## 4 Design of IVM-ReU  
This study basically used Horizon, Cinder, Nova, Swift, Neutron, Glance, and Keystone services provided by OpenStack, and added the IVM-ReU feature that provides computing services to users through cloud computing. Figure 3 presents the overall structure of the used OpenStack and IVM-ReU architecture proposed in this study. As verified in Figure 3, basic services provided by the OpenStack and IVM-ReU functions proposed in this study are closely connected. First, Nova, which provides computing services, is a core  
Figure 3: OpenStack and IVM–ReU architecture. Figure 2: Computing service control flow in IVM–ReU. function of OpenStack, which creates and manages instances by provisioning computing resources as needed to users. Swift is an object storage service, which consists of account and container, whose data are managed via a database, and object, which is directly stored in a storage space. Here, object performs tasks, such as the searching, downloading, and uploading of actual data and files, and container is a unit of storage space that can search objects or create and delete containers. In addition, account searches containers and manages user accounts. Glance manages virtual machine images to be used in various hypervisors, and provides image services that manage operating systems to be run in each of the virtual machines. KeyStone, which is an authentication service, manages the use of resources, such as computing, networking, and storage, inside the physical server through user authentication. Neutron provides network services for other OpenStack services, such as assigning IP addresses to created instances and managing them. Cinder, which is a block storage service, can create, add, and delete a volume, which is a storage space, in instances created through Nova, and it updates the modified volume information in the database. The created volumes may be allocated to single or multiple instances. Horizon is an OpenStack Dashboard service for users to request a task easily and rapidly through the web user interface (UI) and monitor the process and results in relation to basic services of OpenStack, such as resource and event management services such as instance creation, deletion, and management, performance, and log viewing [20]. The IVM-ReU architecture proposed in this study is intended to provide computing services to users who want to process a large-scale job based on the basic services of OpenStack. It largely consists of Viewer, IVM-ReU Manager, Idle VM (Worker) Manager, and IVM-ReU Task Saver Manager. Viewer visualizes a procedure that provides computing resources and services to users as a form of web UI by using the Horizon service of OpenStack, consisting of Setting, which represent basic settings such as user registration, login history, application registration and deletion, etc. to provide computing services that process a large-scale job in parallel through IVM-ReU; VM Information, which displays static, dynamic information and the states about all virtual machines created in OpenStack or idle virtual machines classified based on the idle-state determination criteria; Job Submission, by which users submit their jobs; and Result, which shows the processed task results. IVM-ReU Manager, which provides computing services to users by selecting and managing idle virtual machines from the created virtual machines, consists of Idle Virtual Machine Selection, Idle Virtual Machine Pool, Computing Service with Idle Virtual Machine, Fault Tolerance, and IVM-ReU Task Saver Connection. Idle Virtual Machine Selection classifies virtual machines by searching for machines in an idle state. Idle Virtual Machine Pool collects and manages static and dynamic information, such as IP addresses, available memory size, and the status of idle virtual machines selected, to provide computing services to users through the use of cloud computing resources.