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[Intro Music]

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In the previous lecture, I

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explained to you what happened when we

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have created a new hello-world

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container from hello-world image.

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And basically executable file

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called 'hello' was executed; and

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responsibility of this file is

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simply to bring text to the

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terminal. That's it. And afterwards

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container was stopped. Now, let me

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explain you what container actually

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is. On this diagram, you see image

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layers here. And you already know

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that every image is read-only. All

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'fs' layers in the image are read-

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only. You're not able to modify

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them. When you create a new

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container from a specific image,

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Docker creates a very small

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writeable layer, and you are able to

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read from that layer, and write to

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that layer, create new files and

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folders. And every container has

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its own writeable layer. In this

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example, there are two different

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containers, Container A and

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Container B created from the same

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image. Let me quickly do that

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actually. Let me go to terminal, and

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create one more hello-world

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container. 'docker run hello-world',

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and I see same result. And if I'll

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enter 'docker ps -a' command now,

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I will see that there are now two

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different stopped containers, and

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those two different containers used

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same hello-world image, here and

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here. But the command that was

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executed inside of every of those

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containers was the same. It was

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launch of the 'hello' file. Alright,

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let's proceed. After creation of the

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container, Docker actually launches

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specific process, specific

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process inside of the container.

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And as long as at least one process

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is running inside of the container,

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container is considered as running.

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But as soon as process ends,

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container is stopped. Let me

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explain you all of this using

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another diagram. On this diagram

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there is Docker Host. It is a

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Linux computer. There is Docker

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Engine installed inside. Also every

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Linux machine has a Linux Kernel,

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and such resources as CPU, RAM,

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network and hard drive. And let's

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suppose that we have already pulled

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some image from Docker Hub. Here

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was this image located on the hard

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drive of this Docker Host. And this

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image, let's assume has two

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different 'fs' layers, 'image fs 1',

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and 'image fs 2'. And again, those

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image layers are located on hard

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drive of this Docker Host. When you

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enter 'docker run', some image command,

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Docker will create additional

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writeable layer for new container.

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That is the first action. And this

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writeable layer will be created here

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on the hard drive of this Docker

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Host. Next you know that every

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image has instructions, which are

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executed when new container is

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created. And in case with

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hello-world image this command was

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run 'hello' executable file. And

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that means that new process on this

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Docker Host is created for a

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particular new container, like this.

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New process is created. This

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process does some job. And

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afterwards if this process ends,

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Docker removes this container from

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memory. And in such case, when

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there are no processes running

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inside of the container, container

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will be stopped by Docker. But

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those files, writeable 'fs' layer

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for this container remains on the

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hard drive. But if you remove a

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particular stopped container, then

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Docker of course removes this

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writeable 'fs' layer as well. It

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means that you are able to create

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multiple containers like so. Create

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Container 1 with additional

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writeable 'fs' layer, but notice

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that we don't need to create those

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layers because those layers are

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already present on hard drive. And

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this container will actually reuse

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those 'fs' layers. Any processes

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running inside of this container

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will reuse those 'fs' layers.

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Alright, writeable 'fs' layer was

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created, process was launched. And

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now let's suppose that this process

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is a long running process, like

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database process; or for example,

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we execute some scripts, perform

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some long running actions, and so

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on; and in such case when process

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is running, container is kept

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running, and there is process in

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memory of this Docker Host. And of

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course, Linux Kernel manages all

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requests to resources like CPU, RAM,

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network from this process. All

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requests come via Linux Kernel.

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Afterwards, when this container is

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running, let's suppose that you

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want to create a new container

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using same image. In such case

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again, Docker creates new writeable

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'fs' layer for new container.

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Those layers remain here. This

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Container 2 will simply reuse

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those two 'fs' layers. And next it

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creates new process for new

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container. And again, this process

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communicates with resources like

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CPU, RAM, network and hard drive

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via common Linux Kernel. Container 1

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and Container 2 communicate via

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common Linux Kernel. But

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processes are completely

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independent. They are different,

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but running on the same Docker

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Host. Alright, this is what happens

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when you create new containers

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based on a specific image. And key

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point here is that 'fs' layers of

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basic image that was used for

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creation of those containers are

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reused, they are not copied to any

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new container. Every new container

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gets just new writeable 'fs' layer,

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that's it. And when process is

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stopped in specific container,

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Docker stops container as well and

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removes it from memory. But files

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of this container still remain on

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the hard drive until you enter

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'docker rm' command, and remove

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specific stopped container. As you

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see from this diagram, Docker

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utilizes the resources of Docker

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Host very efficiently. First it

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reuses files on the hard drive.

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There is no need to copy a file

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system layers from specific image

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when you create new containers. For

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every new container, Docker creates

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just new writeable 'fs' layer,

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all remaining underlying 'fs'

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layers are reused from this image.

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And next Docker does not keep a

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container running if there are no

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processes running inside of the

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container. It immediately stops

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container when there are no

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processes running. Alright, let's

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now go to terminal and let's remove

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containers that we have created

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before. Basically picture in our

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case is like this, there are no

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containers running, but there are

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still two containers on hard drive,

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but those containers are stopped.

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Let's go to terminal. And here

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again, list containers that were

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stopped. There are no containers

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running. And now let's use 'docker

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rm' command in order to remove

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stopped container. And let's first

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remove this container. Let's take

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its ID, and let's remove it like

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so. And let's remove second

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container, but let me use alternative

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version of this command, 'docker

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container rm'; and you're also able

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to use a randomly generated name in

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order to perform some actions with

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container. Let me copy this name

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and paste here as argument for 'rm'

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command; and second container was

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removed as well. And now there are

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no containers that are currently

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running or stopped. Great. But I

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still have image, 'docker images',

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on the list, hello-world image and

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busybox image. It means that now

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picture is following. There are no

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'fs' layers for containers, but there

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is still image on hard drive with

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corresponding 'fs' layers. Basically,

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there are two different images at

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the moment. And you're able of

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course, remove images as well using

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'docker rmi' command. And here you

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could type either IMAGE ID or NAME

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of the image in order to remove it.

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Let's remove busybox image. Deleted.

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And let's remove hello-world image.

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hello-world. hello-world image was

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removed as well. And now if I'll

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have a look at the list of images,

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I will see empty list. And that

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means that at the moment, there

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are no images on hard drive, on

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local Docker Host. That's how

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picture looks like at the moment.

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Alright guys, I hope you enjoyed

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this lecture. And basically that's

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all for this section, where I

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have made small introduction into

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images and containers. And next let

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me briefly summarize what we have

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covered in this section. I'll see

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you next. Bye-Bye.

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[no audio]