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Docker

MSSE 2017

Goals

• Gain an understanding of Docker
• Potentially Dockerize your application
• Prepare your application for the cloud

What is Docker

• Project that automates deployment of applications inside software containers
• Layer of abstraction on operating system virtualization
• Docker is not a VM

How it works

• Docker containers have extremely light weight components of an operating system
• The other things they require to run obtained from the host VM
• Can only be run on Linux as Docker takes advantage of cgroups and kernel namespaces to achieve isolation

Benefits

• Extremely lightweight containers that can run virtually anything
• Isolation from other containers on the host machine
• Ability to install from an exact specification
• On demand instantiation of new containers

Running Docker

• Docker used to run inside of a Virtualized Linux image (Docker Machine)
• Docker now has native application support for Windows and Mac
• Install at -> http://www.docker.com

Components of Docker

• Docker Engine
• Images
• Containers
• Docker Registry

Docker Engine

• Client CLI
• Rest Layer
• Docker Daemon

Images

• An instance of a application
• Images are immutable
• Ability to start from a pristine, working application
• See docker images docker images

Images include

• Code
• Networking
• Configuration
• Dependencies
• Can include a small subset of an operating system

Image properties

• Repository (name)
• Tag (Version - latest by default)
• Image ID
• Size

Image sizes

• Docker reuses data from other containers
• What’s listed in images is usually not representative of total container space
• Usually much smaller

Starting a container from a given image

• Docker will look locally for the image name
• If it’s not found - will pull from the public docker repository
• If a version is not specified, will run latest
• Run a image via docker run $IMAGE_NAME:$VERSION
• Starts a container from the given image

Example - Running a image

# docker run -ti ubuntu:latest ls

# docker run -ti ubuntu:latest bash

Containers

• An interactive instance of the application you started
• Created from images
• Isolated from other processes and containers

Container Commands

• See a list of running containers docker ps
• See all created containers docker ps -a
• See last container exited docker ps -l

Container Naming

• Containers have unique ID
• Container name is generated unless you specify --name=example

Container properties

• Containers do not overwrite images
• When main process exits, containers stop (-d (detached) to keep running)
• Stopping a container does not remove or delete it (can specify –rm)

docker run --rm -ti ubuntu sleep 5

Immutability in Docker

• Docker images are immutable
• Docker containers can be “committed into new images”
• This new image will contain your saved work

Docker flow

• New container from an image
• Container is changed in some meaningful way
• Docker commit is run
• A new image is created

Commit example

# docker run -it ubuntu bash

//In container
# touch NEW_FILE
# exit
//Outside of container

//Commit the container with a new image name
# docker commit $NAME or $ID new-image

//List the new image
# docker images

Exposing ports

• Use -p when running a docker to expose a port

docker run -p 8081:8080 somecontainer

• 8081 is the port you would connect to
• 8080 is the port your connection is redirected to

Let docker determine port

• Docker will pick an available port for you if you don’t specify one
• Use -p when running a docker to expose a port

docker run -p 8080 somecontainer

• docker port $CONTAINER_NAME
• 8080 is the port your connection is redirected to

Docker files

• Files with instructions for creating a docker image
• Each line represents an instruction to build a container
• Each line creates a new image
• Final image is saved in docker images

Docker file Commands

• FROM the base origin of the image
  • Generally some kind of system
  • Can be another docker image
• ADD - Add a local file to the docker image
• RUN - Run a command on the system
  • Sometimes used to do updates

Couple more

• ENV - Adds environment variable to the container
• EXPOSE - Exposes the internal port
• ENTRYPOINT - Execute command

Example - Spring Boot Docker File

• ./gradlew build //Make sure you’ve build the jar

FROM java:alpine
ADD build/libs/week-12-forms-0.0.1-SNAPSHOT.jar app.jar
RUN sh -c 'touch /app.jar'
ENV JAVA_OPTS=""
ENTRYPOINT [ "sh", "-c", "java $JAVA_OPTS -Djava.security.egd=file:/dev/./urandom -jar /app.jar" ]

Creating the image

docker build -t msse/week12forms:1.0 .

• -t tag name - $NAME:$Version
• . < Very important. Location of Dockerfile

Run the container

docker run --rm -ti -p 8080:8080 msse/week12forms:1.0

• Starts up the application accessible at port 8080

Attaching to a running container

• docker exec --ti ubuntu bash

Docker Networking

• Docker has private networks
• docker network create $NAME
• Starts a name server

docker run --rm -ti --net=$NAME --name example-server <container>
docker run --rm -ti --net=$NAME --link example-server <container2> --name example-server2

• example-server2 now gets network information for example-server one

Docker Compose

• Let’s you run many containers at once
• Useful for testing local environment
• Not for production - can’t really scale
• Run via docker-compose up

Docker Registry

• Repository for images
• Can be private for organizations
• Docker hub - public repository
  • Can create an account and host your own images

Docker and the Cloud

• Docker is generally used with service orchestration
  • Service Discovery
  • Starting/Stopping Containers

Docker reference

• Docker has many many more moving parts
• Docker swarm
• Lots of other features
  • Shared file systems
  • Other networking options
• https://docs.docker.com/engine/reference/builder/