--- title: Client-Side Engine order: 3 layout: page --- [[architecture.client-side]] = Client-Side Engine ((("Client-Side Engine"))) The user interface of a server-side Vaadin application is rendered in the browser by the Vaadin Client-Side Engine. It is loaded in the browser when the page with the Vaadin UI is opened. The server-side UI components are rendered using __widgets__ (as they are called in GWT) on the client-side. The client-side engine is illustrated in <>. [[figure.architecture.client-side]] .Vaadin Client-Side Engine image::img/clientside-arch-hi.png[width=80%, scaledwidth=100%] The client-side framework includes two kinds of built-in widgets: GWT widgets and Vaadin-specific widgets. The two widget collections have significant overlap, where the Vaadin widgets provide a bit different features than the GWT widgets. In addition, many add-on widgets and their server-side counterparts exist, and you can easily download and install them, as described in <>. You can also develop your own widgets, as described in <>. The rendering with widgets, as well as the communication to the server-side, is handled in the [classname]#ApplicationConnection#. Connecting the widgets with their server-side counterparts is done in __connectors__, and there is one for each widget that has a server-side counterpart. The framework handles serialization of component state transparently, and includes an RPC mechanism between the two sides. Integration of widgets with their server-side counterpart components is described in <>.