> Markdown version of [Client-Side Engine](https://vaadin.com/docs/v7/framework/architecture/architecture-client-side). Section index: [llms.txt](https://vaadin.com/docs/v7/framework/llms.txt)

# <a id="architecture.client-side"></a>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 Google Web Toolkit) on the client-side. The client-side engine is illustrated in [Vaadin Client-Side Engine](#figure.architecture.client-side).

[Image: clientside arch hi]

Vaadin Client-Side Engine

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 ["Using Vaadin Add-ons"](https://vaadin.com/docs/v7/framework/addons/addons-overview.md#addons.overview). You can also develop your own widgets, as described in ["Client-Side Vaadin Development"](https://vaadin.com/docs/v7/framework/clientside/clientside-overview.md#clientside.overview).

The rendering with widgets, as well as the communication to the server-side, is handled in the 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 ["Integrating with the Server-Side"](https://vaadin.com/docs/v7/framework/gwt/gwt-overview.md#gwt.overview).