3.1.1. Application Tiers and Blocks

The framework enables building multi-tiered applications with the distinct client, middle and database tiers. Further on, we will consider mainly the middle and client tiers, so the words "all tiers" will refer to these tiers only.

An application can have one or more blocks on each tier. A block is a separate executable program interacting with other blocks of the application. Usually, a block is a web application running on JVM.

AppTiers
Figure 1. Application Tiers and Blocks
Middleware

The middle tier contains core business logic of the application and provides access to the database. It is represented by a separate web application running on a Java servlet container. See Middleware Components.

Web Client

A main block of the client tier. It contains the interface designed primarily for internal (back-office) users. It is represented by a separate web application running on a Java servlet container. The user interface is based on the Vaadin framework. See Generic User Interface.

Web Portal

An additional block of the client tier. It can contain an interface for external users and entry points for integration with mobile devices and third-party applications. It is represented by a separate web application running on a Java servlet container. The user interface is based on the Spring MVC framework. See Portal Components.

Frontend UI

An optional UI layer designed for external users. Unlike Web Portal, it is a client-side application (for example, a JavaScript application executed in the web browser). It communicates with the middleware via REST API running either in Web Client or in Web Portal blocks. It can be powered by React or other libraries/frameworks. See Frontend User Interface.

Middleware is the mandatory block for any application. User interface can be implemented by one or several blocks, such as Web Client and Web Portal.

All of the Java-based client blocks interact with the middle tier uniformly via HTTP protocol which enables deploying the middle tier arbitrarily, behind a firewall as well. In the simplest case, when the middle tier and the web client are deployed on the same server, local interaction between them can bypass the network stack for better performance.