Oozie is an Apache open source project, originally developed at Yahoo. Oozie is a general purpose scheduling system for multistage Hadoop jobs.

• Oozie allow to form a logical grouping of relevant Hadoop jobs into an entity called Workflow. The Oozie workflows are DAG (Directed cyclic graph) of actions.
• Oozie provides a way to schedule Time or Data dependent Workflow using an entity called Coordinator.
• Further you can combine the related Coordinators into an entity called Bundle and can be scheduled on a Oozie server for execution.

Oozie support most of the Hadoop Jobs as Oozie Action Nodes like: MapRedude, Java, FileSystem (HDFS operations), Hive, Hive2, Pig, Spark, SSH, Shell, DistCp and Sqoop. It provides a decision capability using a Decision Control Node action and Parallel execution of the jobs using Fork-Join Control Node. It allow users to configure email option for Success/Failure notification of the Workflow using Email action.

    <done-flag>_SUCCESS</done_flag> 

The above snippet in coordinator.xml for input dataset signals the presence of input data. That means coordinator action will be in WAITING state till _SUCCESS file is present in the given input directory. Once it is present, workflow will start execution.