Event based / Map reduce

Map reduce

Map reduce is a pattern based on the map and reduce functions usually found in functional languages. The idea is very simple. The map function is applied in parallel to each object in a set, then the results are collected and combined by the reduce function to get the output. This pattern is great for applicable problems and another advantage come from the fact that when incorporated into a framework, the programmer only need to provide the map and reduce functions. One of the aspects of this pattern that we are currently looking at in one of our cloud computing project is to incorporate a reschedule function. Basically when the operations are been carried out independently on different servers, if a map or a reduce fails we want to reschedule the failed operation without having to reschedule all the other tasks.

Event based

This pattern is similar to the observer pattern. The system is composed of components that can post events (announcers) or listen to events (listeners) and a medium which is the central piece of the solution acting as the liaison between the listeners and announcers. The medium dispatches the received events to affected listeners and allows listener to register and listen to events of their choice. I think the main difference with the observer pattern is the presence of the medium. In the observer pattern the subject maintains a list of its observers and directly notifies them of state changes without passing through an intermediate medium.