Hi Mike, It's analysis* and aggregation, not just aggregation so it's a bit more complex. Each row in the input generates at least one new row of data when we are done.
For our data sizes (~1 billion 2-3kb rows per job now and growing) we originally did normal inserts, but then we switched to bulk imports - it was much faster and a lot less stress on the regionservers. Bulk importing uses a reducer, so even if we went through and changed our M/R pipelines to use a temp table for organized intermediate data, the most efficient way to populate the temp table would be via the bulk loader - using a reducer anyway. -chris * Sorry to be broad but for business reasons I can't talk to much about the analysis details. On Sep 16, 2011, at 1:11 PM, Michael Segel wrote: > > Chris, > > I don't know what sort of aggregation you are doing, but again, why not write > to a temp table instead of using a reducer? > > > > >> Subject: Re: Writing MR-Job: Something like OracleReducer, JDBCReducer ... >> From: [email protected] >> Date: Fri, 16 Sep 2011 11:58:05 -0700 >> To: [email protected] >> >> If only I could make NY in Nov :) >> >> We extract out large numbers of DNA sequence reads from HBase, run them >> through M/R pipelines to analyze and aggregate and then we load the results >> back in. Definitely specialized usage, but I could see other perfectly valid >> uses for reducers with HBase. >> >> -chris >> >> On Sep 16, 2011, at 11:43 AM, Michael Segel wrote: >> >>> >>> Sonal, >>> >>> You do realize that HBase is a "database", right? ;-) >>> >>> So again, why do you need a reducer? ;-) >>> >>> Using your example... >>> "Again, there will be many cases where one may want a reducer, say trying >>> to count the occurrence of words in a particular column." >>> >>> You can do this one of two ways... >>> 1) Dynamic Counters in Hadoop. >>> 2) Use a temp table and auto increment the value in a column which contains >>> the word count. (Fat row where rowkey is doc_id and column is word or >>> rowkey is doc_id|word) >>> >>> I'm sorry but if you go through all of your examples of why you would want >>> to use a reducer, you end up finding out that writing to an HBase table >>> would be faster than a reduce job. >>> (Again we haven't done an exhaustive search, but in all of the HBase jobs >>> we've run... no reducers were necessary.) >>> >>> The point I'm trying to make is that you want to avoid using a reducer >>> whenever possible and if you think about your problem... you can probably >>> come up with a solution that avoids the reducer... >>> >>> >>> HTH >>> >>> -Mike >>> PS. I haven't looked at *all* of the potential use cases of HBase which is >>> why I don't want to say you'll never need a reducer. I will say that based >>> on what we've done at my client's site, we try very hard to avoid reducers. >>> [Note, I'm sure I'm going to get hammered on this when I head to NY in Nov. >>> :-) ] >>> >>> >>>> Date: Fri, 16 Sep 2011 23:00:49 +0530 >>>> Subject: Re: Writing MR-Job: Something like OracleReducer, JDBCReducer ... >>>> From: [email protected] >>>> To: [email protected] >>>> >>>> Hi Michael, >>>> >>>> Yes, thanks, I understand the fact that reducers can be expensive with all >>>> the shuffling and the sorting, and you may not need them always. At the >>>> same >>>> time, there are many cases where reducers are useful, like secondary >>>> sorting. In many cases, one can have multiple map phases and not have a >>>> reduce phase at all. Again, there will be many cases where one may want a >>>> reducer, say trying to count the occurrence of words in a particular >>>> column. >>>> >>>> >>>> With this thought chain, I do not feel ready to say that when dealing with >>>> HBase, I really dont want to use a reducer. Please correct me if I am >>>> wrong. >>>> >>>> Thanks again. >>>> >>>> Best Regards, >>>> Sonal >>>> Crux: Reporting for HBase <https://github.com/sonalgoyal/crux> >>>> Nube Technologies <http://www.nubetech.co> >>>> >>>> <http://in.linkedin.com/in/sonalgoyal> >>>> >>>> >>>> >>>> >>>> >>>> On Fri, Sep 16, 2011 at 10:35 PM, Michael Segel >>>> <[email protected]>wrote: >>>> >>>>> >>>>> Sonal, >>>>> >>>>> Just because you have a m/r job doesn't mean that you need to reduce >>>>> anything. You can have a job that contains only a mapper. >>>>> Or your job runner can have a series of map jobs in serial. >>>>> >>>>> Most if not all of the map/reduce jobs where we pull data from HBase, >>>>> don't >>>>> require a reducer. >>>>> >>>>> To give you a simple example... if I want to determine the table schema >>>>> where I am storing some sort of structured data... >>>>> I just write a m/r job which opens a table, scan's the table counting the >>>>> occurrence of each column name via dynamic counters. >>>>> >>>>> There is no need for a reducer. >>>>> >>>>> Does that help? >>>>> >>>>> >>>>>> Date: Fri, 16 Sep 2011 21:41:01 +0530 >>>>>> Subject: Re: Writing MR-Job: Something like OracleReducer, JDBCReducer >>>>> ... >>>>>> From: [email protected] >>>>>> To: [email protected] >>>>>> >>>>>> Michel, >>>>>> >>>>>> Sorry can you please help me understand what you mean when you say that >>>>> when >>>>>> dealing with HBase, you really dont want to use a reducer? Here, Hbase is >>>>>> being used as the input to the MR job. >>>>>> >>>>>> Thanks >>>>>> Sonal >>>>>> >>>>>> >>>>>> On Fri, Sep 16, 2011 at 2:35 PM, Michel Segel <[email protected] >>>>>> wrote: >>>>>> >>>>>>> I think you need to get a little bit more information. >>>>>>> Reducers are expensive. >>>>>>> When Thomas says that he is aggregating data, what exactly does he >>>>> mean? >>>>>>> When dealing w HBase, you really don't want to use a reducer. >>>>>>> >>>>>>> You may want to run two map jobs and it could be that just dumping the >>>>>>> output via jdbc makes the most sense. >>>>>>> >>>>>>> We are starting to see a lot of questions where the OP isn't providing >>>>>>> enough information so that the recommendation could be wrong... >>>>>>> >>>>>>> >>>>>>> Sent from a remote device. Please excuse any typos... >>>>>>> >>>>>>> Mike Segel >>>>>>> >>>>>>> On Sep 16, 2011, at 2:22 AM, Sonal Goyal <[email protected]> >>>>> wrote: >>>>>>> >>>>>>>> There is a DBOutputFormat class in the >>>>> org.apache,hadoop.mapreduce.lib.db >>>>>>>> package, you could use that. Or you could write to the hdfs and then >>>>> use >>>>>>>> something like HIHO[1] to export to the db. I have been working >>>>>>> extensively >>>>>>>> in this area, you can write to me directly if you need any help. >>>>>>>> >>>>>>>> 1. https://github.com/sonalgoyal/hiho >>>>>>>> >>>>>>>> Best Regards, >>>>>>>> Sonal >>>>>>>> Crux: Reporting for HBase <https://github.com/sonalgoyal/crux> >>>>>>>> Nube Technologies <http://www.nubetech.co> >>>>>>>> >>>>>>>> <http://in.linkedin.com/in/sonalgoyal> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> On Fri, Sep 16, 2011 at 10:55 AM, Steinmaurer Thomas < >>>>>>>> [email protected]> wrote: >>>>>>>> >>>>>>>>> Hello, >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> writing a MR-Job to process HBase data and store aggregated data in >>>>>>>>> Oracle. How would you do that in a MR-job? >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> Currently, for test purposes we write the result into a HBase table >>>>>>>>> again by using a TableReducer. Is there something like a >>>>> OracleReducer, >>>>>>>>> RelationalReducer, JDBCReducer or whatever? Or should one simply use >>>>>>>>> plan JDBC code in the reduce step? >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> Thanks! >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> Thomas >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>> >>>>> >>>>> >>> >> >
