Docker

Docker

On my journey, I learned that my organization makes use of Docker often. Here are my notes on the tool.

Why use it ?

Sometimes OS’s and services are not compatible which leads to issues libraries , OS, & dependencies that will not work together.

Docker runs components, dependencies in its own containers. Just had to build the docker configuration once.

Containers are completed isolated environments, they can have their own network interfaces, etc but they all share the same OS kernel.

Docker uses LXC containers.

OS have two things, an OS kernel and a set of software. The OS kernel interacts with the underlying hardware. Its the software above the OS kernel that makes them different. Could have different compilers, user interface etc.

Sharing the underlying kernel, what does that mean? Docker can run any flavor of OS so long as they are all based on the same kernel.

Each docker container has the additional software that makes these OS's different.

You will not be able to run a window based container with an Linux OS kernel.

When you install Docker on windows you work Linux container on Linux virtual machine on windows.

Unlike hyper visors, Docker not meant to run different OS's, dockers package and ship them anywhere any time as many times as we want.

Docker installs on the OS.

Containers VS Virtual Machines

Utilization is used higher disk space, CPU space when you use a virtual machine. Docker allows it to boot up in seconds.

Docker has less isolation as more resources are shared.

You can see Docker hosted on virtual docker.

Products are containerized.

If you need to run multiple you could, you just need to remember to add a front load balancer. If one fails you can delete it and re-download it.

## How to Use Docker? There is a dockerhub or dockerstore. Where you can use docker run [REGISTRY PRODCUT NAME]

ex: docker run ansible

Container VS Image

An image is a package or template, like a VM template that is used to create one or more containers that are running instances of images that are isolated and have their own enviornments & set of processes.

You can create your own Docker image and push it to the docker repository.

Developers & Ops

In the past Developers would create the requirements and a set of instructions on how hosts must be set up, what prerequists are to be installed on the host and how the dependencies are to be configured, etc.

OPs teams would hit issues as they did not develop, so working with developers to resolve. With docker the operations and developers team work together to transform the guide into a docker file with both of their requirements.The image can now run on any host, and it will work anywhere when deployed in production.

Getting Started

Docker has two editions, community (free docker products), Enterprise (paid products).

Community is available on cloud platforms and on all major OS (Linux, Mac,Windows). If on Windows/OS you will have to run a virtual machine for Linux.

Install

Go here I installed using the convientient script.

$ curl -fsSL https://get.docker.com -o get-docker.sh

$ sudo sh get-docker.sh

With the following command I am pullling an image from dockerup called whalesay

sudo docker pull docker/whalesay

To run this image I ran

sudo docker run docker/whalesay cowsay [INPUT]

(insert my hi friendu photo here)

## Docker Commands docker run runs a container from an image

If the image is not found it will pull the image from Docker hub, for the first time.

docker ps lists all the containers that are running some basic information about them- name, container ID, names, ports and status. Each container gets a random name and ID.

docker ps -a lists all the containers including previously stopped or exited containers.

docker stop [CONTAINER ID OR CONTAINER NAME] will stop the running container.

docker rm [CONTAINER ID OR CONTAINER NAME] will remove the docker container compeltely.

docker images lists available images and their sizes

docker rmi [IMAGE NAME] removes the images, but you must stop and remove all dependent container before it can be removed.

docker pull [IMAGE] only pull the image and not run the container

containers are not meant to host OS's, they are designed to carry computations or run the images. A container only lives as long as the process is alive inside.

docker exec [IMAGE NAME] [COMMAND] will run a command within the container.

docker run -d [IMAGE NAME] will run the container detached.

docker run --name [DESIRED NAME] [IMAGE] will run a container and name it your desired name

docker run [IMAGE NAME] would run the latest image

docker run [IMAGE NAME]:[VERSION NUMBER] this would be considered a tag and docker would know to pull an image of this version

Docker containers by default do not listen to standard input. It runs in a noninteractive mode. To have input you must include the -i argument in your command, this will run docker in interactive mode. No prompts will run with the -i argument as the prompt is on the terminal. The prompt must be set by adding -it argument rather than just -i.

-t stands for a psudo terminal.

Every docker container gets assigned an IP by default, its internal and can only be accessed within the docker host.

If you have an image running on an internal docker , users outside the internal docker cannot access. To ammend this you can use the IP of the docker host, but you have to map the port inside the docker container to a free port.

*Need to review and rewrite port mapping section.

docker run -p 38282:8080 kodekloud/simple-webapp:blue this command would run docker with the container host being 38282 on port 8080 with the tag blue for image simple-webapp

Data is persistent in a docker container. All data is gone if you delete the container. Best practice is to map directories to the docker host. It will mount directories inside the container and stored on the external volume and remains even if you delete the docker container.

docker run -v /opt/datadir:/var/lib/mysql mysql this command would mount it to the var lib and then its specified to the directory inside the container. This is important because if you delete the Docker container the information remains if mounted in another point.

docker inspect [CONTAINER NAME OR ID] gives you properites on a running container.

docker logs [CONTAINER ID OR NAME] shows you the logs (the content written to the standard out of the container)

docker run -it this command attaches to the terminal and in interactive mode on the designated container.

Enviornment Variables

Found under the config line in the container.