The one-dimensional model of the three-tier architecture was split into a multi-dimensional SOA, where inter-service communication was enabled through ESB using the Simple Object Access Protocol (SOAP) and other web services standards.

SOA, along with ESB, could be used to break down a large three-tier application into services, where applications were built using these reusable services. The services could be dynamically discovered using service metadata through a metadata repository. With SOA, each functionality is built as a coarse-grained service that's often deployed inside an application server. 

Multiple services need to be integrated to create composite services that are exposed through the ESB layer, which becomes a centralized bus for communication. This can be seen in the following diagram:

The preceding diagram shows the consumer and provider model connected through the ESB. The ESB also contains significant business logic, making it a monolithic entity where the same runtime is shared by developers in order to develop or deploy their service integrations.

In the next section, we'll talk about API gateways. The concept of the API gateway evolved around 2008 with the advent of smartphones, which provide rich client applications that need easy and secure connectivity to the backend services.