Introduction to MapReduce API

This section explains basics of the Hazelcast MapReduce framework. While walking through the different API classes, we will build the word count example that was discussed earlier and create it step by step.

The Hazelcast API for MapReduce operations consists of a fluent DSL like configuration syntax to build and submit jobs. JobTracker is the basic entry point to all MapReduce operations and is retrieved from com.hazelcast.core.HazelcastInstance by calling getJobTracker and supplying the name of the required JobTracker configuration. The configuration for JobTrackers will be discussed later, for now we focus on the API itself. In addition, the complete submission part of the API is built to support a fully reactive way of programming.

To give an easy introduction to people that are already used to Hadoop, we decided to create the class names as familiar as possible to their counterparts on Hadoop. That means while most users will recognize a lot of similar sounding classes, the way to configure the jobs is more fluent due to the already mentioned DSL like styled API.

While building the example, we will go through as much options as possible, e.g. we create a specialized JobTracker configuration (at the end). Special JobTracker configuration are not required, as for all other Hazelcast features you can use "default" as the configuration name, but special configurations offer better options to predict behavior of the framework while execution.

The full example is available here as a ready to run Maven project.


The JobTracker is used to create Job instances whereas every instance of com.hazelcast.mapreduce.Job defines a single MapReduce configuration. The same Job can be submitted multiple times, no matter if executed in parallel or after the previous execution is finished.

ATTENTION: After retrieving the JobTracker, be aware of the fact that it should only be used with data structures derived from the same HazelcastInstance. Otherwise you can get unexpected behavior

To retrieve a JobTracker from Hazelcast, we will start by using the "default" configuration for convenience reasons to show the basic way.

import com.hazelcast.mapreduce.*;

JobTracker jobTracker = hazelcastInstance.getJobTracker( "default" );

JobTracker is retrieved using the same kind of entry point as most of other Hazelcast features. After building the cluster connection, you use the created HazelcastInstance to request the configured (or default) JobTracker from Hazelcast.

Next step will be to create a new Job and configure it to execute our first MapReduce request against cluster data.


As mentioned in the last section, a Job is created using the retrieved JobTracker instance. A Job defines exactly one configuration of a MapReduce task. Mapper, combiner and reducers will be defined per job but since the Job instance is only a configuration, it is possible to be submitted multiple times, no matter if executions happening in parallel or one after the other.

A submitted job is always identified using a unique combination of the JobTracker's name and a, on submit-time generated, jobId. The way for retrieving the jobId will be shown in one of the later sections.

To create a Job, a second class com.hazelcast.mapreduce.KeyValueSource is necessary. We will have a deeper look at the KeyValueSource class in the next section, for now it is enough to know that it is used to wrap any kind of data or data structure into a well defined set of key-value pairs.

Below example code is a direct follow up of the example of the JobTracker section and reuses the already created HazelcastInstance and JobTracker instances.

We start by retrieving an instance of our data map and create the Job instance afterwards. Implementations used to configure the Job will be discussed while walking further through the API documentation, they are not yet discussed.

ATTENTION: Since the Job class is highly depending on generics to support type safety, the generics change over time and may not be assignment compatible to old variable types. To make use of the full potential of the fluent API, we recommend to use fluent method chaining as shown in this example to prevent the need of too much variables.

IMap<String, String> map = hazelcastInstance.getMap( "articles" );
KeyValueSource<String, String> source = KeyValueSource.fromMap( map );
Job<String, String> job = jobTracker.newJob( source );

ICompletableFuture<Map<String, Long>> future = job
    .mapper( new TokenizerMapper() )
    .combiner( new WordCountCombinerFactory() )
    .reducer( new WordCountReducerFactory() )

// Attach a callback listener
future.andThen( buildCallback() );

// Wait and retrieve the result
Map<String, Long> result = future.get();

As seen above, we create the Job instance and define a mapper, combiner, reducer and eventually submit the request to the cluster. The submit method returns an ICompletableFuture that can be used to attach our callbacks or just to wait for the result to be processed in a blocking fashion.

There are more options available for job configuration like defining a general chunk size or on what keys the operation will be operate. For more information, please consolidate the Javadoc matching your used Hazelcast version.


The KeyValueSource is able to either wrap Hazelcast data structures (like IMap, MultiMap, IList, ISet) into key-value pair input sources or to build your own custom key-value input source. The latter option makes it possible to feed Hazelcast MapReduce with all kind of data like just-in-time downloaded web page contents or data files. People familiar with Hadoop will recognize similarities with the Input class.

You can imagine a KeyValueSource as a bigger java.util.Iterator implementation. Whereas most methods are required to be implemented, getAllKeys is optional to implement. If implementation is able to gather all keys upfront, it should be implemented and isAllKeysSupported must return true, that way Job configured KeyPredicates are able to be evaluate keys upfront before sending them to the cluster. Otherwise, they are serialized and transferred as well to be evaluated at execution time.

As shown in the example above, the abstract KeyValueSource class provides a number of static methods to easily wrap Hazelcast data structures into KeyValueSource implementations already provided by Hazelcast. The data structures' generics are inherited into the resulting KeyValueSource instance. For data structures like IList or ISet, the key type is always String. While mapping, the key is the data structure's name whereas the value type and value itself are inherited from the IList or ISet itself.

// KeyValueSource from com.hazelcast.core.IMap
IMap<String, String> map = hazelcastInstance.getMap( "my-map" );
KeyValueSource<String, String> source = KeyValueSource.fromMap( map );
// KeyValueSource from com.hazelcast.core.MultiMap
MultiMap<String, String> multiMap = hazelcastInstance.getMultiMap( "my-multimap" );
KeyValueSource<String, String> source = KeyValueSource.fromMultiMap( multiMap );
// KeyValueSource from com.hazelcast.core.IList
IList<String> list = hazelcastInstance.getList( "my-list" );
KeyValueSource<String, String> source = KeyValueSource.fromList( list );
// KeyValueSource from com.hazelcast.core.IList
ISet<String> set = hazelcastInstance.getSet( "my-set" );
KeyValueSource<String, String> source = KeyValueSource.fromSet( set );


The com.hazelcast.mapreduce.PartitionIdAware interface can be implemented by the KeyValueSource implementation if the underlying data set is aware of the Hazelcast partitioning schema (as it is for all internal data structures). If this interface is implemented, the same KeyValueSource instance is reused multiple times for all partitions on the cluster node. As a consequence, the close and open methods are also executed multiple times but once per partitionId.


Using the Mapper interface, you will implement the mapping logic. Mappers can transform, split, calculate, aggregate data from data sources. In Hazelcast, it is also possible to integrate data from more than the KeyValueSource data source by implementing com.hazelcast.core.HazelcastInstanceAware and requesting additional maps, multimaps, list, sets.

The mappers map function is called once per available entry in the data structure. If you work on distributed data structures that operate in a partition based fashion, then multiple mappers work in parallel on the different cluster nodes, on the nodes' assigned partitions. Mappers then prepare and maybe transform the input key-value pair and emit zero or more key-value pairs for reducing phase.

For our word count example, we retrieve an input document (a text document) and we transform it by splitting the text into the available words. After that, as discussed in the pseudo code, we emit every single word with a key-value pair of the word itself as key and 1 as the value.

A common implementation of that Mapper might look like the following example:

public class TokenizerMapper implements Mapper<String, String, String, Long> {
  private static final Long ONE = Long.valueOf( 1L );

  public void map(String key, String document, Context<String, Long> context) {
    StringTokenizer tokenizer = new StringTokenizer( document.toLowerCase() );
    while ( tokenizer.hasMoreTokens() ) {
      context.emit( tokenizer.nextToken(), ONE );

The code is pretty basic and just splits the mapped texts into their tokens and iterate over the tokenizer as long as there are more tokens and emits a pair per word. What is to note, we're not yet collecting multiple occurrences of the same word but just fire every word on its own.

LifecycleMapper / LifecycleMapperAdapter

The LifecycleMapper interface or its adapter class LifecycleMapperAdapter can be used to make the Mapper implementation lifecycle aware. That means it will be notified when mapping of a partition or set of data begins and when the last entry was mapped.

Only special algorithms might have a need for those additional lifecycle events to perform preparation, cleanup or emit additional values.

Combiner / CombinerFactory

As stated in the introduction, a Combiner is used to minimize traffic between the different cluster nodes when transmitting mapped values from mappers to the reducers by aggregating multiple values for the same emitted key. This is a fully optional operation but is highly recommended to be used.

Combiners can be seen as an intermediate reducer. The calculated value is always assigned back to the key for which the combiner initially was created. Since combiners are created per emitted key, not the Combiner implementation itself is defined in the jobs configuration but a CombinerFactory that is able to create the expected Combiner instance.

Due to the fact that Hazelcast MapReduce is executing mapping and reducing phase in parallel, the Combiner implementation must be able to deal with chunked data. That means, it is required to reset its internal state whenever finalizeChunk is called. Calling that method creates a chunk of intermediate data to be grouped (shuffled) and sent to the reducers.

Combiners can override beginCombine and finalizeCombine to perform preparation or cleanup work.

For our word count example, we are going to have a simple CombinerFactory and Combiner implementation similar to the following one:

public class WordCountCombinerFactory
    implements CombinerFactory<String, Long, Long> {

  public Combiner<Long, Long> newCombiner( String key ) {
    return new WordCountCombiner();

  private class WordCountCombiner extends Combiner<Long, Long> {
    private long sum = 0;

    public void combine( Long value ) {

    public Long finalizeChunk() {
      return sum;

    public void reset() {
      sum = 0;

As mentioned before, the Combiner must be able to return its current value as a chunk and reset the internal state by setting sum back to 0. Since combiners are always called from a single thread, no synchronization or volatility of the variables is necessary.

Reducer / ReducerFactory

Reducers doing the last bit of algorithm work. This can be aggregating values, calculating averages or anything else that is expected by the algorithm to work.

Since values arrive in chunks, the reduce method is called multiple times for every emitted value of the creation key. This also can happen multiple times per chunk if no Combiner implementation was configured for a job configuration.

In difference of the combiners, a reducers finalizeReduce method is only called once per reducer (which means once per key). So, a reducer does not need to be able to reset its internal state at any time.

Reducers can override beginReduce to perform preparation work.

Again for our word count example, the implementation will look similar to the following code snippet:

public class WordCountReducerFactory implements ReducerFactory<String, Long, Long> {

  public Reducer<Long, Long> newReducer( String key ) {
    return new WordCountReducer();

  private class WordCountReducer extends Reducer<Long, Long> {
    private volatile long sum = 0;

    public void reduce( Long value ) {
      sum += value.longValue();

    public Long finalizeReduce() {
      return sum;

Different from combiners, reducer tends to switch threads if running out of data to prevent blocking threads from the JobTracker configuration. They are rescheduled at a later point when new data to be processed arrives but unlikely to be executed on the same thread as before. Due to this fact, some volatility of the internal state might be necessary.


A Collator is an optional operation that is executed on the job emitting node and is able to modify the finally reduced result before returned to the user's codebase. Only special use cases are likely to make use of collators.

For an imaginary use case, we might want to know how many words were all over in the documents we analyzed and for this case, a Collator implementation can be given to the submit method of the Job instance.

A collator would look like the following snippet:

public class WordCountCollator implements Collator<Map.Entry<String, Long>, Long> {

  public Long collate( Iterable<Map.Entry<String, Long>> values ) {
    long sum = 0;

    for ( Map.Entry<String, Long> entry : values ) {
      sum += entry.getValue().longValue();
    return sum;

The definition of the input type is a bit strange but due to the fact that Combiner and Reducer implementations are optional, the input type heavily depends on the state of the data. As stated above, collators are non-typical use cases and the generics of the framework always help in finding the correct signature.


A KeyPredicate can be used to pre-select if a key should be selected for mapping in the mapping phase. If the KeyValueSource implementation is able to know all keys upfront to execution, the keys are filtered before the operations are divided to the different cluster nodes.

It is also possible to be used to select only a special range of data (e.g. a time-frame) or similar use cases.

A basic KeyPredicate implementation to only map keys containing the word "hazelcast" might look like the following code class:

public class WordCountKeyPredicate implements KeyPredicate<String> {

  public boolean evaluate( String s ) {
    return s != null && s.toLowerCase().contains( "hazelcast" );

TrackableJob and Job Monitoring

A TrackableJob instance can be retrieved after submitting a job. It is requested from the JobTracker using the, per JobTracker, unique jobId. It can be used to get runtime statistics of the job. At the moment, the information available are limited to the number of processed (mapped) records and the processing state of the different partitions or nodes (if KeyValueSource is not PartitionIdAware).

To retrieve the jobId after submission of the job, use com.hazelcast.mapreduce.JobCompletableFuture instead of the com.hazelcast.core.ICompletableFuture as variable type for the returned future.

Below snippet will give a quick introduction on how to retrieve the instance and the runtime data. For more information, please have a look at the Javadoc corresponding your running Hazelcast version.

IMap<String, String> map = hazelcastInstance.getMap( "articles" );
KeyValueSource<String, String> source = KeyValueSource.fromMap( map );
Job<String, String> job = jobTracker.newJob( source );

JobCompletableFuture<Map<String, Long>> future = job
    .mapper( new TokenizerMapper() )
    .combiner( new WordCountCombinerFactory() )
    .reducer( new WordCountReducerFactory() )

String jobId = future.getJobId();
TrackableJob trackableJob = jobTracker.getTrackableJob(jobId);

JobProcessInformation stats = trackableJob.getJobProcessInformation();
int processedRecords = stats.getProcessedRecords();
log( "ProcessedRecords: " + processedRecords );

JobPartitionState[] partitionStates = stats.getPartitionStates();
for ( JobPartitionState partitionState : partitionStates ) {
  log( "PartitionOwner: " + partitionState.getOwner()
          + ", Processing state: " + partitionState.getState().name() );

NOTE: Caching of the JobProcessInformation does not work on Java native clients since current values are retrieved while retrieving the instance to minimize traffic between executing node and client.

JobTracker Configuration

The JobTracker configuration is used to setup behavior of the Hazelcast MapReduce framework.

Every JobTracker is capable of running multiple MapReduce jobs at once and so one configuration is meant as a shared resource for all jobs created by the same JobTracker. The configuration gives full control over the expected load behavior and thread counts to be used.

The following snippet shows a typical JobTracker configuration. We will discuss the configuration properties one by one:

<jobtracker name="default">
  <!-- Queue size 0 means number of partitions * 2 -->
  • max-thread-size: Configures the maximum thread pool size of the JobTracker.
  • queue-size: Defines the maximum number of tasks that are able to wait to be processed. A value of 0 means unbounded queue. Very low numbers can prevent successful execution since job might not be correctly scheduled or intermediate chunks are lost.
  • retry-count: Currently not used but reserved for later use where the framework will automatically try to restart / retry operations from an available save point.
  • chunk-size: Defines the number of emitted values before a chunk is sent to the reducers. If your emitted values are big or you want to better balance your work, you might want to change this to a lower or higher value. A value of 0 means immediate transmission but remember that low values mean higher traffic costs. A very high value might cause an OutOfMemoryError to occur if emitted values not fit into heap memory before being sent to reducers. To prevent this, you might want to use a combiner to pre-reduce values on mapping nodes.
  • communicate-stats: Defines if statistics (for example about processed entries) are transmitted to the job emitter. This might be used to show any kind of progress to a user inside of an UI system but produces additional traffic. If not needed, you might want to deactivate this.
  • topology-changed-strategy: Defines how the MapReduce framework will react on topology changes while executing a job. Currently, only CANCEL_RUNNING_OPERATION is fully supported which throws an exception to the job emitter (will throw a com.hazelcast.mapreduce.TopologyChangedException).