Oozie is developed on a client-server architecture. Oozie server is a Java web application that runs Java servlet container within an embedded Apache Tomcat. Oozie provides three different type of clients to interact with the Oozie server: Command Line, Java Client API and HTTP REST API.
Oozie server does not store any in-memory information of the running jobs. It relies on RDBMS to store states and data of all the Oozie jobs. Every time it retrieves the job information from the database and stores updated information back into the database.
Oozie Server (can) sits outside of the Hadoop cluster and performs orchestration of the Hadoop jobs defined in a Oozie Workflow job.