What are Containers?
Containers are lightweight, standalone, and executable software packages that include everything needed to run a piece of software, including the code, runtime, system tools, libraries, and settings.
Containers solve the "it works on my machine" problem by packaging applications with their dependencies, ensuring consistent execution across different environments. They provide an isolated execution environment that:
- Ensures consistent behavior across development, testing, and production
- Allows applications to run reliably when moved from one computing environment to another
- Enables microservices architecture by isolating individual components
- Facilitates DevOps practices with repeatable deployments
- Maximizes resource utilization compared to traditional deployment methods
At their core, containers rely on Linux kernel features like namespaces for isolation and cgroups for resource limitations. Docker adds a layer of tooling that makes these capabilities accessible and manageable.
Container vs Virtual Machines
Unlike virtual machines, containers virtualize the operating system instead of hardware. This makes them more lightweight and portable.
Key Differences:
Feature | Containers | Virtual Machines |
---|---|---|
Virtualization | OS-level virtualization | Hardware virtualization |
Size | Typically MBs | Typically GBs |
Startup time | Seconds | Minutes |
Resource overhead | Minimal | Significant |
Isolation | Process isolation | Complete isolation |
Operating system | Shares host OS kernel | Runs complete OS |
Density | High (dozens to hundreds per host) | Low (typically 5-10 per host) |
Containers provide near-native performance with minimal overhead, while VMs offer stronger isolation but with greater resource requirements. Many modern infrastructures use both technologies together, leveraging containers for applications and VMs for infrastructure segmentation.
Key Concepts
Images
Docker images are read-only templates used to create containers. They contain:
- Application code
- Runtime environment
- Dependencies
- System tools
- System libraries
Images are built in layers, with each layer representing a set of filesystem changes. This layered approach enables efficient storage and transfer, as identical layers can be shared across multiple images. Images are defined by Dockerfiles, which contain instructions for building the image.
Containers
Containers are running instances of Docker images that can be:
- Started
- Stopped
- Moved
- Deleted
Each container has its own:
- Filesystem (with a writable layer on top of the read-only image)
- Network interface with its own IP address
- Process namespace (isolation from other processes)
- Resource allocation (CPU and memory limits)
Containers are ephemeral by design, meaning they can be created and destroyed without affecting the application's functionality.
Registry
A registry stores Docker images. Docker Hub is the default public registry.
Registries serve as centralized repositories for storing and distributing Docker images. They enable:
- Version control for container images
- Collaboration among team members
- Continuous integration and deployment workflows
- Distribution of public and private images
Besides Docker Hub, there are many registry options including:
- GitHub Container Registry
- Amazon Elastic Container Registry (ECR)
- Google Container Registry (GCR)
- Azure Container Registry (ACR)
- Self-hosted options like Harbor or Nexus
Basic Container Commands
Each of these commands supports additional options that allow for fine-grained control of container behavior. Use docker <command> --help
to explore the full range of options for any command.
Best Practices
- Use official images when possible
- Official images are maintained, security-patched, and follow best practices
- They provide a consistent base for your applications
- Their provenance is verified, reducing supply chain risks
- Keep containers ephemeral
- Design containers to be stateless and easily replaceable
- Store persistent data in volumes, not inside containers
- Implement proper startup and shutdown handling
- Enable seamless scaling and high availability
- Use .dockerignore files
- Exclude unnecessary files from the build context
- Improve build performance and reduce image size
- Prevent sensitive files from being accidentally included
- Example: exclude
.git
,node_modules
, logs, and temp files
- Minimize the number of layers
- Combine related commands in a single RUN instruction
- Use multi-line commands with && for installation and cleanup
- Remember that each instruction in a Dockerfile creates a layer
- Fewer layers often result in smaller images
- Use multi-stage builds for smaller images
- Separate build environment from runtime environment
- Include only necessary artifacts in the final image
- Dramatically reduce image size by excluding build tools
- Improve security by reducing attack surface
Container Lifecycle
Understanding the container lifecycle is essential for effective containerization:
- Creation: Container is created from an image using
docker create
ordocker run
- Running: Container is executing its main process
- Paused: Container can be temporarily paused with
docker pause
- Stopped: Container has exited but still exists and can be restarted
- Removed: Container is permanently deleted with
docker rm
Each state transition can trigger events that can be monitored and acted upon, enabling sophisticated container orchestration workflows.
Container Networking
Containers can communicate through several network modes:
- Bridge: Default network mode with internal Docker network
- Host: Container shares host's network namespace
- None: Container has no network interface
- Overlay: Multi-host networking (for Docker Swarm)
- Macvlan: Container appears as a physical device on your network
- Custom networks: User-defined bridge networks for container isolation
Proper network configuration is crucial for containerized applications, especially in microservices architectures.