Abstract: A network-based messaging system comprises multiple agents to communicate messages between multiple users in real time using, for example, an XML document synchronization model. Each agent has properties defined in XML and can subscribe to properties of other agents. Each agent can notify other agents which subscribe to it of changes to its properties. The agents communicate using an XML or alternative extensible data interchange protocol. The agents include device agents to represent each of multiple user devices, which may include computers on a wireline network and mobile devices on a wireless network. The agents also include persona agents to represent each user. The persona agents collect information about the properties of other agents and publish the information to other, subscribing agents. Each persona agent comprises properties to maintain state information for each device used by the corresponding user. Most of the agents reside in a centralized agent system.