What is an Image

What is an Image:

App binaries and dependencies.

Metadata about the image data and how to run the image.

Official Definition: "An Image is an ordered collection of root filesystem changes and the corresponding execution parameters for use within a container runtime."

Not a complete OS. there's not actually a complete OS. There's no kernel. There's no kernel modules.

It's really just the binaries that your application needs because the host provides the kernel.

That's one of the distinct characteristics around containers that makes it different from a virtual.

It's not booting up a full operating system.

It's really just starting an application, and that image can be really small.

Small as one file.

Even you make it as Big as a CentOS, Ubuntu distro with apt, and apache, PHP, and more installed.

The Mighty Hub using DOCKER Hub Registry Images:

Basics of Docker Hub (hub.docker.com)

Find official and other good public images

Download images and basics of image tags

Register and login to hub.docker.com and search for the images (Eg: nginx) 

There you can see the,

Supported tags and respective Dockerfile links

Note: // All these tags in the same line are targeting same image.

Eg:

• 1.17.5, mainline, 1, 1.17, latest   
• 1.17.5-perl, mainline-perl, 1-perl, 1.17-perl, perl
• 1.17.5-alpine, mainline-alpine, 1-alpine, 1.17-alpine, alpine
• 1.17.5-alpine-perl, mainline-alpine-perl, 1-alpine-perl, 1.17-alpine-perl, alpine-perl
• 1.16.1, stable, 1.16    /// Stable version as on date.
• 1.16.1-perl, stable-perl, 1.16-perl
• 1.16.1-alpine, stable-alpine, 1.16-alpine
• 1.16.1-alpine-perl, stable-alpine-perl, 1.16-alpine-perl

You can pull the image by using the tags.

Eg: docker pull nginx, docker pull nginx:1.17.5.

"Explorer" tab shows you the official images,

Images and the Layers:

Image layers

Union file system

history and inspect commands

copy on write

Image layers, this is a fundamental concept of how Docker works. It uses something called the union file system to present a series of file system changes as an actual file system.

The history and inspect commands and see how we can use them to understand

PS C:\Users\MOHI> docker image ls

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE

ubuntu              latest              775349758637        10 days ago         64.2MB

httpd               latest              d3017f59d5e2        11 days ago         165MB

nginx               alpine              b6753551581f        2 weeks ago         21.4MB

nginx               latest              540a289bab6c        2 weeks ago         126MB

alpine              latest              965ea09ff2eb        2 weeks ago         5.55MB

mysql               latest              c8ee894bd2bd        3 weeks ago         456MB

centos              latest              0f3e07c0138f        5 weeks ago         220MB

centos              7                   67fa590cfc1c        2 months ago        202MB

ubuntu              14.04               2c5e00d77a67        5 months ago        188MB

hello-world         latest              fce289e99eb9        10 months ago       1.84kB

elasticsearch       2                   5e9d896dc62c        14 months ago       479MB

PS C:\Users\MOHI> docker history nginx:latest

IMAGE               CREATED             CREATED BY                                      SIZE                COMMENT

540a289bab6c        2 weeks ago         /bin/sh -c #(nop)  CMD ["nginx" "-g" "daemon…   0B

<missing>           2 weeks ago         /bin/sh -c #(nop)  STOPSIGNAL SIGTERM           0B

<missing>           2 weeks ago         /bin/sh -c #(nop)  EXPOSE 80                    0B

<missing>           2 weeks ago         /bin/sh -c ln -sf /dev/stdout /var/log/nginx…   22B

<missing>           2 weeks ago         /bin/sh -c set -x     && addgroup --system -…   57MB    // Some data changes so size differs

<missing>           2 weeks ago         /bin/sh -c #(nop)  ENV PKG_RELEASE=1~buster     0B

<missing>           2 weeks ago         /bin/sh -c #(nop)  ENV NJS_VERSION=0.3.6        0B

<missing>           2 weeks ago         /bin/sh -c #(nop)  ENV NGINX_VERSION=1.17.5     0B

<missing>           3 weeks ago         /bin/sh -c #(nop)  LABEL maintainer=NGINX Do…   0B      //Only Meta data changes since 0KB

<missing>           3 weeks ago         /bin/sh -c #(nop)  CMD ["bash"]                 0B

<missing>           3 weeks ago         /bin/sh -c #(nop) ADD file:74b2987cacab5a6b0…   69.2MB

What do I mean by image layers?

It's actually transparent completely to you when you're using Docker, but when you start digging into certain commands, like the history command, the inspect and commit, you start to get a sense that an image. 

Every image starts from the very beginning with a blank layer known as scratch.

Then every set of changes that happens after that on the file system, in the image, is another layer.

You might have one layer, you might have dozens of layers and some layers maybe no change in terms of the file size.

You'll notice on here that we actually have a change here that was simply a metadata change about.

we're starting with one layer. Every layer gets its own unique SHA that helps the system identify.

What happens if I have another image that's also using the same version of nginx?

Well, that image can have its own changes, on top of the same layer that I have in my cache. This is where the fundamental concept of the cache of image layers saves us a whole bunch of time and space. Because we don't need to download layers we already have, and remember it uses a unique SHA for each layer so it's guaranteed to be the exact layer it needs. It knows how to match them between Docker Hub and our local cache. As we make changes to our images, they create more layers.

If we decide that we want to have the same image be the base image for more layers, then it's only ever storing one copy of each layer.

In this system, really, one of the biggest benefits is that we're never storing the same image data more than once on our file system.

It also means that when we're uploading and downloading we don't need to upload and download the same layers that we already have on the other side.