Docker
Description
Docker is a tool, or sets of tools, that facilitate the packaging of an application into a so-called container. Whereas a VM runs on emulated hardware which contains an OS, a container runs on the host's kernel. Currently, when we talk about the kernel, we mean the the Linux kernel. When running on macOS or Windows, Docker will use a Linux VM to run the containers. This is because Docker uses Linux kernel functionalities such as cgroups and namespaces among others.
Dockerfile example
FROM centos:7
RUN yum update -y && yum install httpd httpd-tools -y
EXPOSE 80
CMD [ "/usr/sbin/httpd", "-D", "FOREGROUND" ]
Mount volume and expose port
docker run -d -p 8080:80 -v <host folder>:<container folder>:<options> <docker image>
Questions
Understand and be able to explain the following concepts
What is Docker
Docker is a technology or software that allows us to manipulate and build containers easily.
How are containers different from virtual machines?
Containers run on the host's kernel whereas virtual machines run on emulated hardware.
What is the difference between the Docker CLI and the Docker engine?
Actually, the Docker engine is composed a these three components: the Docker daemon (dockerd), the Docker CLI (docker) and a REST API with which programs (including the CLI) can communicate with dockerd.
Why can we say that Docker is based on a client-server architecture?
We can say it's a client-server architecture because the client (docker) sends commands to the server (dockerd) through the REST API.
What is the difference between a Docker image and a Docker container?
A Docker image is a read-only template with instructions for creating and running a Docker container.
How does one create a Docker image?
A Docker image is created using the docker build command which uses a file named Dockerfile. This file contains all necessary instructions to build the image.
How does one create a Docker container?
A Docker container is created using the docker run command which takes the name of an image as argument.
What is Dockerhub?
Dockerhub is a repository of Docker images.
Explain what happens when you type docker run -it --rm alpine /bin/sh.
With this command we run the /bin/sh command inside a new container created from the alpine image. -i and -t means we want an interactive pseudo-TTY and --rm means we want the container to be automatically removed when it exits.
Explain how port mapping works in Docker.
By default, a container connects to the bridge network and exposes all of its ports to other containers, but none to the outside world. This is done using the --publishor -p option.
Explain how to use the -p xx:yy option when using docker run.
xx represents the localhost port we want to use to connect to the yy port of the container. Whilst we can chose the value for xx we cannot chose the value for yy, it is defined by the Dockerfile (using EXPOSE) and the software running inside the container.
Be able to perform the following operations
Write a Dockerfile to define an image that contains a TCP server written in Java
FROM java:8
COPY my-server.jar /somewhere/my-server.jar
Run multiple containers from the same image
docker run [options] <image-name> [command]
Obtain the IP address assigned to the each of these containers
docker inspect <container-name> | grep IPAddress
Send requests to the TCP server running in the containers, with nc or telnet
Use -p option when running the container and then do nc localhost <port-no>.
Log into a running container and explore the file system
docker exec -it <container-name> /bin/sh