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What is map side join and reduce side join?

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18 September 2014


What is map side join and reduce side join?

Two different large data can be joined in map reduce programming also. Joins in Map phase refers as Map side join, while join at reduce side called as reduce side join.  Lets go in detail, Why we would require to join the data in map reduce. If one Dataset A has master data and B has sort of transactional data(A & B are just for reference). we need to join them on a coexisting common key for a result. It is important to realize that we can share data with side data sharing techniques(passing key value pair in job configuration /distribution caching) if master data set is small. we will use map-reduce join  only when we have both dataset is too big to use data sharing techniques.

Joins at Map Reduce is not recommended way. Same problem can be addressed through high level frameworks like Hive or cascading. even if you are in situation then we can use below mentioned method to join.

Map side Join

Joining at map side performs the join before data reached to map. function It expects a strong prerequisite before joining data at map side. Both joining techniques comes with it’s own kind of pros and cons. Map side join could be more efficient to reduce side but strict format requirement is very tough to  meet natively. however if we would prepare this kind of data through some other MR jobs, will loose the expected performance over reduce side join.

  • Data should be partitioned and sorted in particular way.
  • Each input data should be divided in same number of partition.
  • Must be sorted with same key.
  • All the records for a particular key must reside in the same partition.

Reduce Side Join

Reduce side join also called as Repartitioned join or Repartitioned sort merge join and also it is mostly used join type. This type of join would be performed at reduce side. i.e it will have to go through sort and shuffle phase which would incur network overhead. to make it simple  we are going to add the steps needs to be performed for reduce side join. Reduce side join uses few terms like data source, tag and group key lets be familiar with it.

  • Data Source  is referring to data source files, probably taken from RDBMS
  • Tag would be used to tag every record with it’s source name, so that it’s source can be identified at any given point of time be it is in map/reduce phase. why it is required will cover it later.
  • Group key is referring column to be used as join key between two data sources.

As we know we are going to join this data on reduce side we must prepare in a way that it can be used for joining in reduce phase. let’s have a look what are the steps needs to be perform.

Map Phase

Expectation from routine map function is emit, (Key, value), while to joining at reduce side join we would design map in a way so that it could emit, (Key, Source Tag+Value) of every record for each data source. This output will then go for sort and shuffle phase, as we know these operation would based on key, so it will club all the values from all source at one place regarding a particular key. and this data would reach to reducer

Reduce Phase

Reducer will create a cross product of every record of map out put for one key and will handover to combine function.

Combine function

whether this reduce function is going to perform inner join or outer join  would be decided in combine function. And desired ouput format will also be decided at this place

Please do not get confuse with combiner both are different.


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