Returned if you have reached the maximum number of resources that can be created.
      OperationAbortedException - Returned if multiple requests to update the same resource were in conflict.
      ServiceUnavailableException - Returned if the service cannot complete the request.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      ResourceAlreadyExistsException - Returned if the specified resource already exists.
    • createLogGroup

      CreateLogGroupResult createLogGroup(CreateLogGroupRequest createLogGroupRequest)

      Creates a new log group with the specified name. The name of the log group must be unique within a region for an AWS account. You can create up to 500 log groups per account.

      You must use the following guidelines when naming a log group:

      • Log group names can be between 1 and 512 characters long.
      • Allowed characters are a-z, A-Z, 0-9, '_' (underscore), '-' (hyphen), '/' (forward slash), and '.' (period).

      Parameters:
      createLogGroupRequest -
      Returns:
      Result of the CreateLogGroup operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceAlreadyExistsException - Returned if the specified resource already exists.
      LimitExceededException - Returned if you have reached the maximum number of resources that can be created.
      OperationAbortedException - Returned if multiple requests to update the same resource were in conflict.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • createLogStream

      CreateLogStreamResult createLogStream(CreateLogStreamRequest createLogStreamRequest)

      Creates a new log stream in the specified log group. The name of the log stream must be unique within the log group. There is no limit on the number of log streams that can exist in a log group.

      You must use the following guidelines when naming a log stream:

      • Log stream names can be between 1 and 512 characters long.
      • The ':' colon character is not allowed.

      Parameters:
      createLogStreamRequest -
      Returns:
      Result of the CreateLogStream operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceAlreadyExistsException - Returned if the specified resource already exists.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • deleteDestination

      DeleteDestinationResult deleteDestination(DeleteDestinationRequest deleteDestinationRequest)

      Deletes the destination with the specified name and eventually disables all the subscription filters that publish to it. This will not delete the physical resource encapsulated by the destination.

      Parameters:
      deleteDestinationRequest -
      Returns:
      Result of the DeleteDestination operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      OperationAbortedException - Returned if multiple requests to update the same resource were in conflict.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • deleteLogGroup

      DeleteLogGroupResult deleteLogGroup(DeleteLogGroupRequest deleteLogGroupRequest)

      Deletes the log group with the specified name and permanently deletes all the archived log events associated with it.

      Parameters:
      deleteLogGroupRequest -
      Returns:
      Result of the DeleteLogGroup operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      OperationAbortedException - Returned if multiple requests to update the same resource were in conflict.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • deleteLogStream

      DeleteLogStreamResult deleteLogStream(DeleteLogStreamRequest deleteLogStreamRequest)

      Deletes a log stream and permanently deletes all the archived log events associated with it.

      Parameters:
      deleteLogStreamRequest -
      Returns:
      Result of the DeleteLogStream operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      OperationAbortedException - Returned if multiple requests to update the same resource were in conflict.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • deleteMetricFilter

      DeleteMetricFilterResult deleteMetricFilter(DeleteMetricFilterRequest deleteMetricFilterRequest)

      Deletes a metric filter associated with the specified log group.

      Parameters:
      deleteMetricFilterRequest -
      Returns:
      Result of the DeleteMetricFilter operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      OperationAbortedException - Returned if multiple requests to update the same resource were in conflict.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • deleteRetentionPolicy

      DeleteRetentionPolicyResult deleteRetentionPolicy(DeleteRetentionPolicyRequest deleteRetentionPolicyRequest)

      Deletes the retention policy of the specified log group. Log events would not expire if they belong to log groups without a retention policy.

      Parameters:
      deleteRetentionPolicyRequest -
      Returns:
      Result of the DeleteRetentionPolicy operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      OperationAbortedException - Returned if multiple requests to update the same resource were in conflict.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • deleteSubscriptionFilter

      DeleteSubscriptionFilterResult deleteSubscriptionFilter(DeleteSubscriptionFilterRequest deleteSubscriptionFilterRequest)

      Deletes a subscription filter associated with the specified log group.

      Parameters:
      deleteSubscriptionFilterRequest -
      Returns:
      Result of the DeleteSubscriptionFilter operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      OperationAbortedException - Returned if multiple requests to update the same resource were in conflict.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • describeDestinations

      DescribeDestinationsResult describeDestinations(DescribeDestinationsRequest describeDestinationsRequest)

      Returns all the destinations that are associated with the AWS account making the request. The list returned in the response is ASCII-sorted by destination name.

      By default, this operation returns up to 50 destinations. If there are more destinations to list, the response would contain a nextToken value in the response body. You can also limit the number of destinations returned in the response by specifying the limit parameter in the request.

      Parameters:
      describeDestinationsRequest -
      Returns:
      Result of the DescribeDestinations operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • describeDestinations

      DescribeDestinationsResult describeDestinations()
      Simplified method form for invoking the DescribeDestinations operation.
      See Also:
    • describeExportTasks

      DescribeExportTasksResult describeExportTasks(DescribeExportTasksRequest describeExportTasksRequest)

      Returns all the export tasks that are associated with the AWS account making the request. The export tasks can be filtered based on TaskId or TaskStatus.

      By default, this operation returns up to 50 export tasks that satisfy the specified filters. If there are more export tasks to list, the response would contain a nextToken value in the response body. You can also limit the number of export tasks returned in the response by specifying the limit parameter in the request.

      Parameters:
      describeExportTasksRequest -
      Returns:
      Result of the DescribeExportTasks operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • describeLogGroups

      DescribeLogGroupsResult describeLogGroups(DescribeLogGroupsRequest describeLogGroupsRequest)

      Returns all the log groups that are associated with the AWS account making the request. The list returned in the response is ASCII-sorted by log group name.

      By default, this operation returns up to 50 log groups. If there are more log groups to list, the response would contain a nextToken value in the response body. You can also limit the number of log groups returned in the response by specifying the limit parameter in the request.

      Parameters:
      describeLogGroupsRequest -
      Returns:
      Result of the DescribeLogGroups operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • describeLogGroups

      DescribeLogGroupsResult describeLogGroups()
      Simplified method form for invoking the DescribeLogGroups operation.
      See Also:
    • describeLogStreams

      DescribeLogStreamsResult describeLogStreams(DescribeLogStreamsRequest describeLogStreamsRequest)

      Returns all the log streams that are associated with the specified log group. The list returned in the response is ASCII-sorted by log stream name.

      By default, this operation returns up to 50 log streams. If there are more log streams to list, the response would contain a nextToken value in the response body. You can also limit the number of log streams returned in the response by specifying the limit parameter in the request. This operation has a limit of five transactions per second, after which transactions are throttled.

      Parameters:
      describeLogStreamsRequest -
      Returns:
      Result of the DescribeLogStreams operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • describeMetricFilters

      DescribeMetricFiltersResult describeMetricFilters(DescribeMetricFiltersRequest describeMetricFiltersRequest)

      Returns all the metrics filters associated with the specified log group. The list returned in the response is ASCII-sorted by filter name.

      By default, this operation returns up to 50 metric filters. If there are more metric filters to list, the response would contain a nextToken value in the response body. You can also limit the number of metric filters returned in the response by specifying the limit parameter in the request.

      Parameters:
      describeMetricFiltersRequest -
      Returns:
      Result of the DescribeMetricFilters operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • describeSubscriptionFilters

      DescribeSubscriptionFiltersResult describeSubscriptionFilters(DescribeSubscriptionFiltersRequest describeSubscriptionFiltersRequest)

      Returns all the subscription filters associated with the specified log group. The list returned in the response is ASCII-sorted by filter name.

      By default, this operation returns up to 50 subscription filters. If there are more subscription filters to list, the response would contain a nextToken value in the response body. You can also limit the number of subscription filters returned in the response by specifying the limit parameter in the request.

      Parameters:
      describeSubscriptionFiltersRequest -
      Returns:
      Result of the DescribeSubscriptionFilters operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • filterLogEvents

      FilterLogEventsResult filterLogEvents(FilterLogEventsRequest filterLogEventsRequest)

      Retrieves log events, optionally filtered by a filter pattern from the specified log group. You can provide an optional time range to filter the results on the event timestamp. You can limit the streams searched to an explicit list of logStreamNames.

      By default, this operation returns as much matching log events as can fit in a response size of 1MB, up to 10,000 log events, or all the events found within a time-bounded scan window. If the response includes a nextToken, then there is more data to search, and the search can be resumed with a new request providing the nextToken. The response will contain a list of searchedLogStreams that contains information about which streams were searched in the request and whether they have been searched completely or require further pagination. The limit parameter in the request. can be used to specify the maximum number of events to return in a page.

      Parameters:
      filterLogEventsRequest -
      Returns:
      Result of the FilterLogEvents operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • getLogEvents

      GetLogEventsResult getLogEvents(GetLogEventsRequest getLogEventsRequest)

      Retrieves log events from the specified log stream. You can provide an optional time range to filter the results on the event timestamp.

      By default, this operation returns as much log events as can fit in a response size of 1MB, up to 10,000 log events. The response will always include a nextForwardToken and a nextBackwardToken in the response body. You can use any of these tokens in subsequent GetLogEvents requests to paginate through events in either forward or backward direction. You can also limit the number of log events returned in the response by specifying the limit parameter in the request.

      Parameters:
      getLogEventsRequest -
      Returns:
      Result of the GetLogEvents operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • putDestination

      PutDestinationResult putDestination(PutDestinationRequest putDestinationRequest)

      Creates or updates a Destination. A destination encapsulates a physical resource (such as a Kinesis stream) and allows you to subscribe to a real-time stream of log events of a different account, ingested through PutLogEvents requests. Currently, the only supported physical resource is a Amazon Kinesis stream belonging to the same account as the destination.

      A destination controls what is written to its Amazon Kinesis stream through an access policy. By default, PutDestination does not set any access policy with the destination, which means a cross-account user will not be able to call PutSubscriptionFilter against this destination. To enable that, the destination owner must call PutDestinationPolicy after PutDestination.

      Parameters:
      putDestinationRequest -
      Returns:
      Result of the PutDestination operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      OperationAbortedException - Returned if multiple requests to update the same resource were in conflict.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • putDestinationPolicy

      PutDestinationPolicyResult putDestinationPolicy(PutDestinationPolicyRequest putDestinationPolicyRequest)

      Creates or updates an access policy associated with an existing Destination. An access policy is an IAM policy document that is used to authorize claims to register a subscription filter against a given destination.

      Parameters:
      putDestinationPolicyRequest -
      Returns:
      Result of the PutDestinationPolicy operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      OperationAbortedException - Returned if multiple requests to update the same resource were in conflict.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • putLogEvents

      PutLogEventsResult putLogEvents(PutLogEventsRequest putLogEventsRequest)

      Uploads a batch of log events to the specified log stream.

      Every PutLogEvents request must include the sequenceToken obtained from the response of the previous request. An upload in a newly created log stream does not require a sequenceToken.

      The batch of events must satisfy the following constraints:

      • The maximum batch size is 1,048,576 bytes, and this size is calculated as the sum of all event messages in UTF-8, plus 26 bytes for each log event.
      • None of the log events in the batch can be more than 2 hours in the future.
      • None of the log events in the batch can be older than 14 days or the retention period of the log group.
      • The log events in the batch must be in chronological ordered by their timestamp.
      • The maximum number of log events in a batch is 10,000.
      • A batch of log events in a single PutLogEvents request cannot span more than 24 hours. Otherwise, the PutLogEvents operation will fail.

      Parameters:
      putLogEventsRequest -
      Returns:
      Result of the PutLogEvents operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      InvalidSequenceTokenException
      DataAlreadyAcceptedException
      ResourceNotFoundException - Returned if the specified resource does not exist.
      OperationAbortedException - Returned if multiple requests to update the same resource were in conflict.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • putMetricFilter

      PutMetricFilterResult putMetricFilter(PutMetricFilterRequest putMetricFilterRequest)

      Creates or updates a metric filter and associates it with the specified log group. Metric filters allow you to configure rules to extract metric data from log events ingested through PutLogEvents requests.

      The maximum number of metric filters that can be associated with a log group is 100.

      Parameters:
      putMetricFilterRequest -
      Returns:
      Result of the PutMetricFilter operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      OperationAbortedException - Returned if multiple requests to update the same resource were in conflict.
      LimitExceededException - Returned if you have reached the maximum number of resources that can be created.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • putRetentionPolicy

      PutRetentionPolicyResult putRetentionPolicy(PutRetentionPolicyRequest putRetentionPolicyRequest)

      Sets the retention of the specified log group. A retention policy allows you to configure the number of days you want to retain log events in the specified log group.

      Parameters:
      putRetentionPolicyRequest -
      Returns:
      Result of the PutRetentionPolicy operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      OperationAbortedException - Returned if multiple requests to update the same resource were in conflict.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • putSubscriptionFilter

      PutSubscriptionFilterResult putSubscriptionFilter(PutSubscriptionFilterRequest putSubscriptionFilterRequest)

      Creates or updates a subscription filter and associates it with the specified log group. Subscription filters allow you to subscribe to a real-time stream of log events ingested through PutLogEvents requests and have them delivered to a specific destination. Currently, the supported destinations are:

      • An Amazon Kinesis stream belonging to the same account as the subscription filter, for same-account delivery.
      • A logical destination (used via an ARN of Destination) belonging to a different account, for cross-account delivery.
      • An Amazon Kinesis Firehose stream belonging to the same account as the subscription filter, for same-account delivery.
      • An AWS Lambda function belonging to the same account as the subscription filter, for same-account delivery.

      Currently there can only be one subscription filter associated with a log group.

      Parameters:
      putSubscriptionFilterRequest -
      Returns:
      Result of the PutSubscriptionFilter operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ResourceNotFoundException - Returned if the specified resource does not exist.
      OperationAbortedException - Returned if multiple requests to update the same resource were in conflict.
      LimitExceededException - Returned if you have reached the maximum number of resources that can be created.
      ServiceUnavailableException - Returned if the service cannot complete the request.
    • testMetricFilter

      TestMetricFilterResult testMetricFilter(TestMetricFilterRequest testMetricFilterRequest)

      Tests the filter pattern of a metric filter against a sample of log event messages. You can use this operation to validate the correctness of a metric filter pattern.

      Parameters:
      testMetricFilterRequest -
      Returns:
      Result of the TestMetricFilter operation returned by the service.
      Throws:
      InvalidParameterException - Returned if a parameter of the request is incorrectly specified.
      ServiceUnavailableException - Returned if the service cannot complete the 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.