errai 에서 제공하는 client -server 방식

1. 기본적으로 comet을 이용한 MessageBus로 구현이 되어 있다.
1. Messages can be received asynchronously and arbitriraily by declaring callback services within the client bus. As ErraiBus maintains an open COMET channel at all times, these messages are delivered in real time to the client as they are sent. This provides built-in push messaging for all client service
2. websocket이 지원이 되면 Errai Bus가  comet이 아닌 WebSocket을 이용한다.
1. ErraiBus has support for WebSocket-based communication. When WebSockets are enabled, capable web browsers will attempt to upgrade their COMET-based communication with the server-side bus to use a WebSocket channel

출처 : http://docs.jboss.org/errai/2.2.0.CR1/errai/reference/html_single/#sid-5931280_MessagingAPIBasics-SendingMessageswiththeClientBus