Table of contents
In Docker, containers and images are fundamental concepts, and understanding the difference between them is crucial for effective containerization. Here's a breakdown of the differences between Docker containers and images:
Docker Image:
Definition:
- An image is a lightweight, standalone, and executable package that includes everything needed to run a piece of software, including the code, runtime, libraries, and system tools.
Composition:
- Images are composed of layers. Each layer represents a set of file changes, configurations, or dependencies. These layers are combined to form the complete image.
Immutability:
- Images are immutable, meaning that once created, their content does not change. Any changes result in the creation of a new image layer.
Source:
- Images are typically built from a Dockerfile, which is a script that contains instructions for building the image, specifying the base image, adding dependencies, and configuring the environment.
Storage:
- Images are stored in a registry, which can be a public registry like Docker Hub or a private registry. Registries allow for the distribution and sharing of images across different environments.
Usage:
- Images serve as a template for creating containers. When a container is instantiated from an image, it becomes a runnable instance of that image.
Docker Container:
Definition:
- A container is a runnable instance of a Docker image. It encapsulates the application and its dependencies, running in an isolated environment.
Dynamic:
- Containers are dynamic and can be started, stopped, and deleted. Each container runs in its own isolated environment, with its own filesystem, processes, and network.
Stateful Changes:
- Containers can have stateful changes, meaning that data written to the container's filesystem or environment variables can be modified during runtime.
Source:
- Containers are instantiated from a Docker image. When a container is created, it is based on the content and configuration defined in the underlying image.
Isolation:
- Containers provide process isolation, allowing applications to run without interfering with each other. Each container has its own isolated filesystem and process space.
Ports and Networking:
- Containers can expose and bind ports to the host machine, enabling communication between the containerized application and external systems. Containers can also be connected to networks to facilitate communication between containers.
Summary:
Image:
A static, immutable, and layered template for running containers.
Stored in registries and shared across environments.
Created from a Dockerfile and composed of layers.
Container:
A dynamic and runnable instance of a Docker image.
Created from an image and runs in an isolated environment.
Can be started, stopped, and deleted, with the ability to have stateful changes.
In essence, an image is a blueprint for a container, and containers are the runtime instances of images. Images provide consistency, portability, and shareability, while containers offer dynamic execution and isolation for applications.