Logically, Hadoop is composed of two pieces: distributed storage (HDFS) and distributed processing (YARN and MapReduce). Although the code is very complex, the overall architecture is fairly easy to understand. A client can access both storage and processing through two dedicated modules; they are then in charge of distributing the job across all the working nodes, as shown in the following diagram:
All the Hadoop modules run as services (or instances); that is, a physical or virtual node can run many of them. Typically, for small clusters, all the nodes run both distributed computing and processing services; for big clusters, ...
