[GitHub] [flink] c4emmmm opened a new pull request #9033: [FLINK-13167] Introduce FlinkML import/export framework

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c4emmmm opened a new pull request #9033: [FLINK-13167] Introduce FlinkML import/export framework
URL: https://github.com/apache/flink/pull/9033
 
 
   ## What is the purpose of the change
   
   Importing/exporting a trained model is an important part in machine learning lifecycle, especially for serving purpose, hence it triggered a heatedly discussion in the previous pull request for FlinkML basic interfaces. Some thoughts about this are summarized in the following doc.
   
   https://docs.google.com/document/d/1B84b-1CvOXtwWQ6_tQyiaHwnSeiRqh-V96Or8uHqCp8/edit?usp=sharing
   
   This pull request introduces the import/export framework based on the discussion. 
   
   ## Brief change log
   
   Import and export framework have almost the same structure so we take export as example. Export framework has 3 basic components, including 2 interfaces for exporter authors to implement, and one util class for end-users to acquire an Exporter.
   
   Interfaces:
    - Exporter: Interface of model exporters. Implementations can export supported models to a specific target model type, such as PMML.
   
    - ExporterFactory: Interface of factories to create Exporters. Each implementation supports only one model type. Typically an ExporterFactory is given a model instance with a property map if needed, and returns an Exporter that supports the model to export to target model type. ExporterFactories are loaded with ServiceLoader framework of Java, and should be registered in the file "META-INF/services/org.apache.flink.ml.api.misc.exportation.ExporterFactory".
   
   Utility:
    - ExporterLoader: Loader to get an Exporter for a specific model format and a model instance. Users would get Exporters via this class rather than directly creating with constructors. This is a concrete util class and no one needs to override it.
   
   Besides, a new enumeration named ModelType is also introduced, listing all types FlinkML may support. This would be extended when a new exporter support a new type.
   
   ## Verifying this change
   
   This change would add tests for Factory and FactoryLoader, TBD.
   
   ## Does this pull request potentially affect one of the following parts:
   
     - Dependencies (does it add or upgrade a dependency): (no)
     - The public API, i.e., is any changed class annotated with `@Public(Evolving)`: (no)
     - The serializers: (no)
     - The runtime per-record code paths (performance sensitive): (no)
     - Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)
     - The S3 file system connector: (no)
   
   ## Documentation
   
     - Does this pull request introduce a new feature? (yes)
     - If yes, how is the feature documented? (JavaDocs)
   

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