MrSim is a basic implementation of the map-reduce algorithm in Java.
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.
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.
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).
See the Source/Examples folder for some examples, and the
Source/MapReduce/doc folder for detailed documentation of the code.
MrSim has been developed and is currently maintained by Sylvain Hallé, associate professor at Université du Québec à Chicoutimi (Canada).