Prevalence layer . Klaus Wuestefeld defines prevalence as “transparent persistence, fault-tolerance and load-balancing of the execution of the business logic of an information system through the use of state snapshots as well as command and query queuing or logging”. A prevalence layer is effectively a simple persistence framework that serializes objects and writes them to log files. From the point of view of developers all objects are cached in memory and the persistence of the objects is truly treated as a background task that the developers don’t need to worry about. Table 2 presents a comparison of the various types of persistence mechanism and provides references to vendors where applicable. Table 3 presents suggestions for when you might use each type of technology. Large organizations will find that they are using several types of persistence mechanism and will even install the products of several different vendors. Not only do you have a choice you might be forced to work with a wide range of databases whether you want to or not.