Docker init or not to init

This repository exists to prove that you need an init process in your Docker container.

Uses Node.js as an example, but this is basically applicable to all runtimes.

You can read more about PID 1, orphaned processes, zombie processes, process adopting in my blog article.

Run it

Run without --init

Runs the docker container without the --init flag to prove that the Node.js runtime doesn't take care of zombie processes.

./run.sh

Run with --init

Runs the docker container with the --init flag to prove that the init process takes care of zombie processes.

./run.sh with-init

What does this do?

1. Build Node.js application image
2. Run image as a container. Boots up the main.js script which starts a child process that runs child.js. The child process starts another child process that runs sub-child.js.
3. Print out the process table before killing any processes.
4. Kill child.js process. Thus sub-child.js becomes an orphan and PID 1 (main.js process or init) adopts it.
5. Kill sub-child.js process. Thus sub-child.js is terminated and becomes a zombie process. In case of init, this zombie process is removed from the process table.

Example output

This is the output of running the bash script without the with-init argument. This shows that there is a zombie process at the end.

$ ./run.sh
[+] Building 1.7s (10/10) FINISHED
....

Currently running processes (before kills):
UID        PID  PPID  C STIME TTY      STAT   TIME CMD
root         1     0  5 19:25 ?        Ssl    0:00 node lib/main.js
root        14     1  0 19:26 ?        Sl     0:00 /usr/local/bin/node /app/lib/child.js
root        21    14  0 19:26 ?        Sl     0:00 /usr/local/bin/node /app/lib/sub-child.js
root        28     0  0 19:26 pts/0    Rs+    0:00 ps -eaf -w 10000

Killed child process

Currently running processes (after killing child):
UID        PID  PPID  C STIME TTY      STAT   TIME CMD
root         1     0  2 19:25 ?        Ssl    0:00 node lib/main.js
root        21     1  3 19:26 ?        Sl     0:00 /usr/local/bin/node /app/lib/sub-child.js
root        46     0  0 19:26 pts/0    Rs+    0:00 ps -eaf -w 10000

Killed sub-child process

Currently running processes (after killing sub-child):
UID        PID  PPID  C STIME TTY      STAT   TIME CMD
root         1     0  1 19:25 ?        Ssl    0:00 node lib/main.js
root        21     1  1 19:26 ?        Z      0:00 [node] <defunct>
root        64     0  0 19:26 pts/0    Rs+    0:00 ps -eaf -w 10000

Found 1 zombie processes!