Docker TWINs without docker

Dockerfile

FROM alpine:3.12
MAINTAINER [email protected]

### Build with:
# docker build . --tag twin-api:latest

### Run with:
# docker run -ti -v /var/run/docker.sock:/var/run/docker.sock twin-api:latest [number of twin copies]

RUN apk add --no-cache curl jq

# Copy Scripts:
COPY *.sh /

# Make Scripts executable:
RUN chmod 755 /*.sh

# Define EntryPoint:
ENTRYPOINT ["/entrypoint.sh"]

DockerTWINs-withoutDocker.md

Docker TWINs without docker

This is a new example on how to use docker along docker without docker, confused?, I'm using the docker API to create and run containers.

In this particular example the image self-replicates itself running multiple images.

To build:

docker build . --tag twin-api:latest

The image uses an alpine based container with curl and jq.

To run:

docker run -ti -v /var/run/docker.sock:/var/run/docker.sock twin-api:latest [number of twin copies]

Example:

$ docker run -ti -v /var/run/docker.sock:/var/run/docker.sock twin-api:latest 5
Twin docker container running with id=41bdabc1db11af51150c347a1e64e5789257cb826e8c5508d4a152521573e0f7 (Iteration=5)
Twin docker container running with id=ee0aa9c45d36df4f076c79ba69bfe2e685279a5e2cbac2118fd6c3896d35fd88 (Iteration=4)
Twin docker container running with id=bfb390da9aa64c73ef738ae6a99edfe6cdea219fc3ea3e358a16d8e581d0a3ba (Iteration=3)
Twin docker container running with id=5da0748c7f03348800196b4a8b8a329924ee34429b0b77df0b475c43134316d0 (Iteration=2)
Twin docker container running with id=03f45e1d597a11d2a895762519596328973b7128fb243f4b185f59532928ab57 (Iteration=1)

One particular useful command to see what happens in the back of the conversation between docker-cli and dockerd is:

socat -v UNIX-LISTEN:/tmp/fake,fork UNIX-CONNECT:/var/run/docker.sock

And on another terminal:

DOCKER_HOST=unix:///tmp/fake docker ps .....

The scripts are self-explanatory.

entrypoint.sh

#!/bin/sh

IT=${1:-1}
# Obtain Instance ID:
INSTANCE_ID=$(cut -c9- </proc/1/cpuset)

docker_run () {
  IMAGE_ID=$1
  ITERATION=$2
  ID=$(curl -s -XPOST --unix-socket /var/run/docker.sock -d "{ \"Image\": \"${IMAGE_ID}\", \"Cmd\": [\"${ITERATION}\"], \"Tty\": true, \"AttachStdout\":true,\"AttachStderr\":true,\"HostConfig\":{\"Binds\":[\"/var/run/docker.sock:/var/run/docker.sock\"]} }" -H 'Content-Type: application/json'  http://localhost/containers/create | jq -r '.Id')

  curl -s -XPOST --unix-socket /var/run/docker.sock "http://localhost/containers/${ID}/attach?stream=1&stdout=1&stderr=1" &
  curl -s -XPOST --unix-socket /var/run/docker.sock "http://localhost/containers/${ID}/start"
  curl -s -XPOST --unix-socket /var/run/docker.sock "http://localhost/containers/${ID}/wait?condition=next-exit" >/dev/null
}

if [ -e /var/run/docker.sock ] ; then
  # Get Image ID:
  IMAGE_ID=$(curl -s --unix-socket /var/run/docker.sock "http://localhost/containers/${INSTANCE_ID}/json" | jq -r '.Config.Image')
  echo "Twin docker container running with id=${INSTANCE_ID} (Iteration=${IT})"
  [ "${IT}" -le 1 ] || docker_run "${IMAGE_ID}" $(( IT - 1 ))
fi