If a data source of a value is a client, then the server needs to be able to indicate if the client has a communication problem and the value is no longer being updated. The current write service does not provide any mechanism for determining if a value is stale. More importantly there is no service that can be used to indicate that the source of values for a node is a client. Servers could easily report status changes for disconnected clients, if there existed some manner to indicate that a value of a node is to be associated with some client connection. The client connection information may need to also take into account redundancy issues (redundant client/redundant network).

From emails:
Matthias - The central server needs a way to know which client is responsible for updating which Variables. This can be done by defining an ExpectedClient object with information about the client. The server can detect through information about the connected clients if the expected client is connected. It would be necessary to define watchdog functionality since the server normally detects disconnected clients slower than clients can detect connection errors with servers. In addition the server needs to know the list of Variables the client is responsible for updating. If the expected client is not connected, the server can set the status of the affected Variables to BAD