This document discusses using mdb and SmartOS for post-mortem debugging of Node.js applications. Post-mortem debugging allows an application to be debugged after a crash without requiring it to restart, providing minimal downtime. Mdb understands both the native and JavaScript layers of Node.js and has commands like ::findjsobjects to help debug issues like memory leaks. The workshop discussed in the document provides more information on using mdb to debug Node.js applications on SmartOS.

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Node.js post-mortem
debugging with mdb and
SmartOS
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Node.js
var http = require('http');
http.createServer(function (req, res) {
res.writeHead(200, {'Content-Type': 'text/plain'});
res.end('Hello Worldn');
}).listen(4242, '127.0.0.1');
Platform built on top of Chrome's JavaScript runtime for easily
building fast, scalable network applications.
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Debugging challenge
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“My application crashes
randomly every few days,
what’s going on?”

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Common debugging tools
• Live debuggers
• How to reproduce the bug in a timely manner?
• Application cannot restart until investigation done.
• No knowledge of both native and JavaScript layer.
• Logging/Tracing
• What if the information you need was not logged?
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Post-mortem debugging
• OS generates a “photograph” of the program when
it crashes for later inspection with debugger.
• Application can restart right away: minimal
downtime.
• Does not require developers to know when a bug or
crash will happen.
• Investigation can happen at anytime, any number of
times, by anyone.
• Great for debugging rare bugs and production
issues!
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Node.js internals
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mdb on SmartOS
• Understands native and JavaScript layers.
• Useful JavaScript specific commands
like ::findjsobjects and ::jsprint.
• ::findjsobjects -r to find which objects reference
which, useful for investigating memory leaks.
• Commands can be piped to other mdb commands
or to other UNIX programs.
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Other use cases
• Finding memory leaks.
• Investigating 100% CPU utilization issues.
• DTrace with mdb.
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Other platforms
• Work on support for platforms with lldb support:
https://github.com/tjfontaine/lldb-v8.
• Using mdb to load Linux core files: http://
www.joyent.com/blog/mdb-and-linux.
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The workshop
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Learn more
• Join us for the workshop tonight :)
• npm install -g node-debug-school or git clone
https://github.com/joyent/node-debug-school.
• https://www.joyent.com/developers/node/debug/
mdb.
• @misterdjules.
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