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I am not looking for these so-called "debugging" solutions which rely on println. I mean to attach a real debugger to a running Hadoop instance, and debugging it from a different machine.
Is this possible? How? jdb?
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On the remote computer, find and start the Remote Debugger from the Start menu. If you don't have administrative permissions on the remote computer, right-click the Remote Debugger app and select Run as administrator. Otherwise, just start it normally.
Starting the Application With Remote Debugging Enabledjar + start a server socket at port 8998 and publish the debugging messages using the Java Debug Wire Protocol (jdwp) there. Other than address, server, and transport, there are other sub-options available for -Xrunjdwp option — for example: suspend.
You can attach the Visual Studio debugger to a running process on a local or remote computer. After the process is running, select Debug > Attach to Process or press Ctrl+Alt+p in Visual Studio, and use the Attach to Process dialog to attach the debugger to the process.
A nicely given at LINK
To debug task tracker, do following steps.
Edit conf/hadoop-env.sh to have following
export HADOOP_TASKTRACKER_OPTS="-Xdebug -Xrunjdwp:transport=dt_socket,address=5000,server=y,suspend=n"
Start Hadoop (bin/start-dfs.sh and bin/start-mapred.sh)
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I've never done it that way as I'd rather my "real" jobs run unhindered by debug-overhead (which can, under circumstances, change the environment conditions anyway): I debug "locally" against a pseudo-instance (normal debugging in eclipse is absolutely no problem), copying specific files from the live environment once I've isolated (by using e.g. counters) where the problem lies.
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