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 >>>>>>> >>>>>>> >>>>>>> >>>>>>> >>>>> >>> >>> >
