DevOps Classroom Series – 22/Aug/2021

Docker Internals

• Questions to understand
  • How are container isolations created?
    • Network
    • Process tree
    • FileSystem
  • How are cpu and memory resources allocated to this isolation
• For more content on containers
  • Refer Here for the list of the blog items
  • Refer Here for docker internals
• In linux we have a kernel feature called as Namespaces that can be used to create isolations.
• To apply resource limits to this isolation we have one more kernel feature which is called cgroups (Control groups)

Lets try to run some java application inside container

• Manual Steps:
  • Creating a ubuntu vm
  • installing java
  • downloading the application(jar file)
  • running the command to start the applciation
  • Access the application by using http url
• Commands

sudo apt update
sudo apt install openjdk-11-jdk -y
wget https://referenceapplicationskhaja.s3.us-west-2.amazonaws.com/spring-petclinic-2.4.2.jar
java -jar spring-petclinic-2.4.2.jar

• Now access the application from http usign http://<public-ip>:8080

• Lets see if we can do the same in containers

• Now access the machine where docker is installed.

• Option 1:

  • Try to search for a docker container with ubuntu image

  docker container run --name exp1 -p 8000:8080 -it ubuntu /bin/bash
  apt update
  apt install openjdk-11-jdk wget -y
  wget https://referenceapplicationskhaja.s3.us-west-2.amazonaws.com/spring-petclinic-2.4.2.jar
  java -jar spring-petclinic-2.4.2.jar
  

  • When we the run the above commands the application runs inside the contaiener are we are able to access the application using http://<host-ip>:8000

• Option 2:

  • Searching for openjdk image has resulted in openjdk:11

  docker container run --name exp2 -p 8001:8080 -it openjdk:11 /bin/bash
  curl https://referenceapplicationskhaja.s3.us-west-2.amazonaws.com/spring-petclinic-2.4.2.jar --output spring-petclinic.jar
  java -jar spring-petclinic.jar
  

  • When we the run the above commands the application runs inside the contaiener are we are able to access the application using http://<host-ip>:8001

• Observations: When we want to run our application inside the container

  • manually executing commands as shown above is not a sensible idea
  • When we are creating images choosing a base container with necessary softwares installed (option 2) will be sensible.

• In Docker if you want to create images we create Dockerfile, which has instructions on how to build the container image and command to be executed when container is execute

• Create a file in a new folder called as Dockerfile and add the below contents

FROM openjdk:11
ADD https://referenceapplicationskhaja.s3.us-west-2.amazonaws.com/spring-petclinic-2.4.2.jar /spring-petclinic.jar
EXPOSE 8080
CMD ["java", "-jar", "/spring-petclinic.jar"]

• Now lets try to build the image using docker image build -t spc .
• Now create the container

docker container run -d --name exp3 -p 8082:8080 spc
docker container run -d --name exp4 -p 8083:8080 spc