The basic idea is to have a table on the source system that gets updated at a predetermined interval. In your capture processes you would capture the update from the heartbeat table. Using tokens you would add some additional information to the heartbeat record to be able to tell which extract process was capturing the update. This additional information would be used downstream to calculate the real lag time between the source and target systems for a given extract and by checking the last update time on the heartbeat at the target you could also determine if data has stopped flowing between the source and target.