3.38. CometD


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What is CometD?

CometD is a set of libraries that facilitates the writing of web applications that perform messaging over the web such as web chat applications.

The role of CometD is to deliver messages over the wire using the best available transport: Websocket or HTTP, independently of the APIs used in the application. It also provides transparent fallback in case WebSocket does not work.

CometD clustering

CometD provides a clustering solution called Oort that allows you to scale horizontally your web applications. With a CometD based system in cluster mode, clients connect to multiple nodes instead of a single node.

Oort clustering is not a high-availability solution. In fact, when a node is down, all the clients are disconnected and then connected to another node by a new handshake. When this happens then if the application did not implement a method to retrieve information, the data build on the client side is lost.

CometD configuration

To configure CometD in either cluster or standalone mode, some parameters are needed. A list of parameters is provided in CometD's Official documentation.


All CometD parameters are configurable in eXo Platform in the exo.properties file by prefixing them with exo.cometd.. For example, to override the maximum size of Websocket messages, a value must be set for the parameter ws.maxMessageSize. Thus, in eXo Platform this value must be set in exo.properties through the exo.cometd.ws.maxMessageSize.

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