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

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

a bid way.

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

before they could even begin development work.

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

customer use your code, there can be many problems.

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

Then came virtual machines which really save the day.

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

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

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

In certain configurations and for

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

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

Because a virtual machine has to emulate all the hardware and

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

And VMs are slower to run in most circumstances.

And because the images contain an entire operating system including packages

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

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

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

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

that promised to automate the creation of VMs.

Vagrant allowed developers to build new VMs using a configuration file

that specified an OS and software packages to install.

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

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

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

Anyone who wanted to work on Vagrant VM had

run the initial bill to create their VM from scratch.

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

The contents of Docker containers run directly on

top of the host operating system.

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

Eliminating that guest OS and emulated hardware

makes Docker containers much more lightweight than virtual machines.

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

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

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

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

docker containers offer many benefits over VMs.

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

a whole separate OS to load.

You can re-use systems resources more efficiently.

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

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

Docker containers can free resources they are not using.

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

ways a VM can.

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

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

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

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

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

There's one limitation you should be aware of.

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

a virtual machine.

Containers usually start up faster than VMs though so

you'll still see a speed benefit from switching.

Docker's architecture has significant differences from traditional virtual

machine architecture.

Docker runs containers on top of light weight virtualized environments without

fully virtualized in the home machine.

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

be far more affordable than a traditional VM.

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

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

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

managing their resources and keeping the whole

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

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

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

Contrast this with Docker,

where containers are run by the Docker Daemon software.

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

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

the app itself.

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

it simply share the host resources like processors, memory and

networking devices between each container.

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

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

those same resources.

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

very heavy processes which the Docker components manage.

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

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

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