<This functionality is not available in the Freeware version of Concordia>
Concordia provides two forms of collaboration: basic and reliable. The basic collaboration framework has been discussed above. Reliability can be optionally increased by persistent agent group state and proxies.
An agent group may optionally save its membership to persistent storage and restore that state, if it is restarted. In this way, an agent group's membership survives failures of the group. The persistence option is enabled with the persistent argument to AgentGroupImpl's constructor. Each time an AgentGroupImpl object is instantiated with the persistence enabled, it attempts to read its persistent storage file and restore its membership state from that file.
Highly reliable applications may also utilize proxies to interface with agent groups. A proxy for an agent group maintains a persistent reference to the group. If the agent group fails, the proxy will be unable to communicate with it and will acquire a new reference to it. The proxy's maintenance of this persistent reference is transparent to the agent group. Hence, the use of proxies shields group members from agent group failures.