# Docker

Docker is a tool that helps you build, test, ship and run applications seamlessly across various machines. It replicates the environment our software needs on any machine. You can get Docker for your machine from https://docs.docker.com/get-docker/

It has grown in popularity over the last decade due to being lightweight and fast as compared to virtual-machines that are bulky and slow. Unlike VMs, docker does not need a full blown OS of its own to be loaded to start and does not compete for resources other than what the application it is running will use. VMs on the other hand are pretty resource intensive on our processors, disks and memory hence running multiple VMs for various applications becomes a challenge in a limited capacity architecture.

```
┌────────────────────────┐ ┌───────────────────────┐
│      ┌───────────┐     │ │      ┌───────────┐    │
│      │   App     │     │ │      │   App     │    │
│      └───────────┘     │ │      └───────────┘    │
│  ┌────────┐ ┌────────┐ │ │  ┌────────┐ ┌───────┐ │
│  │  Libs  │ │  Deps  │ │ │  │  Libs  │ │  Deps │ │
│  └────────┘ └────────┘ │ │  └────────┘ └───────┘ │
│  ┌───────────────────┐ │ │  ┌──────────────────┐ │
│  │      Guest OS     │ │ │  │     Guest OS     │ │
│  └───────────────────┘ │ │  └──────────────────┘ │
│           VM1          │ │           VM2         │
└────────────────────────┘ └───────────────────────┘
┌──────────────────────────────────────────────────┐
│                     Hypervisor                   │
└──────────────────────────────────────────────────┘
┌──────────────────────────────────────────────────┐
│                      Host OS                     │
└──────────────────────────────────────────────────┘
┌──────────────────────────────────────────────────┐
│             Hardware Infrastructure              │
└──────────────────────────────────────────────────┘
              (VM based architecture)

┌────────────────────────┐ ┌───────────────────────┐
│      ┌───────────┐     │ │      ┌───────────┐    │
│      │   App     │     │ │      │   App     │    │
│      └───────────┘     │ │      └───────────┘    │
│  ┌────────┐ ┌────────┐ │ │  ┌────────┐ ┌───────┐ │
│  │  Libs  │ │  Deps  │ │ │  │  Libs  │ │  Deps │ │
│  └────────┘ └────────┘ │ │  └────────┘ └───────┘ │
│        Container1      │ │       Container2      │
└────────────────────────┘ └───────────────────────┘
┌──────────────────────────────────────────────────┐
│                       Docker                     │
└──────────────────────────────────────────────────┘
┌──────────────────────────────────────────────────┐
│                        OS                        │
└──────────────────────────────────────────────────┘
┌──────────────────────────────────────────────────┐
│             Hardware Infrastructure              │
└──────────────────────────────────────────────────┘
            (Docker based architecture)

```

Couple of terms we will encounter frequently are Docker Images and Docker Containers. Images are packages or templates of containers all stored in a container registry such as [Docker Hub](https://hub.docker.com/). Containers are standalone, executable instances of these images which include code, runtime, system tools, system libraries and settings - everything required to get the software up and running. Coming to Docker, it follows a client-server architecture wherein the CLI client communicates with the server component, which here is, the Docker Engine using RESTful API to issue commands.

### The Docker CLI

```
# after installing Docker from https://docs.docker.com/get-docker/
# To list available commands, either run `docker` with no parameters or execute
# `docker help`
$ docker

>>> docker [OPTIONS] COMMAND [ARG...]
       docker [ --help | -v | --version ]

    A self-sufficient runtime for containers.

    Options:
        --config string      Location of client config files (default "/root/.docker")
    -c, --context string     Name of the context to use to connect to the daemon (overrides DOCKER_HOST env var and default context set with "docker context use")
    -D, --debug              Enable debug mode
        --help               Print usage
    -H, --host value         Daemon socket(s) to connect to (default [])
    -l, --log-level string   Set the logging level ("debug"|"info"|"warn"|"error"|"fatal") (default "info")
        --tls                Use TLS; implied by --tlsverify
        --tlscacert string   Trust certs signed only by this CA (default "/root/.docker/ca.pem")
        --tlscert string     Path to TLS certificate file (default "/root/.docker/cert.pem")
        --tlskey string      Path to TLS key file (default "/root/.docker/key.pem")
        --tlsverify          Use TLS and verify the remote
    -v, --version            Print version information and quit

    Commands:
        attach    Attach to a running container
        # […]

$ docker run hello-world
# `docker run <container-name>` is used to run a container, it will pull the
# images from Docker Hub if they don't already exist in your system. Here the
# docker client connects to the daemon which in turn pulls the "hello-world"
# image from the Docker Hub. The daemon then builds a new container from the
# image which runs the executable that produces the output streamed back to the
# client that we see on our terminals.

$ docker run -d ubuntu sleep 60s
# The -d (or --detach) flag is when we want to run a container in the background
# and return back to the terminal. Here we detach an ubuntu container from the
# terminal, the output should be the id and the command exits. If we check
# running containers, we should still see ours there:
# CONTAINER ID   IMAGE     COMMAND       CREATED         STATUS         PORTS     NAMES
# 133261b4894a   ubuntu    "sleep 60s"   3 seconds ago   Up 2 seconds             vigorous_gould

$ docker run <container-id> -p 3000:8000
# The -p (or --publish) flag is used to expose port 8000 inside the container to
# port 3000 outside the container. This is because the app inside the container
# runs in isolation, hence the port 8000 where the app runs is private to the
# container.

$ docker run -i
# or
$ docker run -it
# Docker runs our containers in a non-interactive mode i.e. they do not accept
# inputs or work dynamically while running. The -i flag keeps input open to the
# container, and the -t flag creates a pseudo-terminal that the shell can attach
# to (can be combined as -it)

$ docker ps -a
# The `docker ps` command only shows running containers by default. To see all
# containers, use the -a (or --all) flag
# Running the above command should output something similar in the terminal:
# CONTAINER ID   IMAGE         COMMAND    CREATED         STATUS                     PORTS     NAMES
# 82f84bf6912b   hello-world   "/hello"   9 minutes ago   Exited (0) 9 minutes ago             eloquent_sammet


$ docker stop hello-world
# or
$ docker start hello-world 
# The stop command simply stops one or more containers, and the start command
# starts the container(s) up again! `docker start -a ubuntu` will attach our
# detached container back to the terminal i.e. runs in the foreground

$ docker create alpine
# `docker create` creates a new container for us with the image specified (here,
# alpine), the container does not auto-start unlike `docker run`. This command
# is used to set up a container configuration and then `docker start` to shoot
# it up when required. Note that the status is "Created":
# CONTAINER ID   IMAGE         COMMAND       CREATED             STATUS           PORTS     NAMES
# 4c71c727c73d   alpine        "/bin/sh"     29 seconds ago      Created                   naughty_ritchie

$ docker rm 82f84
# Removes one or more containers using their container ID.
# P.S.: we can use only the first few characters of the entire ID to identify
# containers

$ docker images
# Displays all images and their information, created here means the latest image
# tag updated on Docker Hub:
# REPOSITORY    TAG       IMAGE ID       CREATED         SIZE
# ubuntu        latest    a8780b506fa4   9 days ago      77.8MB
# alpine        latest    9c6f07244728   3 months ago    5.54MB
# hello-world   latest    feb5d9fea6a5   13 months ago   13.3kB

$ docker rmi 
# Removes one or more images from your system which do not have their instances
# (or containers as we know them) running. If the image has an attached
# container, either delete the container first or use the -f (or --force) flag
# to forcefully delete both the container and image.

$ docker pull busybox
# The pull command downloads the specified image on our system from Docker Hub.

$ docker exec -it 7b272 bash
# This command is used to run a command in the running container's default
# directory. Here 7b272 was our ubuntu container and the above command would
# help us interact with the container by opening a bash session.

$docker logs <container-id>
# Displays the information logged by the specified container
# root@7b27222e4bb7:/# whoami
# root
# root@7b27222e4bb7:/# pwd
# /
# root@7b27222e4bb7:/# ls
# bin  boot  dev  etc  home  lib  lib32  lib64 libx3 srv  sys  tmp  usr  var
# root@7b27222e4bb7:/# exit
# exit

# More commands can be found at https://docs.docker.com/engine/reference/commandline/docker/
```

### The Dockerfile

The Dockerfile is a blueprint of a Docker image. We can mention the artifacts from our application along with their configurations into this file in the specific syntax to let anyone create a Docker image of our application.

#### A few things to keep in mind:

* It is always strictly named `Dockerfile` without any extensions
* We have to build our custom image on top of some already available Docker base image. (there is an empty image called `scratch` which literally lets you build an image from scratch)
* All capitalised commands are part of the syntax, they are not case-sensitive but used like a convention
* Below is a sample Dockerfile but you can read in depth from the [official docs](https://docs.docker.com/engine/reference/builder/).

```
FROM <base-image>
# define base image

ENV USERNAME='admin'\
    PWD='****'
# optionally define environmental variables

RUN apt-get update
# run linux commands inside container env, does not affect host env
# This executes during the time of image creation

COPY <src> <target>
# executes on the host, copies files from src (usually on the host) to target
# on the container

ENTRYPOINT ["some-script.sh"]
# executes an entire script as an entrypoint

CMD [<args>,...]
# always part of dockerfile, introduces entry point linux command e.g. 
# `CMD node server.js`
# This executes after image creation only when the container from the image
# is running.
```

#### Build your images

Use the `docker build` command after wrapping your application into a Docker image to run ( or build) it.

```
$docker build <path-to-dockerfile>
# used to build an image from the specified Dockerfile
# instead of path we could also specify a URL
# -t tag is optional and used to name and tag your images for e.g.
# `$docker build -t my-image:0.1 ./home/app`
# rebuild images everytime you make changes in the dockerfile
```

### Push your image to DockerHub

If you want your application’s Docker image to be made publicly available for any Docker user, you might wanna push it to the [Docker Hub](https://hub.docker.com/) which is a registry of Docker images. Make sure you have an account with a username and password on Docker Hub.

When pushing an image to Docker Hub, we must specify our Docker Hub username as part of the source image name. We need to create the target image with the tag name of username/image-name much like GitHub repositories.

```
$docker login
# to login to Docker Hub using your username and password

$docker tag <src-image>[:<src-tag>] <target-image>[:<target-tag>]
# this tags a local src-image to a public target-image
# e.g. `docker tag my-sample-app:1.0.0  akshitadixit/my-sample-app`
# if tags are not specified, they're defaulted to `latest`

$docker push <target-image>[:<target-tag>]
# uploads our image to Docker Hub
# e.g. `docker push akshitadixit/my-sample-app`
# this image will be accessible under your profile's repositories as
# `https://hub.docker.com/r/username/image-name`
```
