pig-commits mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Apache Wiki <wikidi...@apache.org>
Subject [Pig Wiki] Update of "LoadStoreMigrationGuide" by PradeepKamath
Date Wed, 10 Feb 2010 20:22:33 GMT
Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Pig Wiki" for change notification.

The "LoadStoreMigrationGuide" page has been changed by PradeepKamath.
http://wiki.apache.org/pig/LoadStoreMigrationGuide?action=diff&rev1=6&rev2=7

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

  
  = LoadFunc Migration =
  The methods in the old !LoadFunc have been split among a !LoadFunc abstract class and 3
new interfaces - !LoadMetadata (methods to deal with metadata), !LoadPushDown (methods to
push operations from pig runtime into loader implementations) and !LoadCaster with methods
to convert byte arrays to specific types. An example of how a simple !LoadFunc implementation
based on old interface can be converted to the new interfaces will be shown below. The loader
implementation in the example is a loader for text data with line delimiter as '\n' and '\t'
as default field delimiter (which can be overridden by passing a different field delimiter
in the constructor) - this is similar to current !PigStorage loader in Pig.
+ 
+ == Table mapping old API calls to new API calls ==
+ || '''Old Method in !LoadFunc''' || '''Equivalent New Method''' || '''New Class/Interface
in which method is present''' || '''Explanation''' ||
+ || bindTo() || prepareToRead() || !LoadFunc || bindTo() was the old method which would provide
an !InputStream among other things to the !LoadFunc. The !LoadFunc implementation would then
read from the !InputStream in getNext(). In the new API, reading of the data is through the
!InputFormat provided by the !LoadFunc. So the equivalent call is prepareToRead() wherein
the !RecordReader associated with the !InputFormat provided by the !LoadFunc is passed to
the !LoadFunc. The !RecordReader can then be used by the implementation in getNext() to return
a tuple representing a record of data back to pig. ||
+ || getNext() || getNext() || !LoadFunc || The meaning of getNext() has not changed and is
called by Pig runtime to get the next tuple in the data ||
+ || bytesToInteger(),...bytesToBag() ||  bytesToInteger(),...bytesToBag() || !LoadCaster
|| The meaning of these methods has not changed and is called by Pig runtime to cast a !DataByteArray
fields to the right type when needed. In the new API, a !LoadFunc implementation should give
a !LoadCaster object back to pig as the return value of getLoadCaster() method so that it
can be used for casting. If a null is returned then casting from !DataByteArray to any other
type (implicitly or explicitly) in the pig script will not be possible ||
+ || fieldsToRead() || pushProject() || !LoadPushDown || fieldsToRead() was used by old code
to convey to the loader the exact fields required by the pig script -the same semantics are
now achieved through pushProject() of the !LoadPushDown interface. !LoadPushDown is an optional
interface for loaders to implement - if a loader does not implement it, this will indicate
to the pig runtime that the loader is not capable of returning just the required fields and
will return all fields in the data. If a loader implementation is able to efficiently return
only required fields, it should implement !LoadPushDown to improve query performance||
+ || determineSchema() || getSchema() || !LoadMetadata || determineSchema() was used by old
code to ask the loader to provide a schema for the data returned by it - the same semantics
are now achieved through getSchema() of the !LoadMetadata interface. !LoadMetadata is an optional
interface for loaders to implement - if a loader does not implement it || 
  
  == Old Implementation ==
  {{{
@@ -232, +240 @@

  
  }}}
  
- == Table mapping old API calls to new API calls ==
- || '''Old Method in !LoadFunc''' || '''Equivalent New Method''' || '''New Class/Interface
in which method is present''' || '''Explanation''' ||
- || bindTo() || prepareToRead() || !LoadFunc || bindTo() was the old method which would provide
an InputStream among other things to the !LoadFunc to allow it store the stream to use in
getNext(). In the new API, reading of the data is through the !InputFormat provided by the
!LoadFunc. So the equivalent call is prepareToRead() wherein the !RecordReader associated
with the !InputFormat provided by the !LoadFunc is passed to the !LoadFunc. The !RecordReader
can then be used by the implementation in getNext() to return a tuple representing a record
of data back to pig. ||
- || getNext() || getNext() || !LoadFunc ||
- 

Mime
View raw message