DevOps Classroom Series – 02/Feb/2020

Docker Storage Driver

• Refer to storage drivers section in the article.

Docker Volumes

• Volume adds persistence to the docker container
• Volumes add disk sharing across containers and also between container and host
• Volume life cylce is different from containers life cycle
• Example Scenario:
  • Create a docker container with mysql running:
    • For the containers which generate/store data it is important to preserve it, so we have to use volumes
• Application Categories:
  • Stateless Applications:
    • Application which donot store any state and rely on other applications to store/preserve the data
  • Stateful Application
    • Applications which store the data/state locally and for these applications the data has to preserved by us using volumes.
• For official Docs Refer Here

Docker Volume Command line

• Docker has created a sepearte command line for volume
• Volume command line

docker volume create myvol
docker volume ls
docker volume inspect myvol

• Creating the container and mounting the volume

docker container run -it --name cont1 --mount "source=myvol,target=/jars" openjdk:8 /bin/bash

• Check for the mounts available inside container using df -h
• Now lets experiment with generating data inside the volume, remove the container and check in the volumes mount point if the data is still available

# Inside cont1
cd /jars
wget https://war-jar-files.s3-us-west-2.amazonaws.com/gameoflife.war
exit

# In Docker Host
docker container rm -f cont1

# Check for volumes
docker volume ls
docker volume inspect

# copy the mount point dir 
ls <mountpointdir>
# two files should be available (spc.jar and gol.war)

• Now lets mount the myvol to two containers and run the application simultaneously

docker container run --name cont3 --mount "source=myvol,target=/app" -d -p 8080:8080 openjdk:8 java -jar /app/spring-petclinic.jar

docker container run --name cont4 --mount "source=myvol,target=/app" -d -p 8081:8080 openjdk:8 java -jar /app/spring-petclinic.jar

docker container ls

docker volume ls

# remove all the containers
docker container rm -f $(docker container ls -a -q)

• Data from Docker Volume will be removed only when the volume is removed

docker volume rm myvol

• For creating a readonly volume refer here

Bind mount volumes

• Now your docker host has a folder /app you want the same folder in the container

docker container run --name cont5 --mount "type=bind,source=/app,target=/jars" -d -p 8080:8080 openjdk:8 java -jar /jars/spring-petclinic.jar

• For tempfs refer here

Dockerfile

• Inside the Dockerfile we have a instruction called as VOLUME.
• When you use this for every container a new volume is created.
• Lets write the Dockerfile

FROM openjdk:8

RUN mkdir /app && cd /app && wget https://war-jar-files.s3-us-west-2.amazonaws.com/spring-petclinic-2.2.0.BUILD-SNAPSHOT.jar
VOLUME /app
EXPOSE 8080
CMD ["java", "-jar", "/app/spring-petclinic-2.2.0.BUILD-SNAPSHOT.jar"]

• Create two container from the image and verify the volumes

docker volume ls
# inspect the required volume and you should see spring-petclinic-2.2.0.BUILD-SNAPSHOT.jar in the mountpoint dir
docker stats