It’s important to be able to figure out what’s going on when things go wrong in your Node.js production application. Tools are needed to investigate memory leaks, crashes and other "interesting" events in production. The post-mortem community working group (https://github.com/nodejs/post-mortem) is working on these problems. Come and learn about the key issues being worked, and the progress of the working group so far as illustrated through examples and code.

1. Node.js Postmortem
Working Group Update
Yunong Xiao, Netflix
Michael Dawson, IBM

2. Yunong Xiao
Platform Architect, Netflix
@yunongx
http://yunong.io

3. About Michael

4. About The Postmortem Workgroup
Howard Hellyer
@hhellyer
David Pacheco
@davepacheco
Julien Gilli
@mistredjules
Michael Dawson
@mhdawson
Chris Bailey
@seabaylea
Daniel Khan
@danielkhan
Joshua Clulow
@jclulow
Yunong Xiao
@yunong
James Bellenger
@jbellenger
Bradley Meck
@bmeck
Luca Maraschi
@lucamaraschi
David Clements
@davidmarkclements
Richard Chamberlain
@rnchamberlain

5. Mission Statement
The working group is dedicated to the support and improvement of
postmortem debugging for Node.js.

6. Debugging Node.js

7. Debugging Node.js
Test Environment

8. What about production?

11. “The method described in this article was designed to
provide a core dump… with a minimal impact on the
spacecraft… as the resumption of data acquisition from
the spacecraft is the highest priority.”
- Chafin, R. "Pioneer F & G Telemetry and Command Processor Core Dump
Program." JPL Technical Report XVI, no. 32-1526 (1971): 174.

12. Core Dumps: Brief History
● Magnetic core memory
● Dump out the contents
of “core” memory for
debugging
● “Core dump” was coined
● Initially printed on paper
● Postmortem debugging
was born

13. Production Constraints
● Uptime is critical
● Not easily reproducible
● Can’t simulate environment
● Resume normal operations ASAP

14. Postmortem Debugging

15. └─[0] <> node --abort_on_uncaught_exception throw.js
Uncaught Error
FROM
Object.<anonymous> (/Users/yunong/throw.js:1:63)
Module._compile (module.js:435:26)
Object.Module._extensions..js (module.js:442:10)
Module.load (module.js:356:32)
Function.Module._load (module.js:311:12)
Function.Module.runMain (module.js:467:10)
startup (node.js:134:18)
node.js:961:3
[1] 4131 illegal hardware instruction (core dumped) node
--abort_on_uncaught_exception throw.js

16. Where: Inspect stack trace
Why: Inspect heap and stack variable state

17. Generate Core Dump Ad-hoc
root@demo:~# gcore `pgrep node`
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7facaeffd700 (LWP 5650)]
[New Thread 0x7facaf7fe700 (LWP 5649)]
[New Thread 0x7facaffff700 (LWP 5648)]
[New Thread 0x7facbc967700 (LWP 5647)]
[New Thread 0x7facbd168700 (LWP 5617)]
[New Thread 0x7facbd969700 (LWP 5616)]
[New Thread 0x7facbe16a700 (LWP 5615)]
[New Thread 0x7facbe96b700 (LWP 5614)]
0x00007facbea5b5a9 in syscall () from /lib/x86_64-linux-gnu/libc.so.6
Saved corefile core.5602

18. Example

19. Netflix API Request

20. Node to API RPC

21. [2016-09-09T16:25:48.388Z] WARN: reactive
socket/rs-pool/17352 on lgud-yunong:
TcpLoadBalancer._connect: no more free
connections

22. Postmortem Debugging

23. Connection Pool State
> ::findjsobjects -p _connections | ::jsprint
{
"connected": {
},
"connecting": {},
"free": {
"100.82.188.185:7001": [...],
"100.82.37.181:7001": [...],
"100.82.41.121:7001": [...],
"100.82.102.157:7001": [...],
"100.82.106.115:7001": [...],
"100.82.129.239:7001": [...],
"100.82.102.158:7001": [...],
"100.82.74.237:7001": [...],
...
}
}

24. Postmortem Debugging is Critical to Large Scale
Production Node Deployments

25. Postmortem WG
github.com/nodejs/post-mortem/

26. Postmortem WG - Mission
Guide improvements in postmortem
● Interfaces/APIs
● Dump formats
● Tools and Techniques

27. State of key tools today
Heap dump - snapshot of heap
● heapdump module - https://github.com/bnoordhuis/node-heapdump
● Chrome developer tools
● Limitations
● Need to modify application
● Slow to generate (minutes or hours)
● O(N) memory usage
● Limited content
● Output is large

28. State of key tools today
Core dump - memory image
● Creation
○ Crash, signal
○ --abort-on-uncaught-exception
○ Fast (relative to heap dumps)
○ Size matches process memory
● OS debuggers
○ Examination at C/C++ or assembler level
○ No knowledge of Node/v8 structures
● Node core dump inspectors
○ MDB (limited platform support)
○ IDDE (IBM SDK specific)
○ LLNODE (newer, less complete)

29. Example commands
MDB_V8 command LLNODE command IDDE
Print a stack trace jsstack, jsframe v8 bt !stack, !frame
Find objects findjsobjects v8 findjsobjects,
v8 findjsinstances <type>
!jslistobjects
!jsgroupobjects
!jsfindbyproperty
!jsobjectsmatching
Print an object jsprint v8 inspect !jsobject
Print function source jssource v8 source (prints source for a stack
frame)
!jsobject, !string + work
Find constructor for an
object
jsconstructor n/a !jsconstructor
Print elements of a
FixedArray
v8array v8 inspect <instance> !array
Find native memory
backing a buffer
nodebuffer v8 inspect <instance> !nodebuffers

30. How to make this better?
● Improve ease of use
● Common APIs to introspect dumps
● Cross platform support
● Common command set
● Lightweight dump

31. The Postmortem WG is working on...
Common Heap Dump Format
Improved Core Dump Analysis
● Library in C & JS
● Tools: mdb_v8(mdb), llnode(lldb), ...
Node Report

32. Common Heap Dump Format
Enabler for new tools
Generation
● mdb
● llnode
Consumption
● Conversion to existing v8 format - > chrome dev tools
● C/Javascript APIs

33. Core Dump Analysis
Currently working on
● Platform coverage
● Re-use of command implementation
● Common APIs
Soon to be
nodejs/llnode !
https://github.com/nodejs/post-mortem/issues/37

34. Working to get to….

35. Node Report
Lightweight Dump
● Fast
● Small
● Human readable
● Key information to start investigating
● Triggers: exception, fatal error, signal, JavaScript API

36. NodeReport
example - heap out
of memory error
NodeReport content:
● Event summary
● Node.js and OS versions
● JavaScript stack trace
● Native stack trace
● Heap and GC statistics
● Resource usage
● libuv handle summary
● Environment variables
● OS ulimit settings

37. Javascript API
API in Javascript
● More accessible
● Leverages
○ llnode
○ Common Heap Dump (future)

38. JavaScript API - example application

39. Summary
What is postmortem debugging
Example of where it’s helpful
Activities of the working group
● Common heap format
● APIs (C/JS)
● Tools(lldb, mdb_v8, NodeReport)

40. Get Involved !
Great chance to learn
● Low level machine details
● Key debugging techniques
● Different platforms/operating systems
Where
● Most work done through GitHub issues/Pull Requests
● http://github.com/nodejs/post-mortem/

41. Postmortem Debugging is Critical to Large Scale
Production Node Deployments

42. Some production problems are otherwise impossible
Save complete process state for debugging later

43. Copyrights and Trademarks
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