Docker

Question 1

What is Docker?

Answer 1

Docker is a tool for creating, deploying, and running applications easily

Question 2

Why use Docker?

Answer 2

Docker allows to packge applications with all their dependencies and distribute them as individual bundles with that Docker guarantees that the applications will run the same way on every Docker instance

Question 3

What is a Docker image?

Answer 3

A docker image is a file that represents a packaged application with all the dependencies needed to run correctly -> An image is like a java class

Question 4

How are docker images built?

Answer 4

Docker images are built as series of layers and these layers are assembled on top of one another and simply put, a layer is an image

Question 5

What do we need for a Docker image of a Hello World application?

Answer 5

1. Operating System Layer
2. JVM
3. Hello World application

Question 6

What is a docker container?

Answer 6

A docker container is an instance of an image and can be identified by its ID -> A docker container is like an instance of a java class

Question 7

What are the states for a container that docker defines?

Answer 7

Created, restarting, running, removing, paused, exited, and dead

Question 8

What are the advantages of containers?

Answer 8

• Containers are lightweight VMs
• Their behaviors are completely isolated from each other
• Easy way of scaling applications because we can run multiple containers of the same image at the same time

Question 9

What is usually the first layer of a docker image?

Answer 9

The first layer of a docker image is usually the operating system

Question 10

How can we pass environment variables to docker containers?

Answer 10

• Using -env, -e
• Using -env-file
• Using docker compose

Question 11

What ways of interactive exploring are there to check configurations or log files inside a container?

Answer 11

• Running a container with shell access using the docker run command with the -it option
• Spawning a shell in a running container with docker exec
• Non-interactive exploring
• Copying the filesystem

Question 12

What ways are there to copy files to and from a docker container?

Answer 12

• Using the docker cp command
• Volume mounts
• Dockerfile with COPY

Question 13

When can we use the docker cp command?

Answer 13

Docker cp cannot be used to copy between two containers. It can only be used to copy files between the host system and a single container

Question 14

What is a bind mount?

Answer 14

A docker bind mount is a high-performance connection from the container to a directory on the host machine -> It allows the host to share its own file system with the container, which can be made read-only or read-write

Question 15

What is a volume mount?

Answer 15

A volume is a directory on the host machine that is accessible to a container, so that the data is not stored in the container’s filesystem but instead it is stored in the host machine filesystem. The data persists even if the container is deleted or recreated

Question 16

What are use cases for bind mounts?

Answer 16

• Developing and testing applications locally
• Sharing code or configuration files between host and container
• Debugging applications by accessing log files

Question 17

What are use cases for volume mounts?

Answer 17

• Storing application data that needs to persist
• Sharing data between different containers
• Backing up data

Question 18

When and why is it a good idea to use docker compose?

Answer 18

When using docker, the management of several different containers quickly becomes cumbersome, docker compose is a tool that helps to overcome this problem and easily handle multiple containers at once

Question 19

What is important about the YAML configuration file?

Answer 19

Docker compose works by applying many rules declared within a single docker-compose.yml configuration file

Question 20

What are services?

Answer 20

Services refer to the containers’ configuration. Components that can be split in different images are defined as individual services

Question 21

What are networks in docker?

Answer 21

Networks define the communication rules between containers, and between a container and the host

Question 22

What types of volumes exist?

Answer 22

There are three types of volumes: anonymous, named, and host

Question 23

What is the priority order that compose uses?

Answer 23

1. Compose file
2. Shell environment variables
3. Environment file
4. Dockerfile
5. Variable not defined

Question 24

What are the benefits of docker compose?

Answer 24

Docker compose is a powerful tool for defining and running multi-container docker applications. It allows developers to define their application’s services, networks, and volumes in a single YAML file, making it easy to deploy and manage complex applications. Container management is easier with docker’s service profiles, which allow users to define containers’ configurations specifically

Question 25

What are docker compose services?

Answer 25

A docker service is a logical grouping of containers that work together to provide a specific function. In docker compose, a service refers to a containerized application element that can be horizontally scaled. It allows multiple instances of the same service to run simultaneously

Question 26

What are service profiles?

Answer 26

Docker compose files define specific configurations for containers that run in a docker service based on service profiles -> These service profiles allow us to define configurations for specific containers that run as part of a service. As a result, they simplify container management and also maintain the correct configurations

Question 27

What is the main difference between COPY and ADD?

Answer 27

The ADD directive is more powerful in two ways: 1. It can handle remote URLs for its source argument (the COPY directive can only accept local files) and 2. the ADD directive can auto-extract tar files into the image file system

Question 28

When to use ADD or COPY?

Answer 28

We should always prefer COPY over ADD unless we specifically need one of the two additional features of ADD because using ADD to copy remote files into a docker image creates an extra layer and increases the file size

Question 29

When does the run command execute?

Answer 29

The run instruction executes when we build the image

Question 30

When does the cmd command execute?

Answer 30

The cmd instruction specifies a default command that executes everytime when the container starts

Question 31

When does the entrypoint command execute?

Answer 31

The entrypoint behaves similarly to cmd but in addition it allows us to customize the command executed at startup

Question 32

What are tips for creating efficient docker images?

Answer 32

• Base your image on an official one -> Avoid building new images when possible
• Create slimmed down images
• Use slim versions when available
• Use multi-stage builds

Question 33

What is the multi-stage build feature in docker?

Answer 33

The multi-stage build feature in docker allows developers to create a sequence of builds, each with its own set of instructions -> This helps in optimizing the size of the resulting image and one of the challenges in multi-stage builds is copying variables between stages, which allows us to reuse values in subsequent stages of the build process

Question 34

What ways are there to copy variables between stages in a multi-stage docker build?

Answer 34

• Using ARG
• Using ENV
• Using File

Question 35

What is the easiest way to copy variables between stages in a multi-stage docker build?

Answer 35

Using ARG

Question 36

When is it useful to use ARG in the docker build process?

Answer 36

ARG is useful when we need to pass a variable’s value from one stage to another during the build process and when we want to make the value of the variable available during the build process, but not in the final image

Question 37

When is it useful to use ENV in the docker build process?

Answer 37

ENV is used to set variables that are available during the build process and also persist in the resulting image

Question 38

When is it useful to use a file in the docker build process?

Answer 38

Using a file is useful when we need to make the variable available in the final image: We can write the variable’s value to a file in one stage and then copy it to another stage. Furthermore this approach is useful when we want to copy too many variables across different stages because adding all the variables to a file and sharing it across all the stages is easier