I've been deploying MiniDFSCluster for some testing, and while using it/looking through the code I made some notes of where there are issues and improvement opportunities. This is mostly minor as its a test tool, but a risk of synchronization problems is there and does need addressing; the rest are all feature creep.
Field nameNode should be marked as volatile as the shutdown operation can be in a different thread than startup. Best of all,
add synchronized methods to set and get the field, as well as shutdown.
The data dir is set from from System Properties.
This is done in formatDataNodeDirs() corruptBlockOnDataNode() and the constructor.
Improvement: have a test property in the conf file, and only read the system property if this is unset. This will enable
multiple MiniDFSClusters to come up in the same JVM, and handle shutdown/startup race conditions better, and avoid the
"java.io.IOException: Cannot lock storage build/test/data/dfs/name1. The directory is already locked." messages
Messages should log to the commons logging and not System.err and System.out. This enables containers to catch and stream better,
and include more diagnostics such as timestamp and thread Id
Class could benefit from a method to return the FS URI, rather than just the FS. This currently has to be worked around with some tricks involving a cached configuration
waitActive() could get confused if "localhost" maps to an IPv6 address. Better to ask for 127.0.0.1 as the hostname; Junit
test runs may need to be set up to force in IPv4 too.
injectBlocks has a spelling error in the IOException, "SumulatedFSDataset" is the correct spelling