Hypermedia as the Engine of Application State (HATEOAS) is an important principle of the REST application architecture. The principle is that the model of application changes from one state to another by traversing the hyperlinks present in the current set of resource representations (model). Let's learn this principle in detail.

In a RESTful system, there is no fixed interface between the client and the server, as you may see in a conventional client-server communication model such as Common Object Request Broker Architecture (CORBA) and Java Remote Method Invocation (Java RMI). With REST, the client just needs to know how to deal with the hypermedia links present in the response body; next, the call to retrieve the appropriate resource representation is made by using these dynamic media links. This concept makes the client-server interaction very dynamic and keeps it different from the other network application architectures.

Here is an example illustrating the HATEOAS principle. In this example, the http://www.packtpub.com/resources/departments/IT URI returns the following response to the client:

{"departmentId":10, 
"departmentName":"IT", 
"manager":"John Chen, 
"links": [ { 
        "rel": "employees", 
        "href": "http://packtpub.com/resources/departments/IT/employees" 
    } ]"} 

This is the current state of the system. Now, to get the employees belonging to the department, the client traverses the hyperlink present in the response body, namely http://www.packtpub.com/resources/departments/IT/employees. This URI returns the following employee list. The application state now changes into the following form (as represented by the response content):

[{"employeeId":100, 
 "firstName":"Steven", 
 "lastName":"King", 
 "links": [ { 
        "rel": "self", 
        "href": "http://www.packtpub.com/resources/employees/100" 
    }] 
 }, 
{"employeeId":101, 
"firstName":"Neena", 
"lastName":"Kochhar", 
"links": [ { 
        "rel": "self", 
        "href": "http://www.packtpub.com/resources/employees/101" 
     }] 
 }] 

In this example, the application state changes from one state to another when the client traverses the hypermedia link. Hence, we refer to this implementation principle as HATEOAS.