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So why do you need to
know about containers? Well, you need to

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know about them because containers in
Linux are another way to start applications.

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So that's how I would like to
define a container as an application that is

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started in an environment that is isolated
by Linux kernel namespaces from a container image.

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There's two components in that
definition that need some additional explanation.

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First component
is the namespace.

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That is a level of isolation that is implemented
by the Linux kernel. And that makes sure that

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within the container, only files, users and processes that
are relevant to that container are available. And that's

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good news, because that means that your containerized
application cannot go up into the operating system.

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The second component is
a container image that contains

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all dependencies that are
needed to run the application.

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And that means that you suddenly don't have
a problem with dependencies anymore. They're just there.

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Container images are obtained
through the container Registry.

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That's an online resource, little bit like a
repository where anybody can post their container images.

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And a kernel that is provided by
the host operating system as a container

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is really focusing on running the application.
You do not need a kernel inside

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a container to run containers. There's a
couple of requirements on a container host.

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The host that runs a
container has the container runtime.

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The container runtime is a small
program that starts a container. And

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there is a container manager software
which allows you to manage them.

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Common container runtimes are containerd and CRI O.
And common container managers are Docker and podman.

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Now, the funny thing is that nobody ever
thinks about the container runtime that they need.

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You just install either Docker or Podman
to get started with containers, and that's it.

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The runtime
comes with it.

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Container technology itself is standardized by the
oci, which is the Open Containers initiative.

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And this standardization means that the
same container image can be used

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on a standalone platform, in
a distributed environment or in cloud.

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And that also explains why containers
so rapidly have become very popular.

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As a developer, you can develop an application
on your local laptop, you can do the

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testing on your local laptop, and once you
have proved that it works out all right,

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you can run the exact same application in
a distributed platform like a cloud as well.

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And that is why containers have become
the de facto standard for running applications nowadays.