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Let's dive in a little deeper
to get a better understanding of

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what Docker is useful for.

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We'll be going into much more
detail in the remaining stages.

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But for now,
let's try to get a broad overview.

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Developers using Docker don't have
to worry about installing and

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configuring complex supporting
software like databases.

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Or worry about which version of
a language an app is built on.

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When a developer Dockerizes an app,

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all the complexity of building the app is
pushed into what Docker calls containers.

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You can think of these like
shipping containers software.

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Containers are easily built, run, and

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shared by any developer with
access to the Docker file.

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Docker's own website
describes it as the world's

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leading software container platform.

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Docker is used by developers
to ensure their apps work on

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every machine they're deployed to.

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And operations staff use Docker to
better scale systems in production.

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Enterprise companies use Docker to build
agile software delivery pipelines, so

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they can ship new features quickly,
with better security.

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Docker lets enterprises deploy to
both Linux and Windows Server easily.

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Employees joining a new
project no longer have to wait

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hours while the supporting
software installs.

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You don't have to carefully explain
how to setup various services.

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Docker files abstract away the
installation of dependencies, allowing you

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to easily package the app for running and
test or production environments.

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If you've worked with
virtual machines before,

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containers may seem similar to those,
and they are somewhat similar.

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Both allow you to install
a set of apps and

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the services they depend on without mixing
them what the software on the host OS.

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But containers don't emulate
a virtual CPU, memory, and

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other hardware like virtual machines do.

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They run directly on the host
computer's hardware,

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making them more efficient
in many situations.

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To run an app in a Docker container,
you first need to write a Docker file.

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The syntax for these files is simple, and
there are thousands of existing examples

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for you to pull from
when packaging your apps.

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Once a Docker file is made,
you can use the Docker commandline

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interface to build an image
from your Docker file.

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An image is essentially a binary file
that contains the app defined by

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the Docker file.

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Then you can share the image with
others who can run it as a container.

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Suppose we have this Python web app.

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It's a really simple app
consisting of a single file.

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All it does is listen for
browser requests on port 8080 and

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respond with hello from Python.

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But even something this simple
introduces a dependency,

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we have to have Python 3
installed in order to run it.

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And it looks like this machine
only has Python 2 installed.

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So we get an error when we try to launch
this app.py program, python app.py.

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And you can see here the import
error No Module named http.server.

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So let's try using Docker
to create an image that

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bundles the gap together with
the correct version of Python.

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We'll need a file named Docker file
with a capital D and no extension.

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And we'll put this together in the same
directory that contains our app.

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This Docker file will create another new
image based on an Ubuntu Linux image.

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It'll install Python 3 on top of it.

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It'll copy the app,py file,
plus another file that we'll use later

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from the current directory
on the host into the image.

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It'll set app.py to run via Python 3
whenever a container that's based on

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the image starts.

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And it'll ensure that outside
apps can connect to the container

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on port 8080, so
that they can communicate with our app.

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Now let's actually build the image.

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We'll run the docker command
with the build subcommand, and

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use the -t flag to tag the image
with the name sample-web-app.

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Lastly, we'll tell it to look for
a Docker file in the current directory.

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As before, Docker build will go through
the instructions in our Docker file

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one by one.

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Creating an image based on Ubuntu,
installing Python,

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copying our app into the image and
exposing port 8080.

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We can confirm that our image was
created successfully with the docker

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images command.

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We'll see an image with our sample
web app tag in the resulting list.

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Now let's create a container
based on the image and

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try connecting to our Python server.

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We'll use the Docker run command,
we'll publish our exposed port 8080 as

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port 8080 on the host with -p 8080:8080.

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And select the image to use by
providing our tag of sample-web-app.

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Our container will start, and

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we'll run our Python app just like
we specified in the Docker file.

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If we switch to our web browser and
direct it to localhost

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port 8080, it will connect to port
8080 on our host, be passed through to

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port 8080 on the container, and our Python
app will respond with Hello from Python.

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Because the container is still
attached to our terminal,

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we can shut it down by going back to
the terminal and pressing Ctrl+C.

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That will stop the main Python process and
the container will then shut down.

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If we go back to our browser and
Reload, we'll see that we no longer get

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a response because on the container
that was responding before has shutdown.

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Docker is useful for building and
deploying single apps or

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services, but it's even better when you're
building complex distributed systems.

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There are existing tools
such as Docker Compose,

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which you can think of is a Docker
file for multiple Docker containers.

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And Docker Swarm, which allows you to
build, deploy, and monitor multiple

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Docker containers at once, either as
a single service or as a set of services.

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Docker also has a rich networking API.

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And connecting containers
to the outside world or

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other Docker containers is
straightforward and easy.

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Hopefully, now you have a better
understanding of exactly what Docker is.

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Up next, we'll discuss why you should be
using Docker right now for your projects.