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Discussion thread

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https://www.mail-archive.com/dev@flink.apache.org/msg29894.html
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JIRA

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Jira
serverASF JIRA
serverId5aa69414-a9e9-3523-82ec-879b028fb15b
keyFLINK-14132

Release1.10


Google Doc: https://docs.google.com/document/d/17CPMpMbPDjvM4selUVEfh_tqUK_oV0TODAUA9dfHakc/edit?usp=sharingReleased:

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).


Background and Motivation

When discussing how to support Hive built-in functions in the thread of “FLIP-57 Rework FunctionCatalog” (http://apache-flink-mailing-list-archive.1008284.n3.nabble.com/DISCUSS-FLIP-57-Rework-FunctionCatalog-td32291.html), a module approach was raised.

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There are two aspects of the motivation:

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  1. Enable users to integrate Flink with cores and built-in objects of other systems, so users can reuse whatever they are familiar with in other SQL systems seamlessly as core and built-ins of Flink SQL and Table 
  2. Enpower users to write code and do customized developement for Flink table core

Modules Plugins define a set of metadata, including functions, user defined types, operators, rules, etc. Prebuilt plugins modules will be added and provided, or users may choose write their own. Flink will take metadata from plugins modules as extensions of its core built-in system that users can take advantages of. For example, users can define their own geo functions and geo data types and plug them into Flink table as built-in objects. Another example is users can use an out-of-shelf Hive plugin module to use Hive built-in functions as part of Flink built-in functions.

Background - How Presto Supports Plugins


In Presto, users can write their own plugins by implementing the Plugin interface. In order for Presto to pick up the desired plugins at runtime, users have to drop all the plugin jars into a designated directory in Presto installation.

Presto support plugins via SPI. Class name of each plugin is provided to Presto via the standard Java ServiceLoader interface: the classpath contains a resource file named org.presto.spi.Plugin in the META-INF/services directory for discovery. 

Proposal

Scope

In this FLIP we’ll design and develop a generic mechanism for modular plugins in pluggable modules in Flink table core, with a focus on built-in functions.

We’ll specifically create two plugin implementations module implementations in this FLIP

  • CorePluginCoreModule, with existing Flink built-in functions only
  • HivePluginHiveModule, supporting Hive built-in functions and numerous Hive versions

Overall Design

All plugins modules will implement the Plugin module interface. The Plugin module interface defines a set of APIs to provide metadata such as functions, user defined types, operators, rules, etc. Each plugin module can choose to provide all or only a subset of the metadata. All plugins modules are managed by a PluginManagermoduleManager, and all pluggable metadata are loaded on demand in object lookup.

Flink’s existing core metadata will also be a plugin module named as “CorePlugin”“CoreModule”. Since we want to focus on supporting functions thru Pluginsmodules,we’ll only migrate Flink’s existing built-in functions into the CorePlugin CoreModule at this moment as the first step.

All plugin module metadata will be seen as a part of Flink table core, and won’t have namespaces.

Objects in modules are loaded on demand instead of eagerly, so there won't be inconsistency.

Users have to be fully aware of the consequences of resetting modules as that might cause that some objects can not be referenced anymore or resolution order of some objects changes. E.g. “CAST” and “AS” cannot be overriden in CoreModule and users should be fully aware of that.

How to Load

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Modules

To load pluginsmodules, users have to make sure relevant classes are already in classpath.

Java/Scala:

// new APIs to TableEnvironment
interface TableEnvironment {
    // load a module instance to the end of the module list
    void loadModule(String name, Module m);

    // unload a module instance from module list and other modules remain the same relative positions
    void unloadModule(String name); // the type string defined in the module

    // list all the modules' names according to order in module list
    List<String> listModules();
}

// note the following plugins modules will be of the order they are specified
tableEnv.usePlugins(Core.INSTANCE, MyPlugin.INSTANCEloadModule("a", MyModule.INSTANCE);
tableEnv.loadModule("b", new Xxx(properties));
tableEnv.loadModule("c", new Yyy());
tableEnv.unloadModule("a");

Yaml file:

Pluginsmodules: # note the following plugins modules will be of the order they are specified
  - name: core
  -   type type: core
   - type: mypluginname: a
     type: mymodule
   - type name: b
     type: xxx
      property1 property1: value1
      property2 property2: value2
   - type: yyy

...

name: c
     type: yyy


Based on the plugins module type defined in yaml file, SQL CLI will invoke factory service to search the factory class that provides the given plugin module name, and then set them in TableEnvironment.

If users don’t set plugins, the CorePlugin will be used by default.

All plugin metadata are case-insensitive.

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A few clarifications

  • By default, yaml file doesn’t have the “modules” section in effect, and core module will be loaded by default.
  • If users specify the “modules” section in yaml file, modules will be strictly loaded according to that, if CoreModule is not specified there, it won’t be loaded.

In case users forgot to specify core module, “modules” section will be commented out in yaml file as following 

#modules: # note the following modules will be of the order they are specified
#  - name: core
#     type: core

SQL:

  • SHOW MODULES: show module names in the existing module list in order
  • LOAD MODULE 'name' [WITH (‘type’=’xxx’, 'prop'='myProp', ...)] : load a module with given name and append to end of the module list
  • UNLOAD MODULE 'name’ : unload a module by name from module list and other modules remain the same relative positions


NOTE: the SQL syntax has been discussed again and received some modifications, see FLINK-21045 and the discussion thread: http://apache-flink-mailing-list-archive.1008284.n3.nabble.com/DISCUSS-FLINK-21045-Support-load-module-and-unload-module-SQL-syntax-td48398.html


Resolution Order

Object will be resolved to plugins modules in the order they are defined either in program or in yaml configs. When there are objects sharing the same name, resolution logic will go thru plugins modules in order and return whatever the first one is found, the other ones sitting in the back in the order will be ignored. E.g. if plugins modules are set as “xxx, yyy” where xxx and yyy modules both have a function named “f”, then “f” will always be resolved as that in xxx module.

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Besides, users may want to define different resolution orders for different metadata, e.g. “xxx, yyy” for functions, but “yyy, xxx” for data types. They will not be taken in this FLIP too. We can tackle that problem incrementally when there’s a real need from users.

Classes

Image Removed

The following is a generic design with functions as a specific example.

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Image Added

Module Interface

Plugin Module interface defines a set of metadata that a plugin module can provide to Flink. It provides default implementations for all the APIs thus an implementation can implement only what it’s able to supply.

interface Plugin Module {
   default Set<String> listFunctions() { return Collections.EMPTY_SET };

   default Optional<FunctionDefinition> getFunctionDefinition(String name) { return Optional.empty() };
   // user defined types, operators, rules, etc

}

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ModuleFactory interface

PluginFactory ModuleFactory defines a factory that is used for descriptors to uniquely identify a Plugin module in service discovery, and create an instance of the pluginmodule.

interface PluginFactory ModuleFactory extends TableFactory {
   Plugin createPlugin Module createModule(Map<String, String> properties);
}

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CoreModule and

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CoreModuleFactory

CorePlugin CoreModule is a pre-defined singleton plugin module that contains should contain all built-in metadata of Flink core.

We currently only move built-in functions into CorePluginCoreModule.

public class CorePlugin CoreModule implements Plugin Module {
  public static final Core INSTANCE = new Core();

  private CorePluginCoreModule() { }

  @Override
  public Set<String> listFunctions() {
    return BuiltInFunctionDefinitions.getDefinitions().stream()
        .map(f -> f.getName())

        .collect(Collectors.toSet());
  }

  @Override
  public Optional<BuiltInFunctionDefinition> getFunctionDefinition(String name) {
    return BuiltInFunctionDefinitions.getDefinitions().stream()
          .filter(f -> f.getName().equalsIgnoreCaseequals(name))
          .findFirst();
  }
}


class CorePluginFactory CoreModuleFactory {
  @Override
  public Map<String, String> requiredContext() {
    Map<String, String> context = new HashMap<>();
    context.put("type", "core");
    return context;
  }

  @Override
  public List<String> supportedProperties() {
    return Collections.EMPTY_LIST;
  }

  @Override
  public Plugin createPluginModule createModule(Map<String, String> properties) {
    return CorePluginCoreModule.INSTANCE;
  }
}

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ModuleManager

PluginManager ModuleManager is responsible for loading all the Pluginsmodules, managing their life cycles, and resolve plugin module objects.

public class PluginManager ModuleManager {
  private List<Plugin> pluginsLinkedHashMap<String, Module> modules;

  public PluginManager ModuleManager() {
    this.plugins modules = new ArrayList<>(CorePluginLinkedHashMap<>();

    modules.put("core", CoreModule.INSTANCE);
  }

  public   public void setPlugins(List<Plugin> pluginsloadModule(String name, Module module) { ... }

    this.plugins = plugins;
  public void unloadModule(String name) { ... }

  public Set<Set<String>> Set<String> listFunctions() {
    return pluginsmodules.stream()
        .map(p -> p.listFunctions())

        .flatmap(e → e.stream())
        .collect(Collectors.toSet());
  }

  public Optional<FunctionDefinition> getFunctionDefinition(String name) {
    Optional<Plugin> Optional<Module> p = pluginsmodules.stream()
        .filter(p -> p.listFunctions().contains(name))
        .findFirst();

    return p.isPresent() ? p.get().getFunctionDefinition(name) : Optional.empty();
  }


// addUserDefinedTypes(), getUserDefinedTypes(), etc
}


FunctionCatalog

FunctionCatalog will hold PluginManager ModuleManager to resolve built-in functions.

class FunctionCatalog implements FunctionLookup {
  PluginManager pluginManagerModuleManager moduleManager;
  CatalogManager catalogManager;

  public Optional<FunctionLookup.Result> lookupFunction(String name) {

    // search built-in functions in PluginManagerModuleManager, rather than BuiltInFunctionsDefinitions

    // Resolution order depends on FLIP-57: Rework FunctionCatalog

  }

}

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There was some proposals of merging FunctionCatalog with CatalogManager. It will not be considered in this FLIP.

How to Write and Use a Self-Defined

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Module - Using

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HiveModule as an Example

To support numerous Hive versions, we will use the shim approach, which is similar to that of existing HiveCatalog. Letting users explicitly specifying Hive versions is necessary since there are differences in Flink-Hive data conversions among different Hive versions.

public class HivePlugin HiveModule implements Plugin Module {

  private static final Logger LOGGER = LoggerFactory.getLogger(HivePlugin.class)final String hiveVersion;
  private final String hiveVersion;  private final HiveShim hiveShim;

  public HivePluginHiveModule(String hiveVersion) {
    this.hiveVersion = hiveVersion;
     this    this.hiveShim = HiveShimLoader.load(hiveVersion);
  }

  @Override
  public Set<String> listFunctions() {
       ...
  }  

  @Override
  public Optional<FunctionDefinition> getFunctionDefinition(String name) {
      ...
  }
}



public abstract class HivePluginFactory HiveModuleFactory implements PluginFactory ModuleFactory {

  @Override
  public Plugin createPluginModule createModule(Map<String, String> properties) {
    return new HivePluginHiveModule(properties.get("hive-version"));
  }

  @Override
  public Map<String, String> requiredContext() {
    return new HashMap<String, String>() {{
      put("type", "hive");
    }};
  }

  @Override
  public List<String> supportedProperties() {
    return Arrays.asList("hive-version");
  }
}


Java/Scala:

tableEnv.usePlugins(CorePlugin.INSTANCEloadModule("hive", new HivePluginHiveModule("2_.2_.1"));


Yaml file:

pluginsmodules: 

  - type: core

     name: core

  - type: hive

     name: hive

     hive-version: 2.2.1


Limitations and Future Work


As mention above, though this FLIP provides a generic design and mechanism for all plugin object module object types we want to support, we will only implement functions. Other objects can be added incrementally later on.

Reference


Document

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