Program Packaging and Distributed Execution

As described earlier, Flink programs can be executed on clusters by using a remote environment. Alternatively, programs can be packaged into JAR Files (Java Archives) for execution. Packaging the program is a prerequisite to executing them through the command line interface.

Packaging Programs

To support execution from a packaged JAR file via the command line or web interface, a program must use the environment obtained by StreamExecutionEnvironment.getExecutionEnvironment(). This environment will act as the cluster’s environment when the JAR is submitted to the command line or web interface. If the Flink program is invoked differently than through these interfaces, the environment will act like a local environment.

To package the program, simply export all involved classes as a JAR file. The JAR file’s manifest must point to the class that contains the program’s entry point (the class with the public main method). The simplest way to do this is by putting the main-class entry into the manifest (such as main-class: org.apache.flinkexample.MyProgram). The main-class attribute is the same one that is used by the Java Virtual Machine to find the main method when executing a JAR files through the command java -jar pathToTheJarFile. Most IDEs offer to include that attribute automatically when exporting JAR files.

Packaging Programs through Plans

Additionally, we support packaging programs as Plans. Instead of defining a progam in the main method and calling execute() on the environment, plan packaging returns the Program Plan, which is a description of the program’s data flow. To do that, the program must implement the org.apache.flink.api.common.Program interface, defining the getPlan(String...) method. The strings passed to that method are the command line arguments. The program’s plan can be created from the environment via the ExecutionEnvironment#createProgramPlan() method. When packaging the program’s plan, the JAR manifest must point to the class implementing the org.apache.flinkapi.common.Program interface, instead of the class with the main method.

Summary

The overall procedure to invoke a packaged program is as follows:

The JAR’s manifest is searched for a main-class or program-class attribute. If both attributes are found, the program-class attribute takes precedence over the main-class attribute. Both the command line and the web interface support a parameter to pass the entry point class name manually for cases where the JAR manifest contains neither attribute.
If the entry point class implements the org.apache.flinkapi.common.Program, then the system calls the getPlan(String...) method to obtain the program plan to execute.
If the entry point class does not implement the org.apache.flinkapi.common.Program interface, the system will invoke the main method of the class.