Docker-2

Docker Dairy-2

Why do we need Docker?

The list of benefits is the following:
Faster development process. There is no need to install 3rd parties like PostgreSQL, Redis, Elasticsearch. Those can be run in containers.
Handy application encapsulation (you can deliver your application in one piece).
Same behaviour on local machine / dev / stage / production servers.
Easy and clear monitoring.
Easy to scale (if you’ve done your application right it will be ready to scaling not only in Docker).
Supported platforms

Docker’s native platform is Linux, as it’s based on features provided by Linux kernel. However, you can still run it on macOS and Windows. The only difference is that on macOS and Windows Docker is encapsulated into a tiny virtual machine. At the moment Docker for macOS and Windows has reached a significant level of usability and feels more like a native app.

Moreover, there a lot of supplementary apps such as
Kitematic or Docker Machine which help to install and operate Docker on non Linux platforms.
Terminology
Container — running instance that encapsulates required software. Containers are always created from images.
Container can expose ports and volumes to interact with other containers or/and outer world.
Container can be easily killed / removed and re-created again in a very short time.
Image — basic element for every container. When you create an image every step is cached and can be reused (Copy On Write model). Depending on the image it can take some time to build it. Containers, on the other hand can be started from images right away.
Port — a TCP/UDP port in its original meaning. To keep things simple let’s assume that ports can be exposed to the outer world (accessible from host OS) or connected to other containers — accessible only from those containers and invisible to the outer world.
Volume — can be described as a shared folder. Volumes are initialized when a container is created. Volumes are designed to persist data, independent of the container’s lifecycle.
Registry — the server that stores Docker images. It can be compared to Github — you can pull an image from the registry to deploy it locally, and you can push locally built images to the registry.
Docker hub — a registry with web-interface provided by Docker Inc. It stores a lot of Docker images with different software. Docker hub is a source of the “official” Docker images made by Docker team or made in cooperation with the original software manufacturer (it doesn’t necessary mean that these “original” images are from official software manufacturers). Official images list their potential vulnerabilities.This information is available for any logged in user. There are both free and paid accounts available. You can have one private image per account and an infinite amount of public images for free.

Example 1: hello world

It’s time to run your first container:

docker run ubuntu /bin/echo 'Hello world'

Console output:

Unable to find image 'ubuntu:latest' locally  
latest: Pulling from library/ubuntu  
d54efb8db41d: Pull complete  
f8b845f45a87: Pull complete  
e8db7bf7c39f: Pull complete  
9654c40e9079: Pull complete  
6d9ef359eaaa: Pull complete  
Digest: sha256:dd7808d8792c9841d0b460122f1acf0a2dd1f56404f8d1e56298048885e45535  
Status: Downloaded newer image for ubuntu:latest  
Hello world

docker run is a command to run a container.
ubuntu is the image you run, for example, the Ubuntu operating system image. When you specify an image, Docker looks first for the image on your Docker host. If the image does not exist locally, then the image is pulled from the public image registry — Docker Hub.
/bin/echo ‘Hello world’ is the command that will run inside a new container. This container simply prints Hello world and stops the execution.