the timestamp when a worker node register itself to master node
sessionId is assigned by Master node for easy tracking.
sessionId is assigned by Master node for easy tracking. It is possible that sessionId is **NOT** unique, so always use WorkerId for comparison.
WorkerId is used to uniquely track a worker machine.
sessionId is assigned by Master node for easy tracking. It is possible that sessionId is **NOT** unique, so always use WorkerId for comparison.
the timestamp when a worker node register itself to master node