Modules
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YARN consists of multiple modules. The modules are listed below as per the directory structure:

hadoop-yarn-api - Yarn's cross platform external interface

hadoop-yarn-common - Utilities which can be used by yarn clients and server

hadoop-yarn-server - Implementation of the hadoop-yarn-api
hadoop-yarn-server-common - APIs shared between resourcemanager and nodemanager
hadoop-yarn-server-nodemanager (TaskTracker replacement)
hadoop-yarn-server-resourcemanager (JobTracker replacement)

Utilities for understanding the code
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Almost all of the yarn components as well as the mapreduce framework use
state-machines for all the data objects. To understand those central pieces of
the code, a visual representation of the state-machines helps much. You can first
convert the state-machines into graphviz(.gv) format by
running:
mvn compile -Pvisualize
Then you can use the dot program for generating directed graphs and convert the above
.gv files to images. The graphviz package has the needed dot program and related
utilites.For e.g., to generate png files you can run:
dot -Tpng NodeManager.gv > NodeManager.png