Docker Q&A

1. How to monitor the processes?

Process monitoring builtin.

2. Logging and log files.

Stdin and Stdout. Can be forwarded. Syslog and journald driver.

3. Storing and manipulating files.

Create volume.
docker run -v /foo

docker run -v /var/foo:foo

4. Do we need to rebuild the image if the code is changed?

See Docker Rails Dev Demo

5. Why set :bind, '0.0.0.0' ? It does not work without this line.

6. How to run tests when the image is built?

  Dockerfile is not used to run unit tests.

Notes from How to Debug Anything Presentation by James Golick

A blind debugging session.
The website using PHP is down.
What we have to work with

The source code. (No)
Knowledge of the system. (No)
Familiarity with the programming language. (No)
SSH Access. (Yes)

Logging in the real world is often useless.
Find a pid. ps aux | grep apache

sudo strace -ff -s 2048 -p pid

How to read strace output

write(l, "hi\n", 3) = 3

write -> function name
arguments are l, "hi\n" and 3
return value is 3

To learn more about system calls: man 2 write

strace gives lot of output. Work backwards and find the failure. Look for the error message in the strace output. Find the cause by reading the strace output. Find the offender and write down your hypothesis. Prove your hypothesis. Find the offender. Fix the bug.

0. Forget everything you think you know.
1. Get a third party opinion.
2. Refer the 'Linux Performance Tools' diagram for a list of third-party tools.

You can also use strace to start a process.

sudo strace -ff apt-get update

Work backwards, find failure. Find the cause. Confirm your hypothesis. Locate a hook. Stare at the code. Confirm your hypothesis.

2. Locate the correct source code.
3. Identify a hard-coded string to grep for.
4. Stare at the code until it makes sense.
5. Fix whatever is broken.


0. Forget everything you think you know.
1. Get a third party opinion.
2. Locate the correct source code.
3. Identify a hard-coded string to grep for.
4. Stare at the code until it makes sense.
5. Fix whatever is broken.

puts Process.ppid

t = Thread.new do
   sleep 10
end

# Grabbing the pid.
pid = Process.pid

puts pid

# Get the child pids.
pipe = IO.popen("ps -ef | grep #{pid}")

child_pids = pipe.readlines.map do |line|
  puts line
  parts = line.split(/\s+/)
  # puts parts
  # parts[2] if parts[3] == pid.to_s and parts[2] != pipe.pid.to_s
end.compact

# Show the child processes.
# puts child_pids
#
#
# q = Queue.new
# # q.pop
#
t.join

Top 3 Links for July 9th, 2015

1. 4 Easy Ways to Speed Up Your Rails App

2. ActionCable Alpha Release

3. Micro Gem : A Look at the Future by Looking at the Past

Top 3 Links for July 4th, 2015

1. Print & Mail a Letter with Ruby

2. IsoLatte

3. Micro Gem : A Look at the Future by Looking at the Past

Colt Gem Released

Colt is a micro gem used to subscribe to a given plan using the Stripe API. The plans must already exist in your account. The current version can check Stripe credentials and Stripe API version.

 Source Code. You can find the different versions at Rubygems.org