Docker is a platform that enables the encapsulation of software applications within units known as containers. These containers can be visualized as packages that encompass all the necessary components an application requires to execute seamlessly in any environment.
This concept differs from virtual machines, which emulate an entire computer system. While virtual machines replicate hardware, containers mirror the operating system, resulting in a more efficient and faster execution of applications.
Docker is composed of three primary elements:
Docker’s versatility allows developers to build, deploy, and manage applications across various platforms, including personal computers, web-based platforms, or distributed systems. This flexibility enhances productivity and ensures consistent performance across different environments.
Dockerfile: A text file that contains instructions for building a Docker image.
Image: A read-only template with instructions for creating a Docker container. It’s a snapshot of a container.
Container: A runnable instance of an image. You can create, start, stop, move, or delete a container using Docker API or CLI.
Docker Hub: A cloud-based registry service where you can link to code repositories, build your images and test them, store manually pushed images, and link to Docker Cloud.
Docker Compose: A tool for defining and running multi-container Docker applications. With Compose, you use a YAML file to configure your application’s services.
Docker Swarm: A native clustering and scheduling tool for Docker. Swarm allows IT administrators and developers to create and manage a virtual system of Docker nodes and schedule containers.
Volume: A specially-designated directory within one or more containers that bypasses the Union File System to provide several useful features for persistent or shared data.
Service: In a distributed application, different pieces of the app are called “services.” For example, if you imagine a video sharing site, it probably includes a service for storing application data in a database, a service for video transcoding in the background after a user uploads something, a service for the front-end, and so on.
Node: In the context of Docker, a node is a physical or virtual machine that is running Docker Engine.
Stack: A stack is a group of interrelated services that share dependencies, and can be orchestrated and scaled together. A single stack is capable of defining and coordinating the functionality of an entire application (though very complex applications may want to use multiple stacks).
A Dockerfile is a text-based document filled with a sequence of instructions to construct a Docker image. It acts as the architectural plan for creating Docker images and is vital to Docker’s functionality.
The Dockerfile employs a specialized language and outlines the base image to employ, any supplementary software to install, and any configuration modifications to implement. When the
docker build
A Docker image serves as a read-only blueprint, encompassing a series of directives to instantiate a container capable of executing on the Docker platform, offering an efficient method to package up applications and pre-configured server environments, which can be utilized privately or disseminated publicly to other Docker users, constituting the initial step for Docker novices.
Docker Hub is a cloud-based repository service provided by Docker for finding and sharing container images with your team and the Docker community. It’s where you can push or pull Docker images, making it a centralized resource for Docker images.
Here’s how you can use Docker Hub:
docker pull command followed by the name of the image.docker push command followed by the name of the image.Docker Hub is not just a repository for images but also a place where you can build, store, and distribute your images. It’s a fundamental tool for anyone working with Docker and containerization technology.
A Dockerfile is a script composed of text-based instructions, which serves as a blueprint for creating a Docker image. Here’s an example of a Dockerfile for a Node.js application:
1 # Use the official Node.js 14 image from Docker Hub as the base image
2 FROM node:14
3
4 # Set the working directory in the Docker image
5 WORKDIR /usr/src/app
6
7 # Copy package.json and package-lock.json to the working directory
8 COPY package*.json ./
9
10 # Install the application dependencies using npm
11 RUN npm install
12
13 # Copy the rest of the application source code to the working directory
14 COPY . .
15
16 # Expose port 8080 to have it mapped by the Docker daemon
17 EXPOSE 8080
18
19 # Define the command to run the application
20 CMD ["npm", "start"]
21
22Here’s a breakdown of the Dockerfile:
This Dockerfile is a basic example.
Building a Docker Image from a Dockerfile
Create a Dockerfile
A Dockerfile is a text-based document that houses a sequence of instructions Docker utilizes to construct an image. It essentially serves as the blueprint for your application.
Define the Base Image
FROM node:14
Within the Dockerfile, you’ll need to specify the base image you’ll be working from using the FROM command. This could be a language-specific image like node:14 or python:3.7, or an OS-specific image like ubuntu:18.04.
Specify Dependencies
RUN npm install
Next, you’ll employ the RUN command to install any dependencies your application requires. For a Node.js application, this might be executed with npm install.
Copy Application Files
COPY . /app
The COPY command is utilized to transfer files from your local filesystem into the image. For instance, COPY . /app would copy all the files in your current directory to the /app directory in the image.
Define the Command
CMD ["node", "app.js"]
Finally, the CMD command is employed to specify what should be executed when a container is initiated from the image. For a Node.js application, this might be CMD ["node", "app.js"].
Build the Image
docker build -t my-app .With the Dockerfile in place, you can now construct the image. Navigate to the directory containing the Dockerfile and execute docker build -t my-app . This instructs Docker to build an image using the Dockerfile in the current directory, and to tag the image as my-app.
This process is fundamental in Docker.
Obtain the Docker Image: The initial step in executing a Docker container is to confirm you possess the appropriate Docker image. If the image isn’t locally available, you can retrieve it from Docker Hub or another Docker image registry using the command:
docker pull <image_name>For instance, to obtain an Ubuntu image, you would use:
docker pull ubuntuInitiate the Docker Container: Once the image is accessible, you can initiate the container. This is achieved using the command:
docker run -it -d --name <container_name> <image_name>This command generates and starts a new container in the background. For example, to run the Ubuntu image in a container named ‘my_container’, you would use:
docker run -it -d --name my_container ubuntuEngage with the Active Container: If you need to engage with the active container, you can utilize the docker exec command. The command:
docker exec -it <container_ID_or_name> /bin/bashWill launch a bash shell within the container that you can interact with. For instance, to engage with ‘my_container’, you would use:
docker exec -it my_container bashRemember to substitute <image_name>, <container_name>, and <container_ID_or_name> with your specific values.
This procedure enables you to run any application encapsulated in a Docker container, ensuring uniformity across various environments.
Understanding Docker Compose
Docker Compose is a tool engineered to simplify the orchestration of multi-container Docker applications. It employs a YAML file to configure the application’s services, enabling developers to initiate all services with a single command.
The primary role of Docker Compose is to streamline the management of multiple Docker containers. This proves particularly advantageous for applications composed of multiple microservices, as it facilitates each service to operate in isolation, within its own container.
By leveraging Docker Compose, developers can define an entire multi-container application within a single file, and then launch the application with a single command. This not only simplifies the development process but also enhances the portability of the application and eases its deployment.
Writing a docker-compose.yml file
A docker-compose.yml file is a YAML file that defines the services, networks, and volumes for a Docker application. Here’s an example of a docker-compose.yml file for a Node.js application:
1version: '3'
2 services:
3 app:
4 build: .
5 volumes:
6 - .:/usr/src/app
7 ports:
8 - '3000:3000'
9 command: npm start
10 depends_on:
11 - db
12 db:
13 image: mongo
14 volumes:
15 - mongodb_data_container:/data/db
16 volumes:
17 mongodb_data_container:Here’s a breakdown of the docker-compose.yml file:
This docker-compose.yml file is a basic example.
Running Multi-Container Applications
With Docker compose
Define the Application
Start by outlining your application’s services in a docker-compose.yml file. Each service symbolizes a container. You’ll need to specify the image to use, the ports to expose, any volumes to mount, and other configuration specifics for each service.
Build the Services
Once your docker-compose.yml file is set up, navigate to the directory containing the file and use the docker-compose build command to construct all the services. This command interprets the docker-compose.yml file and builds Docker images for each service.
Run the Application
After building the services, use the docker-compose up command to initiate the application. This command starts all the services as defined in the docker-compose.yml file.
Interact with the Application
With the application running, you can interact with it just as you would if it was running outside of Docker. If you’ve exposed any ports in your docker-compose.yml file, you can access the services via these ports.
Stop the Application
When you’re done, use the docker-compose down command to stop and remove all the services.
Running multi-container applications is a common scenario in the development and deployment of modern web services. Docker Compose simplifies this process by allowing the definition of multi-container applications using a YAML file.
Advanced Topics in Docker
Docker is a powerful platform with a wide range of applications. Here are some advanced topics that can enhance your understanding and usage of Docker:
These topics delve into the more complex aspects of Docker and can help you leverage Docker’s full potential in your projects. As you continue to explore Docker, these advanced topics will provide deeper insights and open up new possibilities for using Docker.
Further Resources for Learning
Here are some concise recommendations for Docker learning resources:
These books cover a range of topics from Docker basics to advanced use cases. Happy reading!😊.
