@SdkInternalApi
    final DBSubnetGroup executeModifyDBSubnetGroup(ModifyDBSubnetGroupRequest modifyDBSubnetGroupRequest) {

        ExecutionContext executionContext = createExecutionContext(modifyDBSubnetGroupRequest);
        AWSRequestMetrics awsRequestMetrics = executionContext.getAwsRequestMetrics();
        awsRequestMetrics.startEvent(Field.ClientExecuteTime);
        Request<ModifyDBSubnetGroupRequest> request = null;
        Response<DBSubnetGroup> response = null;

        try {
            awsRequestMetrics.startEvent(Field.RequestMarshallTime);
            try {
                request = new ModifyDBSubnetGroupRequestMarshaller().marshall(super.beforeMarshalling(modifyDBSubnetGroupRequest));
                // Binds the request metrics to the current request.
                request.setAWSRequestMetrics(awsRequestMetrics);
                request.addHandlerContext(HandlerContextKey.CLIENT_ENDPOINT, endpoint);
                request.addHandlerContext(HandlerContextKey.ENDPOINT_OVERRIDDEN, isEndpointOverridden());
                request.addHandlerContext(HandlerContextKey.SIGNING_REGION, getSigningRegion());
                request.addHandlerContext(HandlerContextKey.SERVICE_ID, "RDS");
                request.addHandlerContext(HandlerContextKey.OPERATION_NAME, "ModifyDBSubnetGroup");
                request.addHandlerContext(HandlerContextKey.ADVANCED_CONFIG, advancedConfig);

            } finally {
                awsRequestMetrics.endEvent(Field.RequestMarshallTime);
            }

            StaxResponseHandler<DBSubnetGroup> responseHandler = new StaxResponseHandler<DBSubnetGroup>(new DBSubnetGroupStaxUnmarshaller());

            response = invoke(request, responseHandler, executionContext);

            return response.getAwsResponse();

        } finally {

            endClientExecution(awsRequestMetrics, request, response);
        }
    }
<p>
Modifies an existing DB subnet group. DB subnet groups must contain at least one subnet in at least two AZs in
the Amazon Web Services Region.
</p>

@param modifyDBSubnetGroupRequest
@return Result of the ModifyDBSubnetGroup operation returned by the service.
@throws DBSubnetGroupNotFoundException
        <code>DBSubnetGroupName</code> doesn't refer to an existing DB subnet group.
@throws DBSubnetQuotaExceededException
        The request would result in the user exceeding the allowed number of subnets in a DB subnet groups.
@throws SubnetAlreadyInUseException
        The DB subnet is already in use in the Availability Zone.
@throws DBSubnetGroupDoesNotCoverEnoughAZsException
        Subnets in the DB subnet group should cover at least two Availability Zones unless there is only one
        Availability Zone.
@throws InvalidSubnetException
        The requested subnet is invalid, or multiple subnets were requested that are not all in a common VPC.
@sample AmazonRDS.ModifyDBSubnetGroup
@see <a href="http://docs.aws.amazon.com/goto/WebAPI/rds-2014-10-31/ModifyDBSubnetGroup" target="_top">AWS API
     Documentation</a>
aws-java-sdk-rds/src/main/java/com/amazonaws/services/rds/AmazonRDSClient.java