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The "LoadStoreMigrationGuide" page has been changed by PradeepKamath. http://wiki.apache.org/pig/LoadStoreMigrationGuide?action=diff&rev1=24&rev2=25 -------------------------------------------------- This page describes how to migrate from the old !LoadFunc and !StoreFunc interface (Pig 0.1.0 through Pig 0.6.0) to the new interfaces proposed in http://wiki.apache.org/pig/LoadStoreRedesignProposal and planned to be released in Pig 0.7.0. Besides the example in this page, users can also look at !LoadFunc and !StoreFunc implementation in the piggybank codebase (contrib/piggybank/java/src/main/java/org/apache/pig/piggybank/storage) for examples of migration. For example, !MultiStorage implements a custom !OutputFormat. - *A general note applicable to both !LoadFunc and !StoreFunc implementations is that the implementation should use the new Hadoop 20 API based on org.apache.hadoop.mapreduce package instead of the old org.apache.hadoop.mapred package.* + '''A general note applicable to both !LoadFunc and !StoreFunc implementations is that the implementation should use the new Hadoop 20 API based classes (!InputFormat/!OutputFormat and related classes) in org.apache.hadoop.mapreduce package instead of the old org.apache.hadoop.mapred package.''' The main motivation for these changes is to move closer to using Hadoop's !InputFormat and !OutputFormat classes. This way pig users/developers can create new !LoadFunc and !StoreFunc implementation based on existing Hadoop !InputFormat and !OutputFormat classes with minimal code. The complexity of reading the data and creating a record will now lie in the !InputFormat and likewise on the writing end, the complexity of writing will lie in the !OutputFormat. This enables !Pig to easily read/write data in new storage formats as and when an Hadoop !InputFormat and !OutputFormat is available for them. @@ -21, +21 @@ ||No equivalent method ||relativeToAbsolutePath() ||!LoadFunc ||Pig runtime will call this method to allow the Loader to convert a relative load location to an absolute location. The default implementation provided in !LoadFunc handles this for hdfs files and directories. If the load source is something else, loader implementation may choose to override this. || ||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, this will indicate to the pig runtime that the loader cannot return a schema for the data || ||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 || - ||No equivalent method ||getInputFormat() ||!LoadFunc ||This method will be called by Pig to get the !InputFormat used by the loader. The methods in the !InputFormat (and underlying !RecordReader) will be called by pig in the same manner (and in the same context) as by Hadoop in a map-reduce java program. || + ||No equivalent method ||getInputFormat() ||!LoadFunc ||This method will be called by Pig to get the !InputFormat used by the loader. The methods in the !InputFormat (and underlying !RecordReader) will be called by pig in the same manner (and in the same context) as by Hadoop in a map-reduce java program. '''If the !InputFormat is a hadoop packaged one, the implementation should use the new API based one in org.apache.hadoop.mapreduce. If it is a custom !InputFormat, it should be implemented using the new API in org.apache.hadoop.mapreduce'''|| ||No equivalent method ||setLocation() ||!LoadFunc ||This method is called by Pig to communicate the load location to the loader. The loader should use this method to communicate the same information to the underlying !InputFormat. This method is called multiple times by pig - implementations should bear in mind that this method is called multiple times and should ensure there are no inconsistent side effects due to the multiple calls. || ||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 - in the new API, this is the method wherein the implementation will use the the underlying !RecordReader and construct a tuple || @@ -39, +39 @@ ||No equivalent method ||relToAbsPathForStoreLocation() ||!StoreFunc ||Pig runtime will call this method to allow the Storer to convert a relative store location to an absolute location. An implementation is provided in !LoadFunc (as a static method) which handles this for hdfs files and directories. || ||No equivalent method ||checkSchema() ||!StoreFunc ||A Store function should implement this function to check that a given schema describing the data to be written is acceptable to it || ||No equivalent method ||setStoreLocation() ||!StoreFunc ||This method is called by Pig to communicate the store location to the storer. The storer should use this method to communicate the same information to the underlying !OutputFormat. This method is called multiple times by pig - implementations should bear in mind that this method is called multiple times and should ensure there are no inconsistent side effects due to the multiple calls. || - ||getStorePreparationClass() ||getOutputFormat() ||!StoreFunc ||In the old API, getStorePreparationClass() was the means by which the implementation could communicate to Pig the !OutputFormat to use for writing - this is now achieved through getOutputFormat(). getOutputFormat() is NOT an optional method and implementation SHOULD provide an !OutputFormat to use. The methods in the !OutputFormat (and underlying !RecordWriter and !OutputCommitter) will be called by pig in the same manner (and in the same context) as by Hadoop in a map-reduce java program. The checkOutputSpecs() method of the !OutputFormat will be called by pig to check the output location up-front. This method will also be called as part of the Hadoop call sequence when the job is launched. So implementations should ensure that this method can be called multiple times without inconsistent side effects. || + ||getStorePreparationClass() ||getOutputFormat() ||!StoreFunc ||In the old API, getStorePreparationClass() was the means by which the implementation could communicate to Pig the !OutputFormat to use for writing - this is now achieved through getOutputFormat(). getOutputFormat() is NOT an optional method and implementation SHOULD provide an !OutputFormat to use. The methods in the !OutputFormat (and underlying !RecordWriter and !OutputCommitter) will be called by pig in the same manner (and in the same context) as by Hadoop in a map-reduce java program. '''If the !OutputFormat is a hadoop packaged one, the implementation should use the new API based one in org.apache.hadoop.mapreduce. If it is a custom !OutputFormat, it should be implemented using the new API in org.apache.hadoop.mapreduce'''. The checkOutputSpecs() method of the !OutputFormat will be called by pig to check the output location up-front. This method will also be called as part of the Hadoop call sequence when the job is launched. So implementations should ensure that this method can be called multiple times without inconsistent side effects. || ||bindTo() ||prepareToWrite() ||!StoreFunc ||bindTo() was the old method which would provide an !OutputStream among other things to the !StoreFunc. The !StoreFunc implementation would then write to the !OutputStream in putNext(). In the new API, writing of the data is through the !OutputFormat provided by the !StoreFunc. So the equivalent call is prepareToWrite() wherein the !RecordWriter associated with the !OutputFormat provided by the !StoreFunc is passed to the !StoreFunc. The !RecordWriter can then be used by the implementation in putNext() to write a tuple representing a record of data in a manner expected by the !RecordWriter. || ||putNext() ||putNext() ||!StoreFunc ||The meaning of putNext() has not changed and is called by Pig runtime to write the next tuple of data - in the new API, this is the method wherein the implementation will use the the underlying !RecordWriter to write the Tuple out || ||finish() ||no equivalent method in !StoreFunc - implementations can use commitTask() in !OutputCommitter ||!OutputCommitter ||finish() has been removed from !StoreFunc since the same semantics can be achieved by !OutputCommitter.commitTask() - (!OutputCommitter.needsTaskCommit() should return true to be able to use commitTask()). ||
