Hadoop MapReduce Comprehensive Description

Map Reduce is a really popular paradigm in distributed computing at the moment. The first paper describing this principle is the one by Google published in 2004. Nowadays Map Reduce is a term that everyone knows and everyone speaks about, because it was put as one of the foundations to the Hadoop project. For most of the people Map Reduce is an equivalent to “Hadoop” and “Big Data”, which is completely wrong. But there are some people that understand the simplest case with WordCount and maybe even building an inverted index using Map Reduce.

But being simple as a concept, it has a kind of complicated implementation in Hadoop. I’ve tried to find a comprehensive description of it with a good diagram over the internet, but failed. All the diagrams keep repeating “Map – Sort – Combine – Shuffle – Reduce”. Of course, it is good to know that the framework works this way, but what about dozens of parameters that are tunable for the framework? What happens if you reduce the buffer size of the Map output or increase it? These diagrams don’t offer any help for this. This was the reason for me to build my own diagram and my own description based on the latest source code available in the Hadoop repository.

Hadoop MapReduce Comprehensive Diagram

Hadoop MapReduce Comprehensive Diagram

The overall MapReduce starts with InputFormat. It is the class you specify in the driver application that understands the input and provides you two main interface functions: getSplits that returns set of input data splits and getRecordReader that provides you an iterable interface for reading all the records from a single input split. The size of input split depends on the InputFormat – for text files it is a single block of the HDFS (dfs.blocksize), for gzip-compressed files it is the whole file (as gzip archive is not splittable), etc. Each Mapper processes a single input split, which means in most cases it processes dfs.blocksize data, which equals to 128MB by default.

Your Mapper class function “map” is executed for every key-value pair of the input split, which basically means for each key-value pair returned by the RecordReader defined by InputFormat. For each of them you generate some output and write it to the Context object provided as a parameter of “map” function.

The class responsible for collecting the “map” output is specified by mapreduce.job.map.output.collector.class property and defaults to org.apache.hadoop.mapred.MapTask$MapOutputBuffer implementation. Later in this step I will describe this specific implementation as no other implementation is shipped with Hadoop at the moment.

“Map” function output first goes to the the getPartition function of the Partitioner class. It takes as input the key, the value and the amount of reducers (amount of partition) and returns the number of partition this K-V pair should go to. The default implementation is the following:

public class HashPartitioner<K, V> extends Partitioner<K, V> {
public int getPartition(K key, V value, int numReduceTasks) {
return (key.hashCode() & Integer.MAX_VALUE) % numReduceTasks;

Then the output together with the partition number is written to the circular buffer in memory, which size is defined by the parameter mapreduce.task.io.sort.mb (defaults to 100MB) – it is the total amount of memory allowed for the map output to occupy. If you do not fit into this amount, your data would be spilled to the disk. Be aware that with the change of the default dfs.blocksize from 64MB to 128MB even the simplest identity mapper will spill to disk, because map output buffer by default is smaller than the input split size.

Spilling is performed in a separate thread. It is started when the circular buffer is filled by mapreduce.map.sort.spill.percent percent, by default 0.8, which with the default size of the buffer gives 80MB. By default if your map task for a single input split outputs more than 80MB of data, this data would be spilled to local disks. Spilling works in a separate thread allowing mapper to continue functioning and processing input data while spilling happens. Mapper function is stopped only if the mapper processing rate is greater that spilling rate, so that the memory buffer got 100% full – in this case mapper function will be blocked and wait for the spill thread to free up some space.

Spilling thread writes the data to the file on the local drive of the server where the mapper function is called. The directory to write is determined based on the mapreduce.job.local.dir setting, which contains a list of the directories to be used by the MR jobs on the cluster for temporary data. One directory out of this list is chosen in a round robin fashion. But the data is not just written to the disk. Before this the sorting is performed, and the algorithm used is a QuickSort. The comparator function is defined in the way that it first compares the partition number and only after that the key value, this way the data is sorted by partitions, and within each partition it is sorted by the key.

After the sorting step the Combiner is invoked. It is used to reduce the amount of data written to the disk. There is a corner case when sorter and combiner are not invoked – when the size of a single record produced by mapper is too big to fit in memory (greater than the output buffer size). In this case the record would be written directly to the disk, without combiner and sorter. Output of the combiner function is written to the disk.

Regardless of the amount of data mapper has outputted, when it is finished the “Spill” method is called, which means the data would be written to the disk at least once. It is performing the same task – sorts and combines the data, then outputs it to the file.

Each file outputted by the spill thread has an index, which contains information about partitions in each of the files – where the partition starts and where it finishes. These indexes are stored in memory, and the amount of memory dedicated to this task is mapreduce.task.index.cache.limit.bytes, which equals to 1MB by default. If it is not enough to store the index in memory, the indexes for all the next spill files created would be written to the disk together with the spill file.

When both mapper and the last spilling is finished, the spill thread is terminated and the merge phase starts. During the merge phase, all the spill files should be grouped together to form a single map output file. By default a single merge process can process up to 100 spill files (the parameter responsible for this is mapreduce.task.io.sort.factor). If the amount of spills are greater than this, subsequent merges would be executed to merge all the spills to a single file.

During the merge, if the amount of files being merged is more or equal to min.num.spills.for.combine (3 by default), then the combiner would be executed on top of the “merge” result before writing it to the disk.

The result of the MapTask is a single file containing all the output data of the mapper together with the index file describing the partitions start-stop information for the ReduceTask to be able to easily fetch the part related to the reducer it would run.

Now let’s describe the ReduceTask. Unlike the mappers, the amount of reducers should be specified by the mapreduce.job.reduces parameter, which defaults to 1. The overall logic of the shuffle and merge performed before the reducer will start is defined by the class implementing “Shuffle Consumer Plugin” which is defined by mapreduce.job.reduce.shuffle.consumer.plugin.class property and defaults to org.apache.hadoop.mapreduce.task.reduce.Shuffle. This is the only implementation shipped with Hadoop, so in the later description I would describe only it.

The first thing that the reduce side is doing is starting the “Event Fetcher” thread, which polls the application master for the status of the mappers and listening to the events of mapper execution finish. When it is finished, this information is passed to one of the “Fetcher” threads. The amount of “Fetcher” threads is defined by mapred.reduce.parallel.copies and defaults to 5, which means that a single reduce task might have 5 threads copying data from the mappers in parallel. The fetch is performed using HTTP or HTTPS protocol, the fetcher is connecting to the related datanode URL.

All the data the “Fetcher” downloads from the Mapper side is stored in memory. The amount of memory allocated for this is equal to mapreduce.reduce.shuffle.input.buffer.percent out of the whole reducer memory, which is set by mapreduce.reduce.memory.totalbytes. Having these values default to 0.7 and 1024MB respectively, the total amount of map outputs a single reducer can store in memory by default is 716MB. If this amount of memory is not enough, the fetchers start to save the map outputs to the local disks on the reducer side, in one of the mapreduce.job.local.dir directories.

Mergers are following the fetchers. But they don’t wait for the whole fetching process to finish, they are working in a separate threads. There are 3 types of mergers implemented in Hadoop, and each one of them works in a single separate thread:

  1. InMemory merger. Cannot be disabled, triggered when then memory buffer occupied by the MapTask outputs fetched by this task reaches reduce.shuffle.merge.percent of the total memory allowed to be occupied by this buffer. Executes combiner after the merge. The output is written to the disk. Always invoked at least once;
  2. MemToMem merger. Can be enabled with the setting reduce.merge.memtomem.enabled (disabled by default). It merges the mapper outputs located in the memory and writes the output back to the memory. It is triggered when the amount of distinct MapTask outputs reaches mapreduce.reduce.merge.memtomem.threshold, equal to 1000 by default;
  3. OnDisk merger. Merges the files located on the disks, triggered when the amount of files increases (2 * task.io.sort.factor – 1), but does not merge more than mapreduce.task.io.sort.factor files in a single run.

All of these mergers are followed by the fourth one called finalMerge. This merge is performed in the main thread of the reducer and in a single run groups together all the MapTask outputs left in memory with all the files left on the local disks created by either InMemory or OnDisk mergers. The output data of the final merge is split between the RAM and disks. The amount of RAM allowed to be utilized as a reducer input is mapred.job.reduce.markreset.buffer.percent of the total reducer heap size, but at the moment it defaults to 0.

After all of this preparation the reducer is started and its output is written directly to the HDFS.

Here is the whole description. Feel free to add your comments if I have missed something.


61 thoughts on “Hadoop MapReduce Comprehensive Description

  1. sunil

    your blog says partioning and filling the circular buffer but some books says map output sends to buffer then partition and spill, which one is correct…plz correct me….

    1. 0x0FFF Post author

      You can take a look at the code here: https://github.com/apache/hadoop/blob/branch-2.7.0/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapred/MapTask.java, classes OldOutputCollector and NewOutputCollector. Both of them are calling partitioner.getPartition on insertion the data to the buffer, so “partitioning” itself happens immediately before the data is inserted into the circular buffer

      1. Amit

        Hi I am getting confused with other articles with regards to sequence of processing . i am non java user but wanted to understand the real exact sequence for map reduce .
        Can you please help ?


        1. Amit

          Also please look into the white paper located at “https://hadoop-toolkit.googlecode.com/…/White%20paper-HadoopPerfo”

          1. 0x0FFF Post author

            My description does not conflict with the description given in the article you are referencing to. They are simplifying and omitting some steps for you to get a better understanding of what is happening there on the high level, while my article covers more in-depth steps happening in the MR process. Also be aware that the article you are referencing to is from 2009 year, while my article is from late 2014 – many changes has happened to the code and logic

  2. Abhay Godbole

    Hi Alexey ,
    Excellent article, first time I have read such a detail explanation after Tom White. Thanks for sharing it.
    I have one doubt about the split. When file is stored on HDFS, it gets split into blocks as per the dfs.blocksize. My question is when mapreduce job takes it for processing, the InputFormat’s RecordReader splits it again as per the record boundaries or use the same HDFS splits?

    Please get back


    1. 0x0FFF Post author

      Thank you! I’ve written this article because I wasn’t satisfied with the description given by Tom White plus I didn’t find a good answer for my questions on stackoverflow.

      Regarding your question: data in HDFS is stored in blocks split by binary boundaries, each block is exactly 128MB (by default). When the data is processed, you need to process logical records, not the binary blocks. The way to split the data into logical records is determined by the input format – for TextInputFormat one logical records is a string ending by newline character. This way InputFormat is responsible for splitting the data into input splits, each input split containing even number of logical records. For TextInputFormat it is implemented this way: whole first block of the file is the first input split + the first characters of the second block ending by the first newline character there. Second input split is the block of data starting from the first newline character in the second HDFS block and finishing with the first newline character in the third HDFS block (or EOF). And so on. For sequence files it is a bit different, they have special sync-markers for the InputFormat to be able to effectively split the data into logical records

    1. 0x0FFF Post author

      Mostly the source code itself. I didn’t find enough information in guides/descriptions/books, so source code was the only source of truth

  3. mike

    does each reducer get all the data? i.e if you have 5 reducers, each 5 reducers gets all the data by all the maps? that seems unclear in diagram. I thought each partition data goes to its corresponding reducer and not all.

    1. 0x0FFF Post author

      No, you are wrong. Each reducer reads only part of each mapper’s output related to its partition, which in general might be empty. On the picture I drew the mapper output as 2 files: map output file and index file. Index file shows the reducer how to locate the part of map output file related to its partition

  4. Shuxin Lin

    Really good explanation! it is said that the reducer is always the same as the combiner. but I cant get the point until I read this article about the sort before combiner. Thank you very much!

    1. Dario

      Hello Alexey,
      great article. After reading TW I was wondering how reducers would know which parts of the mapper output file to read from…indexes, of course!

      I have one more doubt: if I would read a mapper output file (which never passed through a combiner function) how would its insides look like?

      1) Values would be grouped per key in a long array:
      key_A, [value_1, value_2, …, value_n]
      key_B, [value_1, value_2, …, value_n]

      key_N, [value_1, value_2, …, value_n]

      2) Values would be ordered per key as simple key/value pairs (until they get to the merge phase on the reducer):
      key_A, value_1
      key_A, value_2

      key_A, value_n
      key_B, value_1
      key_B, value_2

      key_B, value_n

      key_N, value_1
      key_N, value_2

      key_N, value_n

      Thank you

      1. 0x0FFF Post author

        Here is the code responsible for writing map output: https://github.com/apache/hadoop/blob/branch-2.7.1/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapred/IFile.java
        As you see from this code, it writes to the file key-value pairs in a following way:
        “key length in bytes”, “value length in bytes”, “key”, “value”
        “key length in bytes”, “value length in bytes”, “key”, “value”

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  6. Hrishikesh Tiwary

    Great Explanation, but i have a question here.. i understand HashPartioner is called by default but does it happen for Combiner also ? does MapReduce call Combiner also by default even if we dont provide ?
    Combiner has been used at mapper stage when it writes to file and then in Shuffle and Reducer stage. kindly clear this.

    1. 0x0FFF Post author

      No, combiner is optional and by default nothing is called. Combiner is a separate piece of code that should be provided by you, and only in special cases (like WordCount) you can reuse Reducer class for it

  7. tao

    I think the property “mapred.job.reduce.input.buffer.percent” decide the amount of RAM allowed to be utilized as a reducer input. Other than “mapred.job.reduce.markreset.buffer.percent”: The percentage of memory -relative to the maximum heap size- to be used for caching values when using the mark-reset functionality.

    1. 0x0FFF Post author

      Parameter “mapred.job.reduce.input.buffer.percent” is used only in finalMerge step (https://github.com/apache/hadoop/blob/branch-2.7.2/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/task/reduce/MergeManagerImpl.java#L690), that merges results of memtomem and inmemory mergers. It is the percentage of the memory allowed to remain in memory when the final merge starts, and if the size of the segments in memory is greater than this allowed value they would be spilled to disk (https://github.com/apache/hadoop/blob/branch-2.7.2/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/task/reduce/MergeManagerImpl.java#L619)

      markreset parameter is completely different. It is utilized only when you use MarkableIterator to read the values on the reducer side (http://blog.ring.idv.tw/comment.ser?i=373), and is not directly related to the MapReduce implementation. Here is the code for it (https://github.com/apache/hadoop/blob/branch-2.7.2/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapred/BackupStore.java), just a different kind of iterator supporting re-iteration, here is some more details on how it works: http://stackoverflow.com/questions/21464720/how-can-i-use-markableiterator-in-hadoop (but don’t mention the details of the question, it is supported now)

    1. 0x0FFF Post author

      Thank you for the feedback. Generally speaking, this article is not meant to be an introduction to MapReduce. You can find tons of introductions with examples over the internet. This is a comprehensive description – when you already know how it works and how MapReduce processes the data, to get a deeper insight into what’s happening there

  8. hunhun

    I have some puzzles.
    The code
    bufferRemaining = Math.min(distanceTo(bufindex, kvbidx) – 2 * METASIZE,
    softLimit – bUsed) – METASIZE;
    What is meaning “2*METASIZE”? Why “bufferRemaining” equal that?


    1. 0x0FFF Post author

      I’m not the one who has written this code (because I’d create a discriptive diagram before writing this), but I suspect that the buffer itself stores only METASIZE bytes of data, or NMETA integer entries (4 integers in current implementation):
      kvmeta.put(kvindex + PARTITION, partition);
      kvmeta.put(kvindex + KEYSTART, keystart);
      kvmeta.put(kvindex + VALSTART, valstart);
      kvmeta.put(kvindex + VALLEN, distanceTo(valstart, valend));

      So each metadata entry is METASIZE bytes long. bufferRemaining is the number of bytes left in buffer before soft limit is reached, i.e. before the background spilling process is started. In this calculation, “distanceTo(bufindex, kvbidx) – 2 * METASIZE” means the total capacity of the buffer without 2 records, see the comment “The write should block only if there is insufficient space to complete the current write, write the metadata for this record, and write the metadata for the next record” – i.e. we always want to keep at least 2 slots available. The second option under “min” is “softLimit – bUsed”, and this is the calculation of the number of bytes before soft limit (i.e. capacity triggering spilling) is reached. In extreme case with empty buffer and io.sort.spill.percent set to 1.0 (or 100%) this value might equal to the total capacity of the buffer, so the first parameter would be smaller than the second.

  9. Sudhakar

    Wow. Awesome explanation. We had a long running reducer and was trying to figure out why by looking at the log output. Log output is not really intuitive (obviously) but your explanation will surely simplify this. Awesome job again. And kudos to your documentation and diagrammatic skills as well. Way to go!! You should write a book in the future.

  10. Sudhakar

    Is the below statement correct? Should it not be a single REDUCER and not mapper?

    “Having these values default to 0.7 and 1024MB respectively, the total amount of map outputs a single mapper can store in memory by default is 716MB.”

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  12. Zx Yuan

    According to the description of your picture, actually, a Reducer gets its parts of Mapper output at the beginning stage of SHUFFLE. Is that right?

    1. 0x0FFF Post author

      There is no clear definition of boundaries between sort, shuffle and merge. Some even call the whole “sort-shuffle-merge” process a “shuffle”, which is not very precise but correct in general. I prefer to think that “shuffle” starts after the “map” tasks generate their indexed output files, because “shuffle” really means shuffling data across the cluster, while no shuffling is performed before the “map” output file is generated

  13. gooner87

    Is number of spills equal to number of map tasks? i.e. circular buffer memory is allocated per map task? Also, in the flow diagram Combiner is after Sort. Isn’t it combiner followed by sort?

    1. 0x0FFF Post author

      No. Each map task has one or more spills. One happens when there is enough space in circular buffer to fit the mapper output.
      Circular buffer is allocated per map task
      Combiner is effectively doing “reduce” task, i.e. combining different values for the same key. Its input should already be split by key (i.e. >) in order to operate, this is why it consumes sorted data

  14. gooner87

    If we have 1000 spill files then we will have 10 Merge files in the first phase if sort.factor is 100 and in the second phase we will have merge final after merging 10 merge files from the first phase. Am I correct? Also, Since we have merged from 100 spill files there can be entries for the same key hence we are doing combiner after first phase of merging. Am I correct?
    Thanks in advance!!!!!!!

    1. 0x0FFF Post author

      Yes and yes. You might have combiner called multiple times on your input, and even partially combined input, this is why combiner function is associative and commutative

  15. Aashish Soni

    Great blog ! I Have question’s
    1. What is the execution flow of MapReduce when no Combiner and default Hash Partitioner used?
    Map-> Partition-> Sort->Shuffle->Sort->reduce
    Correct me if I am wrong
    2. What is the execution flow of MapReduce when Costum Combiner and custom partitioner is used?
    Map-> Partition->Combiner-> Sort->Shuffle->Sort->Reducer
    Correct me if I am wrong
    3. In case of Identity reducer Shuffle,Sort phase will execute or not ? and happen in case of Identity Mapper?
    4. combiner run on the every mapper output right? and where is execution of partition happen in map or reduce?

    1. 0x0FFF Post author

      1. map->partition->sort->{merge}->shuffle->merge->reduce
      2. map->partition->sort->combine->{merge->[combine->]}shuffle->{merge->[combine->]}->reduce
      3. Shuffle and sort would execute in case of identity mapper, identity reducer and both at the same time. They won’t execute only in case of map-only jobs
      4. Combiner might run many times on top of map output or not run at all – it depends. Partitioning happens on the mapper side

      1. aashish soni

        Thank you very much for your reply ..

        I have some doubts

        1. As per your comment merging is happens between sort and shuffle phase(why did you put merge in bracket). As per my understanding I know, merging happens after the shuffle part and also sorting would perform there. Sorting would perform after merging or not?(Is merging and sorting same?)

        2. Can you please explain to me this , I am unable to understand.

        Can you please guide me .


        1. 0x0FFF Post author

          Merging happens before shuffle if mapper produces more than one output file. Merging on the reducer side happens every time.
          Sorting is ordering elements in array, while merging is joining N sorted arrays together in a single sorted array

          1. aashish soni

            Thanks !

            Could you please to me in more detail ,the execution flow when Costum Combiner and custom partitioner is used?


          2. 0x0FFF Post author

            I described it both in text and on the picture. Feel free to refer to the source code as well. If you have a more specific question – feel free to ask

  16. Giorgio

    Hi Alexey,

    It’s possible that is there a type in the image you provided?
    When you write twice Index Spill file 1, Index Spill file 1, it should be Index Spill file 1, Index Spill file 2, or not?

    Many thanks for great explanation.

  17. wc

    I’ve read several articles describing the differences between Spark and Hadoop in the reduce phase, one being that in Hadoop, mapper outputs are pushed continually to reducers, where as in Spark, reducers pull data from mappers, and that all output from mappers must be available. My understanding was that in order for a reducer operation to occur, all data for that reducer (i.e. partition from each mapper) must be available. Is my understanding of the reducer process correct?

  18. ezysky


    in the reduce side, for memory merge phase, there will be multiple threads for MemToMem Merger? which configuration is used to tune this thread number?

    Also, the merge output is in local file system. why Hadoop can’t output all MtM Merge Output into the same file so that we don’t need to read them in for another merge when the file number exceeds the mapreduce.task.io.sort.factor? thus, we could avoid reading several files in and merge them.

  19. Bogdan

    In the apache documentation, there’s mentioned a Secondary Sort, making use of the Job.setSortComparatorClass(Class) and Job.setGroupingComparatorClass(Class) classes. I’m wondering where this Secondary Sort happens in the grand scheme.

  20. Eugene Ilchenko

    Alexey, thanks a lot for sharing! I love your diagram and detailed explanation – have been searching for something like this for a while.

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  22. Rajaram S

    Really comprehensive information. You have achieved your aim of giving a one stop article about end to end map reduce flow. It would be great if you include what Spilled records in Reducer means. Is it the data on disk which the fetcher thread cant keep in memory?

  23. Jack

    There are so many unobvious components in this diagram which don’t seem why do they exist? So many different mergers and combiners? What the hell a combiner does? What is fetcher? Why should it be there? fetching data is part of a process and shouldn’t be an existing entity. Why does a combiner exist after InMemory merger and not after other mergers? Why after final merge output is split into both ram and disc? What is the reason? It needs explanation. For now just wastes another bunch of minutes.


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