What is the persistence policy
Since the server has no control of when the client will call to get the callbacks, it has to be aware of memory constraints as it manages a growing number of callbacks. The way the callback server does this is through use of a persistence policy. The persistence policy indicates at what point the server has too little free memory available and therefore the callback message should be put into a persistent store. This policy can be configured via the memPercentCeiling attribute (see more on configuring this below).