Tech-Notes

Docker

1. Docker Overview
2. Docker Architecture
3. Core Components
4. Docker Commands
5. Dockerfile
6. Docker Hub
7. Image Creation
8. Building the Image
9. Docker Compose
10. Docker Volume

Docker Overview

Docker is a set of platform as a service products that use OS-level virtualization to deliver software in packages called container

OS Architecture

1. Hardware Layer
2. Kernel Layer
3. Application Layer

Virtualization Techniques:

1. Virtual Machines (VMs) use a Hypervisor to virtualize the kernel layer.
2. Docker uses Docker Engine to perform containerization at the application layer.

Key Concepts

In linux os, two version of same software maintains by using Namespace & ControlGroups(cgroups)

• likewise Docker engine use these feature and perform containerization

Namespaces: allowing multiple instances of the same software to run simultaneously without interference. Control Groups (cgroups): Manage resource allocation (CPU, memory, etc.) for containers.

Docker Architecture

Docker Engine

The Docker Engine is a client-server application that consists of:

1. Server (Docker Daemon or dockerd): Listens for Docker API requests and manages Docker objects.
2. REST API: Communicates over UNIX sockets or a network interface.
3. Command Line Interface (CLI): Users interact with Docker using commands that call the Docker API.

Core Components

1. Image: series of instructions(layers), executed from image’s Dockerfile eg.all artifacts in dockerhub
2. Container: an instance of image or Runtime environment for image

   Docker Commands

Docker Options Command

Command	Description
-v, --version	Print version information and quit
-D, --debug	Enable debug mode
--help	Print usage information
--name	User-defined name for the container. Example: docker run --name=rajCont redis:alpine
-d	Detached mode: runs container in the background, even after ctrl+c
-a	Attached mode: prints output to the terminal
-it	Interactive terminal mode
-e	Set environment variables
-p	Maps ports from the host to the container
--net	Specifies the network where the container will run

Image Commands

Command	Description
docker images	List all images present on the local machine
docker create [IMAGE]	Create a container layer over the specified image, printing the container ID
docker rmi [IMAGE]	Remove a Docker image
docker start [CONTAINER]	Start the specified container
docker run [IMAGE]	Run an image from local or Docker Hub. Example: docker run busybox
docker run [IMAGE]:[VERSION]	Pull a specific version of a Docker image
docker run --name [CONTAINER] [IMAGE]	Run an image with a user-defined container name
docker run [IMAGE] [COMMAND]	Run a command in a new container
docker run -it [CONTAINER] [COMMAND]	Run a command in an interactive terminal
docker run -a [CONTAINER] [COMMAND]	Run a command in attached mode
docker run -d [IMAGE]	Run a container in detached mode
docker run -p [HOST_PORT]:[CONTAINER_PORT] [CONTAINER]	Map host port to container port
docker exec [CONTAINER] [COMMAND]	Execute a command in an already running container
docker exec -it [CONTAINER] /bin/bash	Open a shell in the running container

Container Commands

Command	Description
docker ps	Show running containers
docker ps -a or docker ps --all	Show history of all containers
docker start -a [CONTAINER]	Start and attach to the specified container
docker --restart [CONTAINER]	Restart the specified container
docker stop [CONTAINER]	Gracefully stop a running container
docker kill [CONTAINER]	Forcefully stop a running container
docker pause [CONTAINER]	Pause a running container
docker unpause [CONTAINER]	Unpause a paused container
docker rm [CONTAINER]	Remove a stopped container
docker attach [CONTAINER]	Attach to a running container’s shell
docker run [HOST_PORT]:[CONTAINER_PORT] [CONTAINER]	Map ports from host to container

Other Docker Commands

Command	Description
docker ps --all	List all previously run containers
docker run --rm [CONTAINER]	Remove container after execution
docker system prune	Remove stopped containers and unused resources
docker inspect [CONTAINER]	Show low-level details about a container, like IP

Docker Network Commands

Command	Description
docker network ls	List all networks
docker network create [NETWORKNAME]	Create a new network
docker network create --driver bridge [NETWORKNAME]	Create a new network with a bridge driver

Docker Network Types

1. Closed/None/Null Network: Not allowed to connect to external HTTP.
2. Bridge Network: Default network for containers.
3. Host Network: Open access to all host machine network connections.
4. Overlay Network: Used for Docker Swarm mode, allowing communication between containers on different hosts.

Example Network Commands

• Run a container with no network:
  docker run -d --net none [CONTAINER]

• Show network details:
  docker network ls
  Example outputs:
  1. eth0 - Bridge private
  2. lo - Loopback

• Connect a container to a specific network:
  docker run --net [NETWORK_NAME] [CONTAINER]

• Disconnect a container from a network:
  docker network disconnect [NETWORK_NAME] [CONTAINER]

• Run a container with the host network:
  docker run --net host [CONTAINER]

Dockerfile

A Dockerfile is a text file containing instructions to assemble a Docker image.

It consists of three main parts:

1. Specify the Base Image
2. Commands to Download, Copy, and Install Dependencies
3. Startup Command

Dockerfile Commands

Command	Description
docker build [DOCKERFILE_PATH]	Build an image using the Dockerfile from the specified path, generating an image ID.
docker build -t [DOCKER_ID]/[PROJECT_ID]:[VERSION] [DOCKERFILE_PATH]	Build an image with a customized name. Example: docker build -t rajDock/redis:latest .
docker run [IMAGE_NAME]	Run a container from the specified image (default version is latest).
docker commit -c 'CMD ["redis-server"]' [RUNNING_CONTAINER_ID]	Create a new image from an existing running container.

Example Dockerfile

FROM node:alpine        # Alpine is a lightweight version of an image
COPY ./ ./              # Copy files from the current directory to the container
RUN apt-get update      # Update package index
RUN npm install         # Install dependencies
CMD ["npm", "start"]    # Command to run when the container starts

Dockerfile Instructions:

Instruction	Description	Example
FROM	Set the base image.	FROM alpine
RUN	Executed during the image build process, on top of the current image layer.	RUN apt-get update
CMD	Command executed when the container starts.	CMD echo 'hello world'
ENTRYPOINT	Similar to CMD, but cannot be overridden by command line arguments.	ENTRYPOINT ["redis-server"]
COPY	Copy files from the host file system into the container.	COPY composer.json ./
ADD	Similar to COPY, but can also copy files from URLs and extract tar files.	ADD http://example.com/file.tar.gz ./
ENV	Set environment variables.	ENV name=rajesh
EXPOSE	container listens on the specified network ports at runtime.	EXPOSE 80/tcp
LABEL	Add metadata to the image.	LABEL description="this is cool"
USER	Specify the username or user group to use when running the image.	USER user:group
VOLUME	Create a mount point with specified path and mark it as holding externally mounted volumes.	VOLUME /var/log/
WORKDIR	Set the working directory for any subsequent instructions.	WORKDIR /var/log/
ONBUILD	Adds a trigger instruction to image that will be executed at a later time.	ONBUILD RUN echo "This runs on build"
STOPSIGNAL	Sets the system call signal that will be sent to the container to stop it.	STOPSIGNAL SIGKILL

Docker hub:

cloud-based registry service for sharing and managing Docker image

Command	Description
docker login --username=rajesh	Log in to Docker Hub with the specified username.
docker pull username/imageName	Pull an image from Docker Hub to your local machine.
docker push username/imageName	Push an image from your local machine to Docker Hub.
docker tag user/image:tag user/image:newtag	Add a new tag to an existing image.
docker search searchterm	Search for images on Docker Hub using the specified search term.

Image creation

Two Ways to Create an Image

1. Using docker commit:
   • To create an image from a running container, use the command:

docker commit <container_id> <image_name>:<tag>

1. Using a Dockerfile:
   • Create a Dockerfile with the following commands:

FROM debian:jessie
RUN apt-get update
COPY abc.txt /src/abc.txt
ADD abc.txt /src/abc.txt
WORKDIR /src
USER admin
CMD ["initial command"]

Building the Image

To build an image from a Dockerfile, use the following command:

docker build -t <image_name> .

• If you want to rebuild the image without using the cache, use:

docker build -t <image_name> --no-cache=true

Linking Containers

To link a container to another (e.g., a Redis container), use:

docker run --link redis <container_id>

Docker-compose

tool that simplifies the process of defining and running multi-container Docker applications.

• manage services, networks, and volumes in a single file, typically named docker-compose.yml.

Command	Description
docker-compose -f [YMLFILENAME] up	Start services defined in YML file
docker-compose -f [YMLFILENAME] down	Stop and remove services
docker-compose start	Start existing services
docker-compose stop	Stop running services
docker-compose pause	Pause services
docker-compose unpause	Resume paused services
docker-compose build	Rebuild all images
docker logs -f [CONTAINERID]	View logs for specific container
docker logs -f [CONTAINERNAME]	View logs using container name
docker-compose ps	List running services
docker-compose log [CONTAINER]	Show logs for a specific service

docker-compose.yml

version: '3'

services:
  mongodb:
    image: mongo
    ports:
     - "27017:27017"
    environment:  
     - MONGO_INITDB_ROOT_USERNAME=admin  
     - MONGO_INITDB_ROOT_PASSWORD=password
    volumes:
     - db-data:var/lib/mongo/data  
    build: .
    # build from Dockerfile
    context: ./Path
    dockerfile: Dockerfile
    
    volumes:
     - .db-data

Explanation

• version: Specifies the version of the Docker Compose file format.
• services: Defines the services (containers) that will be created.
  • mongodb: The name of the service.
    • image: Specifies the Docker image to use for the service.
    • ports: Maps the container port to the host port.
    • environment: Sets environment variables for the container.
    • volumes: Defines volume mappings for persistent data storage.
    • build: Indicates that the service should be built from a Dockerfile located in the specified context.

Docker Volume

Volume Type	Command Example	Description
Host Volume	docker run -v [HOST_DIR]:[CONTAINER_DIR] [CONTAINER]	Useful for sharing files between the host and container.
Anonymous Volume	docker run -v [CONTAINER_DIR] [CONTAINER]	Automatically managed by Docker; less control over storage.
Named Volume	docker run -v [ANYNAME]:[CONTAINER_DIR] [CONTAINER]	Creates a named volume for easier data management.