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The "LoadStoreMigrationGuide" page has been changed by PradeepKamath. http://wiki.apache.org/pig/LoadStoreMigrationGuide?action=diff&rev1=8&rev2=9 -------------------------------------------------- This page describes how to migrate from the old !LoadFunc and !StoreFunc interface (as of 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. = 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. + The methods in the old !LoadFunc have been split among a !LoadFunc abstract class which has the main methods for loading data and 3 new interfaces + * !LoadMetadata has methods to deal with metadata - most implementation of loaders don't need to implement this unless they interact with some metadata system. The getSchema() method in this interface provides a way for loader implementations to communicate the schema of the data back to pig. The other methods are concerned with other types of metadata like partition keys and statistics + * !LoadPushDown has methods to push operations from pig runtime into loader implementations - currently only projections .i.e the pushProjection() method is called by Pig to communicate to the loader what exact fields are required in the pig script. The implementation can chose to honor the request or respond that it will not honor the request and return all fields in the data + * !LoadCaster has methods to convert byte arrays to specific types. A loader implementation should implement this if casts (implicit or explicit) from !DataByteArray fields to other types need to be supported. + The main change is that the new !LoadFunc API is based on a !InputFormat to read the data. Implementations can choose to use existing !InputFormats like !TextInputFormat or implement a new one. + == 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 || + || 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 || + || No equivalent method || relativeToAbsolutePath() || 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.|| + || No equivalent method || getInputFormat() || 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 || setLocation() || 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.|| + || No equivalent method || setUDFContextSignature() || This method will be called by Pig both in the front end and back end to pass a unique signature to the Loader. The signature can be used to store into the UDFContext} any information which the Loader needs to store between various method invocations in the front end and back end. A use case is to store RequiredFieldList passed to it in LoadPushDown.pushProjection(RequiredFieldList) for use in the back end before returning tuples in getNext()|| + + 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. == Old Implementation == {{{ @@ -20, +31 @@ */ public class SimpleTextLoader extends Utf8StorageConverter { protected BufferedPositionedInputStream in = null; - protected final Log mLog = LogFactory.getLog(getClass()); long end = Long.MAX_VALUE; private byte recordDel = '\n';
