Containers in depth – understanding how containers work to better work with containers

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© 2018 Brent Laster@BrentCLaster
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Containers in Depth (in 30 minutes):
Understanding how containers work to work
better with containers
Brent Laster

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@BrentCLaster
About me
▪ R&D Director
▪ Global trainer – training (Git, Jenkins, Gradle, Gerrit,
Continuous Pipelines, Containers, Kubernetes, Helm,
Istio)
▪ Author -
▪ OpenSource.com
▪ Professional Git book
▪ Jenkins 2 – Up and Running book
▪ Continuous Integration vs. Continuous Delivery vs.
Continuous Deployment mini-book on Safari
▪ https://www.linkedin.com/in/brentlaster
▪ @BrentCLaster

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@BrentCLaster
Book – Professional Git
▪ Extensive Git reference, explanations, and
examples
▪ First part for non-technical
▪ Beginner and advanced reference
▪ Hands-on labs

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@BrentCLaster
Jenkins 2 Book
▪ Jenkins 2 – Up and Running
▪ “It’s an ideal book for those who are new to
CI/CD, as well as those who have been
using Jenkins for many years. This book will
help you discover and rediscover Jenkins.”
By Kohsuke Kawaguchi, Creator of Jenkins

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@BrentCLaster
Agenda
▪ What containers are and the benefits they provide
▪ How containers are constructed
▪ The differences between layers, images, and containers
▪ What does immutability really mean
▪ The core Linux functionalities that containers are based on
▪ How containers reuse code
▪ The differences between containers and VMs
▪ What Docker really does
▪ The Open Container Initiative
▪ A good analogy for understanding all of this

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What’s in a container?
• A container is a standard
unit of software that
functions like a fully
provisioned machine
installed with all the
software needed to run an
application.
• It’s a way of packaging
software so that
applications and their
dependencies have a self-
contained environment to
run in, are insulated from
the host OS and other
applications - and are
easily ported to other
environments.
• A container is NOT a VM.
A container leverages
several features of the
Linux OS to “carve out” a
self-contained space to
run in. Containers are running instances of images.
Images define what goesinto a container.
Containers are built from images.
What are Containers?

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Benefits of Containers
▪ Easy to create and deploy once image is defined.
▪ Best practices of continuous development, integration, and deployment allow for
frequent and reliable builds and deployments.
▪ Quick/easy rollbacks due to image immutability.
▪ Createdat build/release time instead of at deployment time – so applications are
decoupled from the infrastructure.
▪ Provide observability through application health and signals – not just from the
OS.
▪ Because environment is self-contained, runs the same on all infrastructure –
laptop, desktop, cloud, etc.
▪ Portable across any OS or cloud that supports containers.
▪ Manage application instead of infrastructure
▪ Allows applications such as microservices to be loosely coupled, distributed, and
managed and deployed dynamically. Removes the need for monolithic stacks.
▪ Resourceisolation and (hopefully) effective utilization.
(Paraphrasedfrom: https://kubernetes.io/docs/concepts/overview/what-is-
kubernetes/)

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What is a container image?
▪ A container image is a read-only template used to create a
container.
▪ Container images are like a snapshot of a container.
▪ Images are stored in a “registry” – like repository for images.
▪ Images are generally considered to be “immutable”. That is,
once they are created, the image should not be changed. If a
change is needed, a new image should be created.
▪ Immutability does not imply that containers can’t be changed
by configuration or environment, etc.
▪ Container images are built up from layers.
▪ The docker “build” command is used to create images.

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What is an image layer?
▪ A Docker image is built up from a
series of layers.
▪ Each layer results from an instruction
(modification) in the Dockerfile.
▪ Layers are “stacked” on top of ones
before.
▪ Each layer is only the set of
differences from the last one.
▪ Think of a base layer as being an OS
image.
▪ Each layer is R/O except for last one.
▪ Last/top layer is “container layer”.
▪ Container layer is thin R/W layer.
▪ All changes made to running container
go in that layer.
▪ From https://docs.docker.com/storage/storagedriver/

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Creating and tracking layers to images
3f00bed1e02a | COPY dir:6a0…
ba16edb35eb9 | mysql:5.5.45
24f4da862742 | ENTRYPOINT ["/ent……
19e729dca1ee| CMD ["mysqld"]

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How do layers & images intersect with the
operating system?
▪ Layers can be viewed as single union of all
filesystems - Linux’s Union File System allows
you to stack different file systems and look
through
▪ R/O files can be modified via Copy on Write (cow)
– pulling them up to R/W layer when needed
▪ Other Linux facilities like cgroups and
namespaces help us keep our containers
separate
ccae61fe0274
read-only
}read
write
UNION FILE
SYSTEM
SERVICE
UNION
FILE
SYSTEM
• Layersare basicallylike files tothe O/S
• Image layers are R/O (aka– “immutable”)
• We create a container from an imageviathe runcommand. Adds
another layer – a R/Wone – changescanbe made here(“mutable”)

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Managing Content Across Containers
▪ Major difference between a container
and an image is the top writeable
layer.
▪ All writes (new content or modifying
content) are stored in the writable
layer.
▪ So multiple containers can share
access to the same underlying image
and yet have their own data.
▪ Layering also simplifies
rebuilding/updating images – only
layers that changed need to be
updated
▪ Layers storedlocally on disk (usually
/var/lib/docker)
▪ Docker uses storage drivers to
manage the contents of the image
layers and the writeable layer
▪ From https://docs.docker.com/storage/storagedriver/

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What is Docker?
▪ (Marketing) From docker.com
An open platform for distributed applications for developers and
sysadmins.
Open source project to ship any app as a lightweight
container
▪ (Technical)
Thin wrapper around Linux Container technology
Leverages 3 functionalities from Linux to provide isolated
environment in the OS
» union filesystem - data
» namespaces - visibility (pid)
» cgroups - control groups (resources)
▪ Provides restful interface for service
▪ Provides description format for containers
▪ Provides API for orchestration

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cgroup
cgroups (control groups)
▪ Groups processes for the purpose of system resource management
▪ Limits applications to specific set of system resources
▪ Feature of Linux kernel to track, isolate, and limit resource usage for a set of
processes
▪ Leveraged by Docker engine to share system resources among containers (and
constrain them)
▪ Resources include network, memory, I/O, etc.
System Resources
CPU Memory I/O Network

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namespaces
(credit: https://access.redhat.com/documentation/en-
us/red_hat_enterprise_linux_atomic_host/7/html/overview_of_containers_in_red_hat_systems/introduction_to_linu
x_containers)
▪ Provides isolated instance of a global resource
▪ Appears as separate instance of resources within namespace
PID
(process IDs)
process
identifiers,
lists of
processes
Namespace
A
Namespace
B
Network
network
interfaces
Namespace
A
Namespace
B
Mount
file system
mount points
(R/O, etc.)
Namespace
A
Namespace
B
IPC
(inter-process
communication)
access to inter-
process
communication
Namespace
A
Namespace
B
User
Isolates
userids
Namespace
A
Namespace
B
UTS
(Unix
Timesharing
service)
kernel and
version
identifiers
Namespace
A
Namespace
B

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How Containers from Docker differ from VM
▪ A VM requires a Hypervisor
and a Guest OS to create
isolation; Docker uses Linux
container technologies to run
processes in separate
spaces on the same OS
▪ Because it doesn’t require
the Hypervisor and a Guest
OS, Docker is:
▪ Faster to startup
▪ More portable (can run an
image unchanged in
multiple environments)
Server
Host OS
Docker
Run
time
Runtime
App
l N
Appl
N+1
Ap
pl 2
App
l N
Appl
N+1
Ap
pl 2
Server
Host OS
Hypervisor (such as
Virtual Box)
GuestOS
GuestOS
GuestOS
Run
time
Run
time
Run
time
VM
Envirionment
Docker
Environment
Virtual
Machine
Container

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What is a Dockerfile?
▪ Way to create a Docker
image ( a plan)
▪ A text file that contains (in
order) all of the instructions
to build an image
▪ Has a specific format and
structure
▪ Each instruction in a
Dockerfile can create a
read-only layer

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@BrentCLaster
Dockerfile to Image to Container
Build Run

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Docker Commands

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How do we think about this?
▪ Consider analogy of installing software on a
machine…
▪ And then provisioning systems for users
User R/W
User R/W

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Diving Deeper into Layers

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Containers can reuse layers
▪ Docker commands can reuse layers if they already exist
in the docker file structure
$ docker pull alpine
Using default tag: latest
latest: Pulling from library/alpine
89d9c30c1d48: Pull complete
Digest: sha256:c19173c5ada610a5989151111163d28a67368362762534d8a8121ce95cf2bd5a
Status: Downloaded newer image for alpine:latest
$ docker pull alpine/helm
Using default tag: latest
latest: Pulling from alpine/helm
89d9c30c1d48: Already exists
c14a8f06d505: Pull complete
7de226a1041c: Pull complete
Digest: sha256:ae3be4cdbaf5c6ca5f88c4a28c888fe99dcb1416de41483a2554e9bccec08503
Status: Downloaded newer image for alpine/helm:latest

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Mapping layers to the file system (before 1.10)
(credit: winsock.io/explaining docker-image-ids/)
▪ Layer – alternative definition
▪ A layer is a delta or “diff” that results when commandsare run during the
image build process that produce new or changed files/directories
▪ The new or changed files and directories are stored or ‘committed’ as a
new layer
▪ Prior to Docker 1.10, images and layers were synonymous
ID: fdf9d70e7ba7
Parent: dc89c90bfea
Name: alpine/helm:latest
ID: dc89c90bfea
Parent: ead53b4e9ff
Name: “ ”
ID: ead53b4e9ff
Parent: “ “
Name: “ ”
/var/lib/docker/aufs/diff
fdf9d70e7ba7 …
dc89c90bfea …
ead53b4e9ff …
Image Layers
$ docker inspectalpine/helm:latest
[
{
“Id”: “fdf9d70e7ba72e3a74b
“Parent”: “dc89c90bfea235
…
…
}
]
Inspect

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Mapping layers to the file system (1.10+)
▪ Layers equal to image had challenges
▪ Security – how to tell if image tampered with during
push/pull, etc.
▪ New approach – “Content Addressable IDs”
▪ Layers have no affiliation or notion of any image
» Just collections of directories and files
▪ Layers identified by digest of the form “algorithm:hex-
value”
» Algorithm is method for computing hex value from
layer’s contents (hashing method)
» Hex-value is string computed from applying
algorithm to layer’s contents (hash)
» Example:
sha256:77cae8ab23bf486355d1b31912597053…
▪ Docker image has
» Configuration object – includes ordered list of layer
digests (used to assemble filesystem in container
instead of series of parent images)
» Image ID is digest – computed hash of
configuration object
▪ Diff directories have a name that is a randomly
generated ‘cache ID’
» Docker maintains link from layer to cache id
$ docker inspect alpine/helm:latest
[
{
"Id": "sha256:fdf9d70e7ba76a6af25843
…
"Layers": [
"sha256:77cae8ab23bf486355d1b3
"sha256:fad447c6845e910b04e9391
"sha256:fcaa7782b133863c3ed0f68
]
…
}
]
Inspect
/var/lib/docker/aufs/diff
cc7da7c355e8 …
e4f6b9cf2669…
ecc5d637f8c0 …
Layers

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About the aufs driver
(credit: https://docs.docker.com/storage/storagedriver/aufs-driver/
▪ AUFS – a type of storage driver for union filesystem
▪ Layers multiple directories on a single Linux host and presents
as single directory
▪ Unification process is called “union mount”
▪ Each image and container layer are subdirs in /var/lib/docker

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Images in filesystem
diff – contents of each layer – each stored in a
separate directory
layers– metadata about how image layers are
stacked; contains one file for each layer on docker
host; each file contains the ids of the layers below it
in stack (ancestors)
mnt – mount points – one per container layer – used
to assemble and mount ufs for a container
Layer file contains order of other layers
$ sudo cat /var/lib/docker/aufs/layers/cc7*
ecc5d637f8c036afad722a0518b7b6eff1059564bb671
01d8682ae9b5832b438
e4f6b9cf26699164eea1c218d209d4cf17d1caf34abe8
0b43a9832855d54b3b6
$ docker images
REPOSITORY TAG IMAGE ID
alpine/helm latest fdf9d70e7ba7
alpine latest 965ea09ff2eb

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Container in filesystem
$ docker run -it 965ea sh
/ # echo test > test.txt
diff – differences introduced in
writable container layer
(new/modified files)
layers –metadata about writable
container layer’s parent layers
mnt – mount point for each
container’s ufs – as it appears from
within the container
$ docker ps
CONTAINER ID IMAGE COMMAND
21b9d11a1098 965ea "sh"

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Init layer
▪ Each container has 2 layers in addition to image layers
▪ init layer – based on image layer
» contains subset of files that must always exist in docker
containers
▪ child layer – contains actual content

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Open Container Initiative (OCI)

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Other Players in the Container Space

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That’s all - thanks!

Containers in depth – understanding how containers work to better work with containers

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