I. Introduction
In today's highly competitive world, more and more Big Data is being incorporated into business decision-making processes. Such data not only has a huge volume, but is also constantly generated. Besides, it is usually distributed across multiple sites. Attempting to collect all relevant data in one place in order to apply our favorite algorithm may not be possible. Even if we assume that we could transfer all the data of interest to one place, it would take too much time to process it. When we finally get the result, it will be outdated, since during this period new data will appear.