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Docker - an introduction

Need for Docker

Pre-virtualization had a number of problems - huge cost of VMs - slow deployment, hard to manage - VMWare and VirtualBox provided virtualization software which improved the process - things improved when Azure, AWS started providing VMs for hire. They provided pay-as-you-go model. Also deployment was extremely fast.

Virtualization changed the game as they allowed to almost eliminate the virtual OS. - all containers run on the same linux kernel. However, isolation is provided at runtime. - containers pack up only the needed parts of the OS. Thus they are faster to deploy, scale up - containers are fully portable

Docker architecture

There are 2 distinct parts - docker client - you interact with this layer, typically via CLI apps - docker daemon - the background process which powers the containers. We don't talk to this directly, but only via the docker client.

Typically, the docker client and daemon run on the same machine, but it is possible from the client to connect to a remote daemon.

Images and Containers, Registries and Repositories

Images are read-only templates composed by stacking up other images. They are hosted on docker registry.

Containers is an instance of an image.

Registry is where we store our images. You can host this yourself or use Docker's public registry called DockerHub. Repositories are repos within the registry where images of a particular type are stored.

Docker Hub - is a public registry. Official repos are encouraged to be used as they are vetted by Docker and ensure they get security updates frequently. Generally, user created repos have a prefixing username/repo_name/image_name. Usually, tags are used to specify docker versions.

Docker commands - quickstart

  • list all images held locally - docker images
  • run an image - docker run busybox:1.23 echo "hello world" which is of syntax docker run image <command to run on image> <argument for command> On running this, Docker will try to find the image locally, if not found, will download from docker hub and run. Example output below:
open-geo [master] $ docker run busybox:latest echo "hello atma"
Unable to find image 'busybox:latest' locally
latest: Pulling from library/busybox
697743189b6d: Pull complete 
Digest: sha256:061ca9704a714ee3e8b80523ec720c64f6209ad3f97c0ff7cb9ec7d19f15149f
Status: Downloaded newer image for busybox:latest
hello atma
open-geo [master] $ 
  • if you run the same command agin, it will run faster as there is no downloading.
  • you can do other things like look at the contents of the image by calling docker run busybox:latest ls /

Running docker in interactive mode

  • run the image with -i, -t commands for interactive, TTY mode.
  • docker run -i -t busybox:latest. You can exit by typing exit

Running in foreground vs detached modes

The default mode is to run in foreground. Docker will start the process in container, attaches the console to this process' std output. You terminate the process and the connection together. When running, the terminal cannot be used for other things.

Background / detached mode on the other hand can be stared with -d flag. The container is killed when the root process of container terminates. You can use the terminal to do other things while the container is running.

  • docker run -d busybox:latest sleep 100 will run it in background mode.
  • You can list the running containers using docker ps command. Add the -a to list all containers that are stopped as well.
  • By default, docker remembers both your containers and images. So you can rerun a docker container. If you do not want to retain a container, use docker run --rm option. The container is removed after it is run.
  • to remove a container, run docker rm <name of container>.
  • to run a container with a custom name, run docker run --name <custom_name> <commands>. Then you can verify this using docker ps -a command.
  • When running in background mode, you cannot see the logs. Thus, you can call docker logs <container_name_or_id> to view the logs.

Docker inspect

Use inspect to display low level information about a container or image.

  • to get low level details, run docker inspect <container_id> or docker inspect <conatiner_name>

Port mapping

We can expose a port on the container and map that to port on host using -p option. For instance: docker run -it -p 8888:8080 tomcat:8.0 will run tomcat and map its 8888 port to the default 80 port on host.

Docker images in detail

Docker image layers

A docker image is built as a list of read-only layers. At the bottom is a base layer on which other docker images or custom apps are installed. You can see all the layers of an image using docker history <image name:tag> command.

When a new container is instantiated, a new editable, writable (container) layer is added on top of this read-only image layers. Thus when container is deleted, all changes are lost and the image remains unchanged. Thus different containers instantiated from same image can have different data states emanating from their thin, editable layers.

Build docker image

There are 2 ways of building an image 1. commit changes made to a container (yay!) 2. by writing a Dockerfile.

Build docker image by committing a container

Here we make changes to the container, typically running it in interactive mode. Then after exiting the container (container can be stopped at this point), we call docker commit <container name or id> <username/image_name:tag>. Thus in this ex for example, I did the following - docker run -it debian:jessie which downloads and runs debian OS in interactive mode - from within the container, run apt-get update && apt-get install -y git to install GIT - exit container and to commit, I do docker commit hopeful_ellis atmamani/debian:0.1 - then running docker images will report the atmamani/debian image. - finally, running docker history atmamani/debian:0.1 lists the changes I made. Note, it only states modification as bash, meaning it was a bash command, but not what the command is.

Build docker image using Dockerfile

Dockerfile is a text based manifest. The file should not have any extension and must start with a capital D. Below is a sample Dockerfile

FROM debain:jessie
RUN apt-get update
RUN apt-get install -y git
RUN apt-get install -y vim

Then build it using command docker build -t <username/imagename:tag> <buildcontext> The build context here needs path to the dockerfile on disk. You can also use the build context to package up additional files into the docker image. Docker daemon will create a tarball of these files, send it to the contain, unpack. So I ran docker build -t atmamani/debian:frmdckfile . from the folder containing the Dockerfile.

Once built, you can inspect the history and now it reports it correctly:

docker-trials $ docker history atmamani/debian:frmdckfile
IMAGE               CREATED              CREATED BY                                      SIZE                COMMENT
a9d62a3217ae        25 seconds ago       /bin/sh -c apt-get install -y vim               29.7MB              
a4c2ad02f957        39 seconds ago       /bin/sh -c apt-get install -y git               85MB                
cdfcdde0ccab        About a minute ago   /bin/sh -c apt-get update                       10.3MB              
b6ebaf83dd59        8 days ago           /bin/sh -c #(nop)  CMD ["bash"]                 0B                  
<missing>           8 days ago           /bin/sh -c #(nop) ADD file:e044496893d9e2cbf…   129MB               
docker-trials $ 
Optimizing docker images

One aspect is to reduce the number of layers a docker image has. Each line in the Dockerfile corresponds to a layer. Thus chaining the lines into a single line will reduce this to 1 layer. For instance, the earlier Dockerfile can be trimmed to

FROM debian:jessie
RUN apt-get update && apt-get instal -y git vim

Baking run commands into Docker image

We can bake commands to be run when a container starts up from an image. The preferred way is to use exec commands, an alternate is to specify bash commands. We prefix such with CMD keyword. If you do not specify a CMD, the default is to bring up bash.

FROM debian:jessie
RUN apt-get update && apt-get install -y \
    git \
    python \
CMD ["echo", "helllo world"]

When a container is created, it runs this helllo world with a typo. You can also modify this at runtime, by specifying docker run <image_name> <commands> <arguments> such as docker run atmamani/echoer echo hello hello.

Copying files into the image

FROM debian:jessie
RUN apt-get update && apt-get install -y \
    git \
    python \
COPY ./abc.txt /tmp/abc.txt

The above will copy abc.txt from local dir into the tmp dir of the image. This will be available for all containers spun up from this image.

Pushing images to Docker hub

Some people say don't use latest tag, instead use a real SEMVER version. To tag an image before pushing to hub or production, you need to tag it appropriately. docker tag <image name> <reponame/newname:tag> Thus, docker tag atmamani/debian:frmdckfile atmamani/debian:0.2

Then push using docker push <imagename> as docker push atmamani/debian:0.2

A Hello world app using Flask

Download the repo using git clone -b v0.1 Then build the docker image using docker build -t tut-dockerapp:v0.1 .

Run the container as docker run -d -p 5000:5000 tut-dockerapp:v0.1 this spins up the container and maps the port. You can use the web app at localhost:5000. If for some reason, this is not available or unknown, you can find the IP address of the docker using docker-machine ls or find which port is mapped using docker ps.

To step into a running container, use docker exec -it <cont name or id> <command>, such as docker exec -it keen_noyce bash. The -i flag makes it interactive. To see the running process within the container do ps axu from within the home directory:

dockerapp [(no branch)] $ docker exec -it keen_noyce bash
admin@dd29aef6445a:/app$ cd ~
admin@dd29aef6445a:~$ ps axu
admin        1  0.0  1.2 102852 25080 ?        Ss   19:33   0:00 python
admin       17  0.1  0.1  19964  3680 pts/0    Ss   19:47   0:00 bash
admin       24  0.0  0.1  38380  3216 pts/0    R+   19:47   0:00 ps axu

To exit out of the container's bash, type exit. To stop the container running as daemon, type docker stop <container_name>.

KVP lookup app using Flask

Linking containers using redis

redis is a message broker, inmemory cache, db service that is containerized. We need a redis Python client.

Linking with Docker compose

docker-compose.yml is a YAML file. Verison 3 is up to date. You start with a version number. Then specify the services that needs to be run. The dockerapp contains the main, user facing app, the depends_on contains all other microservices that need to be spun up and the order in which they need to be spun up. A sample looks like:

verison: '3'
    build: .
      - "5000:5000"
      - redis
    image: redis:3.2.0

To build and run using docker compose, run docker-compose up -d from dir that contains the yaml file. Once done, you can run docker ps to see the containers created and being run.

To rebuild a docker compose, you may have to force it if are changing just the python stack. To force rebuild use docker-compose build.