DevOps Classroom notes 06/Jan/2024

Containerization

• To containerize the application, we need to create the docker image of the application.
• To create docker image of the application (manual steps of running application are required), we have two ways
  • create image from running container:
    • should not be adopted
  • Write Dockerfile.
    • Preferred approach
    • repeatable, automatable

First sample Application

• A custom home page on a webserver
• manual steps:
  • install webserver (nginx or apache)
  • custom homepage <h1> This is my home page </h1>
• For an ubuntu distribution

sudo apt update && sudo apt install apache2 -y
echo "<h1> This is my home page </h1>" > /var/www/html/index.html

• For an redhat distribution

sudo dnf update && sudo dnf install httpd -y
sudo systemctl enable httpd.service
sudo systemctl start httpd.service
echo "<h1> This is my home page </h1>" > /var/www/html/index.html

Manual approach using containers

• To create a Docker image find the suitable docker image to start with.
• Docker has a image called as scratch which has nothing.
• Lets take a image called as httpd
• To create a container and login into container during start

docker container run -d -P --name mycontainer -it httpd /bin/bash

• To login into running container

docker container exec -it <name/id> /bin/bash

• Lets create a container and then login interactively

# creates the container and runs in the background
docker container run -d -P --name mycontainer httpd

• Make the changes echo "<h1> This is my home page </h1>" > /usr/local/apache2/htdocs/index.html
• Now create a image from container

docker container commit mycontainer firstimage

• Then to test create a container from first image

docker container run -d -P --name myfirstcontainer firstimage

• This is not recommended as it is not suitable for automation.
• To remove all the containers

# Get all container ids docker container ls -a -q
# pass this to rm -f
docker container rm -f $(docker container ls -a -q)

• To remove all the images

docker image rm $(docker image ls -q)

Dockerfile instruction approach

• We need to create a file filled with instructions for docker to create image
• Dockerfile can be any text but generally Dockerfile is used.
• This file has set of instructions

<INSTRUCTION> <VALUE>

• Some of the instructions are
  • FROM
  • RUN
  • EXPOSE
  • CMD
  • ENTRYPOINT
  • LABEL
  • ADD
  • COPY
  • VOLUME
• For the above appliation the Dockerfile

FROM httpd
EXPOSE 80
RUN echo "<h1> This is my home page </h1>" >  /usr/local/apache2/htdocs/index.html

• Instruction categories
  • Building
    • FROM
    • ADD
    • COPY
    • ARG
  • METADATA
    • LABEL
    • EXPOSE
  • RUNNING
    • CMD
    • ENTRYPOINT

FROM: Choosing the base image

• Docker image needs a base image and this is specified by using FROM Refer Here
• Never use latest tag
• Generally dont use images with hefty sizes in linux.

RUN: Executing a command during image build

• Refer Here for official docs
• Lets download jar file in the docker image with java 17 installed
• jar file location Refer Here
• Base image: FROM amazoncorretto:17-alpine3.17-jdk

FROM amazoncorretto:17-alpine3.17-jdk
RUN wget https://referenceapplicationskhaja.s3.us-west-2.amazonaws.com/spring-petclinic-3.1.0-SNAPSHOT.jar

LABEL: Add metadata

• Refer Here for official docs
• Lets add author name and projects name

EXPOSE

• This instruction exposes ports for tcp or udp Refer Here
• If you dont mention EXPOSE -P will not work

CMD: command to start the application

• CMD is command executed when container is starting.
• Container will be in running state as long as command in CMD is running.
• CMD should have a command which starts your application and doesn’t exist till your application is executing
• If CMD is not written then the base images CMD is considered.

Spring petclinic

• Dockerfile

FROM amazoncorretto:17-alpine3.17-jdk
LABEL author="khaja" 
LABEL project="learning"
RUN wget https://referenceapplicationskhaja.s3.us-west-2.amazonaws.com/spring-petclinic-3.1.0-SNAPSHOT.jar
EXPOSE 8080
CMD ["java", "-jar", "spring-petclinic-3.1.0-SNAPSHOT.jar"]

References

• Install Docker Extension into visual studio code
  

Exercise

• Refer Here for war file
• This requires tomcat 8 or 9 with java 8
• Ensure you are downloading gameoflife.war in webapps folder