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Cloud application architecture is basically service-oriented architecture (SOA), which helps users/clients get their on-demand service. This reference model will help the service providers and end users maintain controlled access and dynamism in real-time applications. This architecture includes three sublayers, namely virtual appliances, agent-based layer and business service provider. Each of these sublayers is discussed below.

1 Virtual appliances: These run with the APIs (application programming interface) of various customer service providers (CSPs) or platforms. It is an instance of virtual environment extension (VEE). Cloud applications are deployed as virtual appliances to make management better. Virtual appliance configuration should be done in such a way that the application and services continue to grow but management overheads do not grow proportionally.VEE (virtual environment extension): Basically, it is fully isolated runtime environment which abstracts away the physical characteristics of the resource and also enables sharing. The VEEs create partitioning by the virtualization layer within a site to optimize the resource utilization.

2 Agent-based layer: In this layer, cloud agents are like brokers between the virtual appliances layer and business service and provider (BSP) layer. The main aim of the cloud agents is the optimal arrangement of VEEs into CSPs configured and managed by the service manager. What does optimal arrangement mean? Suppose there are multiple users in a particular instance of time and all are intended to get the services. Then the cloud broker places the VEEs optimally so that resources can be provisioned in a maximized way. And from the perspective of cost complexity, both cloud provider and cloud user also benefit from this optimal arrangement or optimal placement of VEEs.Cloud brokers have the authority to move throughout the VEEs and are also free to place and move different sites, even remote sites, until the arrangement is satisfied. At broker level, when a service is deployed on the cloud, it is realized as a set of interrelated virtual environment extension or as a VEE group. Hence, all the activities can be done as a whole. Simply put, we can say that cloud brokers are responsible for arranging the deployment order and rollback policies, and maintaining the service workload depending upon the dynamic nature of the applications. And cloud brokers also have the authority to change the capacity of a single VEE or to add and remove the present VEEs from existing group [9].

3 Business service provider (BSP): The BSP layer consists of business service management (BSM), service-level agreement (SLA), service orchestration, and process management. It provides common infrastructure elements for service-level management, metered usage, policy management, license management, and disaster recovery [7]. BSM handles the business service policies. It manages the different services requested by the clients. It also provides the different quality of service (QoS) parameters negotiated in SLA when the business agreement was established. SLA is concerned about business-oriented agreements and laws. SLA contains some specific rules and regulations which are established between the service provider and service user. According to their agreement, cloud users can access the resources and can also renew the agreement for getting further service. Process management schedules and manages the processes. Service orchestration is the component which is responsible for ordered arrangements of the requests. Process management system schedules the processes according to their priority or provisioning strategy of computing and physical resources.