As the times moved forward, enterprises shifted to a new model of application development. This model used to model the applications as a service, where each service used to provide a certain set of business capabilities. So, for example, in an enterprise there will be a payroll service, which will provide all of the necessary functionality related to the management of the employee payroll, such as handling the data for the new employees, keeping a record of how much salary they have got and generating the monthly payslips.

Now, these services needed to be integrated with each other so that the exchange of data between these services could be facilitated. At this point in time, the enterprises needed something that would allow these services to communicate with each other over the network without the bottleneck of handling the different data formats that each service maintains.

The solution to this came in the form of the introduction of the ESB. The ESB model of service-based integration focused on the message oriented style of service communication. When the services have to communicate with each other, they will do it through the use of messages.

Let's take a look at some of the features of the ESB and how it allows for a better integration model:

• Decoupling of applications: The ESB provides the decoupling between the different applications by introducing an arbitrator bus between the applications. The applications now need not talk to each other directly, but rather talk to the bus, which is responsible for facilitating communication between the applications.
• Standardized data format: All of the messages that flow inside a bus usually follow a standard format, such as XML. Every application that needs to communicate with another application now only has to deal with one data format, except for their own, greatly simplifying the process of integration.
• Orchestration: The ESB is responsible for the orchestration of the message flow and making sure that the messages are being delivered to the correct recipients.
• Ability to scale: The ESBs are usually easier to scale, because they do not have any centrally located business logic built inside them that will need to be replicated. The scalability is usually achieved through either vertical scaling, by dedicating more resources to the ESB, or through the use of horizontal scaling, by introducing more instances of the ESB into the infrastructure.

This makes the ESB a great choice for the integration of the applications inside the enterprise while also allowing the flexibility in the integration process.

Now we have an idea about the different approaches to the EAI. So, now it's time for us to move on and understand the different patterns of EAI.