If you are new to PAS for OpenEdge, it may be useful to start by reviewing basic application server concepts. Although application servers fall under the purview of administrators and DevOps engineers, a general understanding of how they work is helpful as you develop services that are eventually deployed to an application server.

What is an application server?

An application server is a runtime framework for web applications. It contains a web server that serves static HTTP content, and a servlet container that hosts the web application. An application sever accepts client requests, passes the requests to the web application, and then sends responses back from the web application to the client application.

An application server also takes care of requirements such as load-balancing, clustering, network security, etc, thus enabling you—the developer—to focus on developing your web application.

Web applications

A web application is a set of services, each of which provides some type of functionality to a client application.

Services

The primary purpose of a service is to expose business logic to a client application. Service interfaces transform requests from a client application into a format that the business logic layer can process. For example, an OpenEdge REST service's interface may transform a JSON request message into an ABL object, a SOAP service may transform a SOAP request into ABL, etc. Each service, therefore, serves a specific type of client.

How application servers, web applications, and services work

Imagine a webpage that displays a list of customer orders to a user. When the user loads the webpage, the web client application code makes a request to the application server. The request URI may look something like this:

http://MyAppServerHost:8110/CustomerWebApp/OrderService/Orders/GetAllCustomerOrders
The URI includes the application server host and port (MyAppServerHost:8110), the web application name (CustomerWebApp) and the service name (OrderService).

The application server receives the request and uses the path parameters in the URI to determine the web application to pass the request to. The web application invokes the service and executes the operation (GetAllCustomerOrders) on the requested resource (Orders).

The result is customer order data that is returned by the application server to the client application. The task of displaying the data in a presentable way to the user is performed by the client application code.

A web application may contain multiple services (OrderService, CustomerService, InventoryService, etc) that are logically grouped based on the functionality they provide. For example, you may have a web application that contains a set of Customer services, another that contains a set of Employee services, and so on.