[ http://issues.apache.org/jira/browse/HADOOP-372?page=comments#action_12422385 ] Doug Cutting commented on HADOOP-372: -------------------------------------
Can you provide more details? Is the intent for the mapred.input.format.class property to become multivalued, a parallel list to mapred.input.dir, and when the latter is longer than the former, the first (or last?) input format is used for unmatched entries? I can imagine how MapTask might create its keys, values and a RecordReader, but how would getSplits() and checkInputDirectories() work? Another approach to implementing this is to write an InputFormat that wraps keys and/or values from files of different types in ObjectWritable. Then map() methods unwrap, introspect and cast. With your approach map methods still need to introspect and cache, this just adds the wrapper. To eliminate the wrapper we'd need to move the getInputKeyClass() and getInputValueClass() methods to RecordReader. These are only called in MapRunner.java, when a RecordReader is already available, so this would be an easy change, and the default implementation could be back-compatible, accessing the job. That's a simpler approach, no? Just add files with different keys and value types, and let the types in the files drive things rather than having to declare them up front. > should allow to specify different inputformat classes for different input > dirs for Map/Reduce jobs > -------------------------------------------------------------------------------------------------- > > Key: HADOOP-372 > URL: http://issues.apache.org/jira/browse/HADOOP-372 > Project: Hadoop > Issue Type: New Feature > Components: mapred > Affects Versions: 0.4.0 > Environment: all > Reporter: Runping Qi > > Right now, the user can specify multiple input directories for a map reduce > job. > However, the files under all the directories are assumed to be in the same > format, > with the same key/value classes. This proves to be a serious limit in many > situations. > Here is an example. Suppose I have three simple tables: > one has URLs and their rank values (page ranks), > another has URLs and their classification values, > and the third one has the URL meta data such as crawl status, last crawl > time, etc. > Suppose now I need a job to generate a list of URLs to be crawled next. > The decision depends on the info in all the three tables. > Right now, there is no easy way to accomplish this. > However, this job can be done if the framework allows to specify different > inputformats for different input dirs. > Suppose my three tables are in the following directory respectively: > rankTable, classificationTable. and metaDataTable. > If we extend JobConf class with the following method (as Owen suggested to > me): > addInputPath(aPath, anInputFormatClass, anInputKeyClass, > anInputValueClass) > Then I can specify my job as follows: > addInputPath(rankTable, SequenceFileInputFormat.class, UTF8.class, > DoubleWritable.class) > addInputPath(classificationTable, TextInputFormat.class, UTF8,class, > UTF8.class) > addInputPath(metaDataTable, SequenceFileInputFormat.class, UTF8.class, > MyRecord.class) > If an input directory is added through the current API, it will have the same > meaning as it is now. > Thus this extension will not affect any applications that do not need this > new feature. > It is relatively easy for the M/R framework to create an appropriate record > reader for a map task based on the above information. > And that is the only change needed for supporting this extension. -- This message is automatically generated by JIRA. - If you think it was sent incorrectly contact one of the administrators: http://issues.apache.org/jira/secure/Administrators.jspa - For more information on JIRA, see: http://www.atlassian.com/software/jira
