This allows cutting the number of connections (interfaces) to only one per subsystem which will connect directly to the ESB. Horizontal integration or Enterprise Service Bus (ESB) is an integration method in which a specialized subsystem is dedicated to communication between other subsystems. From the feature perspective, this method often seems preferable, due to the extreme flexibility of the reuse of functionality. Time and costs needed to integrate the systems increase exponentially when adding additional subsystems. In a case where the subsystems are exporting heterogeneous or proprietary interfaces, the integration cost can substantially rise. The cost varies because of the interfaces that subsystems are exporting. When observed from the perspective of the subsystem which is being integrated, the connections are reminiscent of a star, but when the overall diagram of the system is presented, the connections look like spaghetti, hence the name of this method. Star integration, also known as spaghetti integration, is a process of systems integration where each system is interconnected to each of the remaining subsystems. Reusing subsystems to create another functionality is not possible. On the other hand, cost-of-ownership can be substantially higher than seen in other methods, since in case of new or enhanced functionality, the only possible way to implement (scale the system) would be by implementing another silo. The benefit of this method is that the integration is performed quickly and involves only the necessary vendors, therefore, this method is cheaper in the short term. Vertical integration (as opposed to " horizontal integration") is the process of integrating subsystems according to their functionality by creating functional entities also referred to as silos. In the modern world connected by Internet, the role of system integration engineers is important: more and more systems are designed to connect, both within the system under construction and to systems that are already deployed. System integration involves integrating existing, often disparate systems in such a way "that focuses on increasing value to the customer" (e.g., improved product quality and performance) while at the same time providing value to the company (e.g., reducing operational costs and improving response time).
The system integrator integrates discrete systems utilizing a variety of techniques such as computer networking, enterprise application integration, business process management or manual programming.
System integration is defined in engineering as the process of bringing together the component sub- systems into one system (an aggregation of subsystems cooperating so that the system is able to deliver the overarching functionality) and ensuring that the subsystems function together as a system, and in information technology as the process of linking together different computing systems and software applications physically or functionally, to act as a coordinated whole.