Docker basics

Container vs VM

A container runs natively on Linux and shares the kernel of the host machine with other containers. It runs a discrete process, taking no more memory than any other executable, making it lightweight. By contrast, a virtual machine (VM) runs a full-blown “guest” operating system with virtual access to host resources through a hypervisor. In general, VMs provide an environment with more resources than most applications need.

![containers] (https://i.stack.imgur.com/exIhw.png)

Check your installation

docker run hello-world

Using Docker Containers

Containers are built from images. You can create containers from preexisting images hosted on the docker hub.

Images

• Public repo: https://hub.docker.com
• Build you own images with Dockerfile

Command	Description
docker images	list local images
docker search ubuntu	searches the repo for images
docker pull alpine	download image from https://hub.docker.com
docker pull centos:6	use tag
docker rmi <image_id>	remove image

Quickly run a container

Command	Description
docker run hello-world	quickly run a container
docker run busybox echo "hello from busybox"	echo and exit
docker run -it alpine sh	run container in interactive mode and allocate terminal
docker run -it centos:7 bash	use tag
docker run -d -p 8080:80 httpd:2.4	run an apache server

Work with running containers

Command	Description
docker ps	see running containers
docker port <container_id>	see port mapping for a container
docker exec -it <container_id> bash	enter running container
docker inspect <container_id>	container info, get ip address
docker rm <container_id>	remove container
crtl+p crtl+q	detach current container
docker attach <container_id>	attach container. It attaches to the running process, only use on containers started with /bin/bash
docker kill <container_id>	kill a running container

Using volumes

Command	Description
docker run -it -v /tmp:/container_mount centos:7 bash	mount volume from host to container
docker run -d -p 8080:80 -v "$PWD":/usr/local/apache2/htdocs/ httpd:2.4	run an apache and serve files from current directory

Linking containers

Command	Description
docker run -it -d --rm --name server1 ubuntu:14.04 bash -c "nc -lp 4444"	run a tcp server on port 4444 in a container
docker run -it --rm --link server1 --name server2 ubuntu:14.04 bash	run a 2nd container and link it to the first. run 'nc server1 4444' on server2 to send data. use docker logs to see server1 output
docker logs <container_id>	shows the output of the process running in the container

Cleanup containers/images/volumes

• Docker container files are located here: /var/lib/docker/containers. For every container there is a folder named like the container id, ex: c92aa805eb0127f4ef0f63bba3e10b6988ad3f2e2488ce14664778c2820d1558
• Image files are here: /var/lib/docker/image
• Volumes are here: /var/lib/docker/volumes

Command	Description
docker run -it --rm centos:7 bash	automatically remove container after exit. Check with: docker ps -a
docker rm $(docker ps -a -q)	remove all stoppped containers
docker rmi $(docker images -q)	remove all unsused images

Building Docker Images

• Use container folder from this repo for the below examples

Command	Description
docker build -t node-app .	run this in the Dockerfile location to build a new img
docker images	show all images
docker images -a	show intermidiate images
docker run -p 80:8080 node-app	run a container and map a host port to a container port. 8080 will be in the contaner, 80 will be on the host
docker run -d -p 80:8080 node-app	run in detached mode

Dockerfile

• Dockerfile is used to build a custom image
• Docker image is built up from a series of layers. Each layer represents an instruction in the image’s Dockerfile
• Every command in Dockerfile creates a intermediary image
• Docker skips the lines that have not changed since the last build

Command	Description
FROM	Select base image
MAINTAINER	details about the author
RUN	Run commands at build time. ex: RUN unzip install.zip /tmp/
ADD	Adds files from host to container. Can be used to copy files from URL to container
COPY	Copies new files or directories from src to dest on the container
VOLUME	Adds a volume to the container. You can’t specify the host folder to be munted in a Dockerfile
ENV	Set a environment variable do beused during build time or in the final container
WORKDIR	Sets the working directory for the remainder of the Dockerfile and the final container. If the WORKDIR doesn’t exist, it will be created.
LABEL	Used to set metadata on an image. version,description,etc.. LABEL key=value
ARG	Defines a variable that users can pass at build-time to the builder with: docker build –-build-arg varname=value. Use it in Dokerfile with USER command like this: USER ${user:-some_user}
EXPOSE	It informs Docker that the container listens on the specified network ports at runtime.
USER	Sets the user name or UID when running command in the container
CMD	Command to be run in the container. It will be replaced by the command specified in docker build ...
ENTRYPOINT	Like CMD, with 2 differences. You can’t overwrite it when running the conainer. Averything that is passed at the end of a docker run command will be used as an argument for the ENTRYPOINT command

Note:

• Each entry of the above types in a Dockerfile will be an intermediate docker image. Use as few entries as possible with “&& " between commands.

  • Example: RUN apt-get update && apt-get install -y apache2
  • Example: RUN rm -rf /opt/node.json && rm -rf /opt/solo.rb && rm -rf /opt/environments/ && rm -rf /opt/roles/ && rm -rf /opt/cookbooks/

• find here the full reference for Dockerfile

Example:
FROM node
WORKDIR /usr/src/app
ADD package*.json ./
ADD server.js ./
ADD chinook.db ./
RUN npm install
EXPOSE 8080
CMD [ "npm", "start" ]

Composer

• Compose is a tool for defining and running multi-container Docker applications
• steps
  • create docker-composer.yml
  • docker-compose up
• https://docs.docker.com/compose/compose-file/
• Use compose folder from this repo for the below examples

Command	Description
docker-compose up	run stack defined in docker-compose.yml
docker-compose up -d	run stack in detached mode
docker-compose up -d node-app	run just node-app service
docker-compose down	stop the stack

Registry

A Docker registry is a place to store and distribute Docker images. It serves as a target for your docker push and docker pull commands. The default registry is dockerhub

Image format: <username>/<image_name>:<tag>

• https://hub.docker.com/ - create account
• create a local image with docker build
• docker tag <image_id> <dockerhub_username>/<image_name>
• docker push <dockerhub_username>/<image_name>

Example:

• docker build -t node-app:2 .
• docker tag node-app:2 andreisid/node-app:2
• docker push andreisid/node-app:2

Orchestration

Kubernetes [IN PROGRESS]

Cubernetes components:

• Nods - A node is a worker machine in Kubernetes. The services on a node include Docker, kubelet and kube-proxy.
• Pods - A pod is a representation of an application. It can contain multiple components (webserver,db, cache ...). A pod is usually composed of multiple containers. Its like docker-compose component.
• ReplicaControllers - An object that defines a pod template and control parameters to scale identical replicas of a pod horizontally by increasing or decreasing the number of running copies
• ReplicaSets - ReplicaControllers v2.0. It is used by Deployments
• Deployments - Deployments are one of the most common workloads to directly create and manage. Deployments use replication sets, and it's used for being easier to update and rollback than ReplicaControllers.
• Services = Services are logical bridge between pods/deployments and end-users.

Minikube

Minikube is a simplified kubernetes instance that can run locally, and is very usefull for development purposes.

Command	Description
minikube start --vm-driver=kvm2	start minikube
minikube ssh	ssh to minikube vm
minikube stop	stop vm
minikube delete	remove minikube cluster
minikube service node-app --url	give you the url to access the service
minikube addons list	shows available addons
minikube addons enable ingress	enable loadbalancing feature
minikube dashboard	start kubernetes dashboard

Kubectl

Kubectl is the main cli command to interct with kubernetes

Command	Description
Nod
kubectl get nod	show nod list
kubectl get nod --show-labes	show nods with labes
Pod
kubectl get pod	show pod list
kubectl get pod --show-labels	show pod list with labels
kubectl exec -it <pod_id> -- /bin/bash	enter runnin pod
kubectl describe pod	details about pods
kubectl describe pod <pod_name>	details about the pod
kubectl port-forward <pod_name> 80:8080	pod port 8080, local port 80
kubectl get service	show services
kubectl expose pod node-app --port=80 --target-port=8080	create a service
kubectl expose pod node-app --type=NodePort	create a service
ReplicaController
kubectl create -f node-app-rc.yml
kubectl scale --replicas=4 -f node-app-rc.yml	scale a running pod
kubectl get rc	get the replication controllers
kubectl describe rc <rc_name>	details about the rc
kubectl scale --replicas=3 rc/<replication-controller_name>	scale a running pod
kubectl delete rc/<replication-controller_name>	remove replication controller. All pods are removed
ReplicaSets	usually created with deployments
kubectl get rs	show rs
kubectl describe rs <rs_name>	details about the rs
Deployments
kubectl create -f node-app-deployment.yml
kubectl get deployments	show deployments
kubectl describe pod <pod_name>	details about the pod
kubectl set image deployment/<deployment_name> <container_name>=<image_name>:<tag>	change image in a running deployment
kubectl set image deployment/node-app-deployment node-app=node-app:1	example of changing the image
kubectl edit deployment/Ndeployment_name>	edit the deployment object
kubectl rollout status deployment/node-app-deployment	get the status of a deployment
kubectl rollout history deployment/node-app-deployment	get the history of a deployment
kubectl rollout undo deployment/node-app-deployment	rollback to previous version
kubectl rollout undo deployment/node-app-deployment --to-revision=n	rollback to any version
kubetl expose deployment <deployment_name> --type=NodePort	exposes deployment
Services
kubectl expose pod node-app --type=NodePort	externally expose a pod
minikube service node-app --url	get the external ip of the service
kubectl get pods --namespace kube-system	get the pods running kubernetes system(ex. dns)
kubectl cluster-info	show how the cluster is setup, nodes, master, dns,..