Mar 26, 2017

Docker is Amazing. Here are 10 tips how to use it!

Docker is Amazing!
If you are not sure yet why should you use it, think of it as Digital Ocean tutorials that are as a simple file you can always run over and over:

Step 1: Create a Dockerfile
Dockerfile includes the list of commands you are going to perform to create your image.
The most common commands in this file

FROM And MAINTAINER in the top to select image baseline and the one to blame things
FROM ubuntu:14.04

RUN: run shell command:
RUN apt-get update

ADD: add a file from local directory to target in the docker image
ADD unicorn.rb /app/config/unicorn.rb

ENV: add an environment variable

Step 2: Make sure you have a forever command in the Dockerfile
Docker image stops as soon as its last command stops to run. If you don't have a service or ever running daemon (or you are not going to use this image as a base image), add an infinite loop  
CMD while true; do sleep 1000; done

Step 3: Install docker on target machine
Well, this is why you have Chef/Ansible/Puppet for
sudo yum -y install docker &  sudo service docker start

or in ubuntu:
sudo apt-get update &&\
sudo apt-key adv --keyserver hkp:// --recv-keys 58118E89F3A912897C070ADBF76221572C52609D &&\
sudo apt-add-repository 'deb ubuntu-xenial main' &&\
sudo apt-get update &&\
apt-cache policy docker-engine &&\
sudo apt-get install -y docker-engine

sudo systemctl status docker

Step 4: Build and Tag an Image
sudo docker build -t my_company/my_app .
sudo docker rm my_app

Use the -p flag to map internal port to actual external port while running the docker container
sudo docker create -p 80:80 --name my_app my_company/my_app

Step 5: Run and inspect your image
Start the container
sudo docker start my_app
sudo docker run -d -v /home/ubuntu/java:/home/ubuntu/java bat/spark

Show the running containers
sudo docker ps -a

Show the logs
sudo docker logs my_app

Connect to a running container
sudo docker exec -it my_app /bin/bash

Stop a running container
sudo docker stop my_app

Step 6: Perform tasks on github
If you are going to get code from github and deliver a bundled application use the following:
sudo vi ~/.ssh/id_rsa
sudo chmod 600 ~/.ssh/id_rsa
git clone
cd repository
bundle exec rake assets:precompile

Step 7: Working w/ AWS ECR
AWS ECR is a reporitory solution that can be used to get built images to being deployed in various solutions such CodeBuild

Set credentials
aws configure

Add to ECR
sudo $(aws ecr get-login --region us-east-1)
sudo docker build -t lemonade_app .
sudo docker tag my_app:latest
sudo docker push

Step 8: Attaching Volumes
If you want to attach a volume for the underneath host (for example for DB persistant data) you should define the mapped volume inside the image

RUN mkdir -p /data
VOLUME ["/data"]

and map it when you run the container
sudo docker run -d -v /data:/data bat/spark

Please notice that you can also map volume to another docker container, so you will be able to share data between containers (for example unix sockets):
The source container
sudo docker create -v /dbdata --name dbstore training/postgres /bin/true
The target container
sudo docker run -d --volumes-from dbstore --name db1 training/postgres

Step 9: Adding hosts to /etc/hosts file

Just menation them in the run command
sudo docker run -d --add-host=SERVER_NAME: bat/spark

Step 10: Debugging your application

When modifying files and need to debug, just restart the docker container, and rerun tasks
sudo docker restart [container_id]

Few more tips:
No sudo
"Cannot connect to the Docker daemon. Is the docker daemon running on this host?"
You probably run the command w/o sudo

No Response
sudo docker does not respond even for simple calls like sudo docker ps -a. 
1. Take a look at /var/log/docker
2. If you find out message like this one, delete the container by issuing:
sudo rm -rf /var/run/docker/libcontainerd/containerd/a7764ce62032af2fdb57d3016a68bd079590667ca07
time="2017-03-16T09:42:32.781797550Z" level=info msg="libcontainerd: new containerd process, pid: 2577"
time="2017-03-16T09:42:32.787797862Z" level=fatal msg="open /var/run/docker/libcontainerd/containerd/a7764ce62032af2fdb57d3016a68bd079590667ca073702a7b96ec2068ec7a73/state.json: no such file or directory"

Playing w/ dockers
sudo docker rm spark
sudo docker build -t bat/spark .
sudo docker create  --name spark bat/spark

sudo docker build -t bat/simulator .
sudo docker create  --name simulator bat/simulator
sudo docker run -d -v /home/ubuntu/java:/home/ubuntu/java bat/simulator
sudo docker logs peaceful_shirley

sudo docker build -t bat/processor .
sudo docker create  --name processor bat/processor
sudo docker run -d -v /home/ubuntu/java:/home/ubuntu/java bat/processor

sudo docker ps -a
sudo docker exec -it fervent_ardinghelli /bin/bash

Evaluate Failed Container
If your container fails to start, export its filesystem to a tar file and evaluate the relevant files. In this case we evaluate the unicorn log file:
mkdir content &&\
cd content &&\
sudo docker export [CONTAINTER_ID] > contents.tar &&\

tar -xvf contents.tar

sudo more shared/log/unicorn.stderr.log

Cleanup You Disk from Orphaned Volumes
You will probably keep see that disk fills up as you keep deploying, as old deploy volumes are no longer needed.
Therefore, on every deploy run the following script to clean the disk:
sudo docker volume ls -qf dangling=true | sudo xargs -r docker volume rm

And another recommended cleanup: 
Clean your old and unused images::
#delete old unused images
if [[ $(sudo docker images --filter "dangling=true" -q --no-trunc) ]]; then
   sudo docker rmi $(sudo docker images --filter "dangling=true" -q --no-trunc)


If you can the following error:
/bin/sh: Text file busy
Consider adding the sync command between several commands in the same RUN
RUN chmod 777 /app/scripts/; sync; /app/scripts/

Bottom Line
Docker is really fun, and takes the whole DevOps process to a new level!

Keep Performing,


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