# Ontologies in Cloud Computing--Review and Future Directions
## 1 Introduction  
The cloud has generated enormous interest within academia and other decision-making bodies, shaping their perception of the cloud computing environment. Based on the definition from the National Institutes of Standards and Technology (NIST), \"Cloud computing is a model for enabling ubiquitous, convenient, on-demand network access to a shared pool of configurable computing resources (e.g., networks, servers, storage, applications, services) that can be rapidly provisioned and released with minimal management effort or service provider interaction\" [1]. The word 'cloud' has been used on various occasions, such as in describing large telecommunications networks even in the 1990s [2]. However, during his paper presentation at the Search Engine Conference in 2006, [PERSON] used the world cloud computing for the first time as an approach to Software-as-a-Service (SaaS). It was used to analyze the business model of providing services over the Internet [3, 4, 5]. Cloud computing is regarded as a technique in which IT services are offered using low-cost computing units connected by Internet Protocol (IP) networks [6]. Cloud computing provides a distinct opportunity for enterprises to quickly become competitive and utilize the advantage of information technology (IT) services-based solutions with a reduced (upfront) cost. This also provides the necessary platform for companies to globalize their processes while improving and/or streamlining business operations [7]. Cloud computing is increasingly embedded in the support of critical business functions of enterprises [8]. Cloud computing has the ability to create enormous opportunities for various business strategies and help companies stay focused and become more competitive [9]. The drawback here is that the dispersed nature of the cloud lends itself to different problems with security and privacy threats at the top of the list [10], coupled with regulations and other factors that influence the external business environment in which the organization operates [8]. The baseline for the definition of cloud computing must include scalability and virtualization [11]. Cloud technology has sets of attributes that distinguish it from other technologies; however, services in this category are affected by heterogeneity challenges. As such, cloud services have many challenges to resolve, such as conflicts arising from limited knowledge about cloud resources and service description, security, interoperability, service discovery, and selection [12]. Ontologies are used to model knowledge management systems where such functions as searching, matching, and visualization can be implemented. It improves the interpretation of cloud components and integration of similar definition services, and deciphers dissimilar representations of the equivalent entity [13]. Ontology is employed in cloud services due to its ability to provide a common-access information layer. This capacity can offer an environment that allows it to display the amount of service it has in the cloud more cohesively and hide its differences. Thus, it has a profound effect as it improves solutions even for many of the problems associated with relocation and service configuration [11; 12]. In the present work, our aim is to discuss state-of-the-art ontologies in cloud and multi-cloud computing. Specifically, the contributions of the present work are summarized below:  
1. An extensive review has been carried out to investigate cloud ontology techniques for viable cloud computing. 2. Legacy ontology computing techniques have been matched with the ontology of cloud computing based on shared properties. 3. The current study on cloud computing ontologies is organized into four categories: Cloud security, Cloud resources and services description, Cloud interoperability, and Cloud services discovery and selection. From the systematic review of selected articles, some of the closest articles to this study are the works carried out by [PERSON] and [PERSON] [11], and [PERSON] et al.