DevOps Classroom Series – 23/Jun/2020

Docker References in the blog

• Refer Here

Docker container and Image

• Refer Here
• Refer Here
• Image Concept in Docker

docker image pull openjdk:8
vi Dockerfile

FROM openjdk:8
LABEL author="khaja"
LABEL version="0.2"
LABEL project="QT"
RUN wget https://referenceappkhaja.s3-us-west-2.amazonaws.com/spring-petclinic-2.2.0.BUILD-SNAPSHOT.jar
EXPOSE 8080
ENTRYPOINT ["java"]
CMD ["-jar",  "spring-petclinic-2.2.0.BUILD-SNAPSHOT.jar"]

# build spring petclinic
docker image build -t spc:0.1 .

• Now lets inspect images and focus of Layer

docker image inspect openjdk:8
docker image inspect spc:0.1

• Docker image is collection of docker layers
• When are layers created?. Mostly they are created when the content is changed (Executing RUN,COPY,ADD instruction). Lets experiment to create a new docker image spc:0.2

FROM openjdk:8
LABEL author="khaja"
LABEL version="0.2"
LABEL project="QT"
RUN wget https://referenceappkhaja.s3-us-west-2.amazonaws.com/spring-petclinic-2.2.0.BUILD-SNAPSHOT.jar
RUN mkdir /test
EXPOSE 8080
ENTRYPOINT ["java"]
CMD ["-jar",  "spring-petclinic-2.2.0.BUILD-SNAPSHOT.jar"]

• Now verify the image layers with openjdk:8 and you should see 2 layers.
• So creating too many layers is not optimal. Use RUn statement carefully
• The docker image layers are readonly
• Try to create 2 container with spc:0.2

docker container run -d -P spc:0.2
docker container run -d -P spc:0.2

• When we create a container, an extra R/w layer is created. All the changes done in that container will be stored in R/W layer. When we delete the container the R/W layer gets deleted. So in Docker to preserve changes done inside container we need to take extra measure (Volume)

Stateful and Stateless Application

• Stateful applications use local storage to preserve application data/config
• Stateless applications don’t use local storage, they rather store the state(application/data) in database/external system
• Docker is used in both stateful and stateless applications.
• Ensure extra caution in stateful applications.

Experiments

• Try whoami and pwd commands in the following containers and observe the behavior

docker container run -it openjdk:8 /bin/bash
docker container run -it jenkins /bin/bash
docker container run -it tomcat:8 /bin/bash