When the Flink system is started, it bring up the JobManager and one or more TaskManagers. The JobManager is the coordinator of the Flink system, while the TaskManagers are the worksers that execute parts of the parallel programs. When starting the systen in local mode, a single JobManager and TaskManager are brought up within the same JVM.
When a program is submitted, a client is created that performs the pre-processing and turns the program into the parallel data flow form that is executed by the JobManager and TaskManagers. The figure below illustrates the different actors in the system very coarsely.
An alternative view on the system is given by the stack below. The different layers of the stack build on top of each other and raise the abstraction level of the program representations they accept:
The runtime layer receive a program in the form of a JobGraph. A JobGraph is a generic parallel data flow with arbitrary tasks that consume and produce data streams.
The optimizer and common api layer takes programs in the form of operator DAGs. The operators are specific (e.g., Map, Join, Filter, Reduce, ...), but are data type agnostic. The concrete types and their interaction with the runtime is specified by the higher layers.
The API layer implements multiple APIs that create operator DAGs for their programs. Each API needs to provide utilities (serializers, comparators) that describe the interaction between its data types and the runtime.
The Flink system code is divided into multiple sub-projects. The goal is to reduce the number of dependencies that a project implementing a Flink progam needs, as well as to faciltate easier testing of smaller sub-modules.
The individual projects and their dependencies are shown in the figure below.
In addition to the projects listed in the figure above, Flink currently contains the following sub-projects:
flink-dist: The distribution project. It defines how to assemble the compiled code, scripts, and other resources into the final folder structure that is ready to use.
flink-addons: A series of projects that are in an early version. Currently contains among other things projects for YARN support, JDBC data sources and sinks, hadoop compatibility, graph specific operators, and HBase connectors.
flink-quickstart: Scripts, maven archetypes, and example programs for the quickstarts and tutorials.