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Let's dive in a little deeper and get a better understanding of how you can use Docker. We'll be going into much more detail in the remaining stages, but for now, let's try to get a broad understanding.
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Let's dive in a little deeper
to get a better understanding of
0:00
what Docker is useful for.
0:03
We'll be going into much more
detail in the remaining stages.
0:05
But for now,
let's try to get a broad overview.
0:08
Developers using Docker don't have
to worry about installing and
0:11
configuring complex supporting
software like databases.
0:14
Or worry about which version of
a language an app is built on.
0:17
When a developer Dockerizes an app,
0:21
all the complexity of building the app is
pushed into what Docker calls containers.
0:23
You can think of these like
shipping containers software.
0:28
Containers are easily built, run, and
0:31
shared by any developer with
access to the Docker file.
0:33
Docker's own website
describes it as the world's
0:37
leading software container platform.
0:39
Docker is used by developers
to ensure their apps work on
0:42
every machine they're deployed to.
0:45
And operations staff use Docker to
better scale systems in production.
0:46
Enterprise companies use Docker to build
agile software delivery pipelines, so
0:51
they can ship new features quickly,
with better security.
0:55
Docker lets enterprises deploy to
both Linux and Windows Server easily.
0:58
Employees joining a new
project no longer have to wait
1:03
hours while the supporting
software installs.
1:06
You don't have to carefully explain
how to setup various services.
1:08
Docker files abstract away the
installation of dependencies, allowing you
1:12
to easily package the app for running and
test or production environments.
1:17
If you've worked with
virtual machines before,
1:21
containers may seem similar to those,
and they are somewhat similar.
1:24
Both allow you to install
a set of apps and
1:28
the services they depend on without mixing
them what the software on the host OS.
1:30
But containers don't emulate
a virtual CPU, memory, and
1:36
other hardware like virtual machines do.
1:39
They run directly on the host
computer's hardware,
1:41
making them more efficient
in many situations.
1:44
To run an app in a Docker container,
you first need to write a Docker file.
1:47
The syntax for these files is simple, and
there are thousands of existing examples
1:51
for you to pull from
when packaging your apps.
1:55
Once a Docker file is made,
you can use the Docker commandline
1:58
interface to build an image
from your Docker file.
2:02
An image is essentially a binary file
that contains the app defined by
2:04
the Docker file.
2:08
Then you can share the image with
others who can run it as a container.
2:10
Suppose we have this Python web app.
2:14
It's a really simple app
consisting of a single file.
2:16
All it does is listen for
browser requests on port 8080 and
2:19
respond with hello from Python.
2:24
But even something this simple
introduces a dependency,
2:26
we have to have Python 3
installed in order to run it.
2:30
And it looks like this machine
only has Python 2 installed.
2:34
So we get an error when we try to launch
this app.py program, python app.py.
2:38
And you can see here the import
error No Module named http.server.
2:43
So let's try using Docker
to create an image that
2:49
bundles the gap together with
the correct version of Python.
2:51
We'll need a file named Docker file
with a capital D and no extension.
2:55
And we'll put this together in the same
directory that contains our app.
3:00
This Docker file will create another new
image based on an Ubuntu Linux image.
3:04
It'll install Python 3 on top of it.
3:09
It'll copy the app,py file,
plus another file that we'll use later
3:12
from the current directory
on the host into the image.
3:16
It'll set app.py to run via Python 3
whenever a container that's based on
3:20
the image starts.
3:24
And it'll ensure that outside
apps can connect to the container
3:26
on port 8080, so
that they can communicate with our app.
3:30
Now let's actually build the image.
3:35
We'll run the docker command
with the build subcommand, and
3:37
use the -t flag to tag the image
with the name sample-web-app.
3:41
Lastly, we'll tell it to look for
a Docker file in the current directory.
3:50
As before, Docker build will go through
the instructions in our Docker file
3:54
one by one.
3:58
Creating an image based on Ubuntu,
installing Python,
3:59
copying our app into the image and
exposing port 8080.
4:03
We can confirm that our image was
created successfully with the docker
4:08
images command.
4:12
We'll see an image with our sample
web app tag in the resulting list.
4:15
Now let's create a container
based on the image and
4:21
try connecting to our Python server.
4:23
We'll use the Docker run command,
we'll publish our exposed port 8080 as
4:25
port 8080 on the host with -p 8080:8080.
4:31
And select the image to use by
providing our tag of sample-web-app.
4:39
Our container will start, and
4:47
we'll run our Python app just like
we specified in the Docker file.
4:49
If we switch to our web browser and
direct it to localhost
4:53
port 8080, it will connect to port
8080 on our host, be passed through to
4:59
port 8080 on the container, and our Python
app will respond with Hello from Python.
5:06
Because the container is still
attached to our terminal,
5:11
we can shut it down by going back to
the terminal and pressing Ctrl+C.
5:14
That will stop the main Python process and
the container will then shut down.
5:18
If we go back to our browser and
Reload, we'll see that we no longer get
5:23
a response because on the container
that was responding before has shutdown.
5:28
Docker is useful for building and
deploying single apps or
5:32
services, but it's even better when you're
building complex distributed systems.
5:35
There are existing tools
such as Docker Compose,
5:40
which you can think of is a Docker
file for multiple Docker containers.
5:43
And Docker Swarm, which allows you to
build, deploy, and monitor multiple
5:47
Docker containers at once, either as
a single service or as a set of services.
5:51
Docker also has a rich networking API.
5:56
And connecting containers
to the outside world or
5:59
other Docker containers is
straightforward and easy.
6:02
Hopefully, now you have a better
understanding of exactly what Docker is.
6:05
Up next, we'll discuss why you should be
using Docker right now for your projects.
6:09
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