[ http://issues.apache.org/jira/browse/HADOOP-372?page=comments#action_12430678 ] arkady borkovsky commented on HADOOP-372: -----------------------------------------
The issue as stated in the title is an important useful feature on it's own. However the actual problem that motivated it, as described in the original post, does not necessarily imply the suggested solution. The problem Runping describes looks like a case of Join to me. The suggested solution would help to implement a work-aorund using MapReduce. But is this the best way of allocating development resources? Implementing Join using MultiJobInput in Map step and SplitIterator in Reduce step is a temporary work-around solution. Would it make sense to define interfaces in terms of actual problem (Join)? In short term it still may be implemented piggy-backing MapReduce using two-level keys and MultiJobInput. But the user would be able to think in more appropriate terms. > 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 > Assigned To: Owen O'Malley > > 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
