MrSim

MrSim is a basic implementation of the map-reduce algorithm in Java.

What is MapReduce?

MapReduce is a programming model for processing large data sets, and the name of an implementation of the model by Google. MapReduce is typically used to do distributed computing on clusters of computers. See Wikipedia for a detailed description of MapReduce.

There exist many implementations of the map-reduce model, the most popular probably being Apache Hadoop.

What is MrSim?

MrSim is a simple implementation of map-reduce in Java, intended for a pedagogical illustration of the programming model. It originates from frustrating experiences using other frameworks, which require a lengthy and cumbersome setup before running even the simplest example. In most cases those examples are entangled with technical considerations (distributed file system, network configuration) that distract from learning the map-reduce programming model itself.

MrSim aims at providing a simple framework to create and test map-reduce jobs using using a minimal setup (actually no setup at all), using straightforward implementations of all necessary concepts. This entails some purposeful limitations to the system:

It is not optimized in any way, and should not be used to run serious map-reduce computations
It only offers sequential processing of the map-reduce tuples in a single process

In counterpart, MrSim offers interesting features from a pedagogical point of view:

It runs out of the box, simply add the classes (or the jar) to your classpath
The centralized processing makes it easy to perform step-by-step debugging of a map-reduce job (down to the core implementatios of the framework, since all source code is provided)
The map-reduce environment itself is made of less than 250 lines of code
The examples and underlying implementation are simple and easy to understand

Surprisingly, MrSim also offers a few features that large-scale map-reduce implementations (such as Hadoop) don't have:

Inheritance is fully supported when declaring the types for tuple keys and values. This means that a mapper working with tuples of type (K,V) will properly accept a tuple of type (K',V') if K' is a descendant of K and V' is a descendant of V. This does not work in Hadoop.
Tuples output by reducers can be sent directly as input to mappers, making multiple iterations of map-reduce cycles possible. Again, Hadoop does not support this: tuples produced by reducers must be sent serialized to an output collector, and then be re-read from an input collector and converted back into tuples.

As a rule, don't expect any fancy features to be introduced if they interfere with the system's current simplicity.

Compiling and Installing MrSim

First make sure you have the following installed:

The Java Development Kit (JDK) to compile. MrSim was developed and tested on version 6 of the JDK, but it is probably safe to use any later version. Moreover, it most probably compiles on the JDK 5, although this was not tested.
Ant to automate the compilation and build process

Download the sources for MrSim from GitHub or clone the repository using Git:

git clone git@github.com:sylvainhalle/MrSim.git

Compile the sources by simply typing:

ant

This will produce a file called mrsim.jar in the folder. This file is stand-alone and can be used as a library, so it can be moved around to the location of your choice and included in the build path of the project.

In addition, the script generates in the doc folder the Javadoc documentation for using MrSim. This documentation is also embedded in the JAR file. To show documentation in Eclipse, right-click on the jar, click "Properties", then fill the Javadoc location (which is the JAR itself).

How to use MrSim?

See the Source/Examples folder for some examples, and the Source/MapReduce/doc folder for detailed documentation of the code.

Who maintains MrSim?