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From Daniel Barclay <dbarc...@maprtech.com>
Subject Re: The meaning of the methods in StoragePlugin and EasyFormatPlugin
Date Mon, 31 Aug 2015 19:00:49 GMT
Edmon,

I see you're working on adding documentation about creating storage
plug-ins.

I was looking into that myself a little while ago, but wasn't able to
continue.

Below are some notes on the detailed requirements I had extracted from
the code.  Hopefully they'll be helpful in filling in your documentation
of what's required to create a storage plug-in.

Daniel

--------------------------------------------------------------------------------
				Storage Plug-In Notes

Pieces needed for/aspects of a storage plug-in:

* Need storage plug-in configuration class:
   - per StoragePluginConfig (abstract class (currently))
     - (org.apache.drill.common.logical.StoragePluginConfig)
   - public class
   - public no-argument constructor? (modulo Jackson/@JsonTypeName?)
   - one per storage plug-in type?
   - What about Jackson serialization?
     - TODO: REFINE: plug-in type name (for ?) defaults to simple class name; can
       be specified by some kind of NAME property (caps?  any?)
      - what are requirements on serializability?

* Need storage plug-in class:
   - per StoragePlugin (interface)
     - (org.apache.drill.exec.store StoragePlugin)
   - public class (not clearly needed)
   - public constructor ...( SomeStoragePluginConfig, DrillContext, String),
     where:
     - StoragePluginConfig is _specific_ implementation class of
       StoragePluginConfig
   - multiple storage plug-ins can share one storage plug-in class--one
     constructor per StoragePluginConfig class


* Class path scanning requirement:
   - StoragePluginConfig and StoragePlugin implementation classes are found by
     classpath scanning
   - Need drill-module.conf file in root of classpath subtree containing classes
     to be found.
   - Normally need to append name of each package (immediately?) containing
     implementation classes to configuration property
     drill.exec.storage.packages.

* bootstrap-storage-plugins.json
   - Normally need to have bootstrap-storage-plugins.json file in same classpath
     root.
   - Normally have default configuration for plug-in in same classpath root's
     bootstrap-storage-plugins.json file.
   - Format seems to Jackson's serialization of some kind of list of
     StoragePluginConfig:
     - Jackson seems to follow Java Beans getter/setting mapping rules
       (verified only for simple values--String, boolean)
     - (What else?)

* Schema, ROUGH:
   - Calcite's Schema
   - Drill's AbstractSchema
   - implementations of interface Calcite's Table must be subclasses of Drill's
     DrillTable


(Document that old code doesn't follow recently clarified terminology in (most
of) user documetation:
- "storage plug-in" refers to the code itself (what plugs into Drill)
- "storage plug-in configuration" refers to the configuration associated with
   names such as "cp" and "dfs"--different configurations of the file-system
   plug-in
- "storage plug-in configuration name" refers to names such as "cp" and "dfs"
- "storage plug-in type name" refers to ... (e.g., "file", "hive")
- (old terms in code: "storage engine" (sometimes) means storage plug-in
   configuration name)
)

Pending questions:
- Q: What does the @JsonTypeInfo annotation on StoragePluginConfig do?
   Specifically, how exactly does it relate to "type" in 'type: "file"' and to
   "NAME" and "name" fields (JavaBeans/Jackson properties?) on plug-in classes?

   @JsonTypeInfo(use = JsonTypeInfo.Id.NAME,
     include = JsonTypeInfo.As.PROPERTY, property="type") on  StoragePluginConfig
     specify that JavaBeans/Jackson property named "type" on subclasses

- Q: What exactly does SystemTablePluginConfig's _public_ NAME field do?
- Q: What exactly does SystemTablePluginConfig's _public_ INSTANCE field do?


--------------------------------------------------------------------------------




-- 
Daniel Barclay
MapR Technologies

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