Abstract: An initiating engine is instantiated on a first machine to initiate a request for diagnostic information about one or more managed node machines. The request includes an expression to be evaluated and is received by at least one expression engine, each of which can be located on the first machine, on the managed node machine, or on another machine. Each expression engine processes the request by periodically evaluating the expression, obtaining the data required for the expression from one or more managed node machines, and transmitting the results back to the initiating engine. The expression engine can be configured to transmit the results only when the results are different than a previous evaluation of the expression. Furthermore, to detect a failed connection to the expression engine, the initiating engine maintains a heartbeat function to monitor the activities of the expression engine.