Docker contd…

Docker Image Layers

• Lets run the simple apline image with sleep 1d as cmd
  
• Docker image is collection of readonly layers
• Each docker container gets a writable layer for all changes done w.r.t storage and image layers are reused across containers
  
• When we make any change in existing file which is part of image gets copied into writable layer and then modified (copy on write strategy)
• When you delete the container writable layer is deleted.

How are layers getting created

• Docker Image layers are create during image creation
• Image automatically gets parent layers (FROM)
• Any instruction that leads to change in storage will create a layer.
• Having too many layers is not a good idea, layers are created for reusability.
• On a broader note, the following instructions majorly lead to layer creation
  • RUN
  • ADD | COPY
• Generally we club lots of commands in a RUN statement

RUN apt update && \
    apt install openjdk-17-jdk -y && \
    .... \
    ... \
    ... \

How does Docker deal with layers

• Docker Storage drivers deal with layers and make them available as disks to container
• Right now overlay2 is used in docker
• refer the following articles
  • Refer Here for image creation and layers and Refer Here for storage drivers

Example

• Try finding a docker commad to run mysql container with
  • username: qtdevops
  • password: qtdevops
  • database: qtlms

docker run -d --name db \
    -e MYSQL_ROOT_PASSWORD=qtdevops \
    -e MYSQL_PASSWORD=qtdevops \
    -e MYSQL_USERNAME=qtdevops \
    -e MYSQL_DATABASE=qtlms \
    -p 3306:3306 \
    mysql:8.0

• How to persist the data i.e. data should not be lost post container deletion => Volumes