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[MUSIC]

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In the last stage we took a look at
what Docker is and why people use it.

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In this stage we'll look at how Docker can
streamline your software development in

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a bid way.

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Long ago, developers had to install all of
their apps dependencies on the OS itself

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before they could even
begin development work.

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This may work for your purposes, but
when it comes time to let a team mate or

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customer use your code,
there can be many problems.

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It's not always easy to reproduce what
worked on your machine on their machine.

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Then came virtual machines
which really save the day.

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VMs essentially emulated entire computers
with in a developer's computer,

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VMs were great because they allow
developers to install their app's

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dependencies in a virtual machine
keeping it isolated from main OS.

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In certain configurations and for

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certain apps, they can even be faster
than running on native hardware.

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But this isn't the case most of the time.

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Because a virtual machine has
to emulate all the hardware and

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firmware it runs on,
it can be slow to start up.

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And VMs are slower to run
in most circumstances.

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And because the images contain an entire
operating system including packages

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an app may not need, the disk images
are big and impractical to distribute.

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Despite these drawbacks, virtual
machines made it much easier to build

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reproducible environments for apps to
run on, so they exploded in popularity.

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And in 2010, a software project
named Vagrant came along,

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that promised to automate
the creation of VMs.

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Vagrant allowed developers to build
new VMs using a configuration file

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that specified an OS and
software packages to install.

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But Vagrant still suffered from
the imparent problem of virtual machines,

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distributing applications was still
difficult, it wasn't practical to

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pass the VM from developer to developer or
developer to customer.

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Anyone who wanted to
work on Vagrant VM had

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run the initial bill to
create their VM from scratch.

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That was the situation when Docker and
the concept of containers arrived in 2013.

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The contents of Docker
containers run directly on

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top of the host operating system.

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Unlike a virtual machine where they run
on top of a guest operating system.

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Eliminating that guest OS and
emulated hardware

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makes Docker containers much more
lightweight than virtual machines.

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A Docker still offers many of the same
benefits of Virtual Machines,

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because they're isolated in a container,
the services running in docker containers

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can't interfere with each other,
or with services on the host.

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Because there's no need for
a full guest OS,

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docker containers offer
many benefits over VMs.

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They start up faster,
because you don't have to wait for

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a whole separate OS to load.

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You can re-use systems
resources more efficiently.

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A virtual machine, usually has to receive
some parts of the host resources and

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can't release them when it's
not actively using them.

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Docker containers can free
resources they are not using.

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Containers can usually take of
the hard role in the host machine and

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ways a VM can.

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For example a VMK usually use the GPU
to run codes but a container is up

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can make full use of it, and
there's less redundancy with containers.

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Whereas VMs usually need the entire
guess OS in order to run successfully,

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containers runs strip down versions of
the OS with unnecessary parts removed.

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That means you can run more containers
on the same host than you can with VMs.

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There's one limitation
you should be aware of.

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Depending on the app you're running
a container may not be having faster than

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a virtual machine.

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Containers usually start up
faster than VMs though so

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you'll still see a speed
benefit from switching.

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Docker's architecture has significant
differences from traditional virtual

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machine architecture.

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Docker runs containers on top of light
weight virtualized environments without

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fully virtualized in the home machine.

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Because docker doesn't virtualizing
entire OS, it can run much quicker and

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be far more affordable
than a traditional VM.

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Both virtual machines and containers
run on top of a host operating system.

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To run virtual machines, the host OS
runs software called a hypervisor.

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A hypervisor manages the life cycle of
VMs, starting and stopping them as needed,

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managing their resources and
keeping the whole

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setup secure by isolating the VMs
from the host OS and from each other.

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An entire guest operating system runs
within each virtual machine, and

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your apps dependencies as well as the app
itself are run within the guest OS.

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Contrast this with Docker,

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where containers are run by
the Docker Daemon software.

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Docker containers omit most of
the guest OS except for a few packages.

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Meaning most of the container resources
are devoted to your apps dependencies and

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the app itself.

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Docker doesn't have to run
the entire guest OS to run an app,

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it simply share the host resources
like processors, memory and

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networking devices between each container.

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Even better Docker does an exclusively
hold the resources of the host OS.

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When resources are released from a
container other Docker containers can use

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those same resources.

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Rather than entire virtual machines,
it's simpler to think of containers of

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very heavy processes which
the Docker components manage.

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This is what makes Docker a faster and
better way to deploy absent services.

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So that's how Docker containers
differ from virtual machines.

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Up next we'll look at
the details of how Docker works.