Apache Spark is built around a distributed collection of immutable Java Virtual Machine (JVM) objects called Resilient Distributed Datasets (RDDs for short). As we are working with Python, it is important to note that the Python data is stored within these JVM objects. More of this will be discussed in the subsequent chapters on RDDs and DataFrames. These objects allow any job to perform calculations very quickly. RDDs are calculated against, cached, and stored in-memory: a scheme that results in orders of magnitude faster computations compared to other traditional distributed frameworks like Apache Hadoop.
守护线程 非守护线程