I keep rereading the Docker documentation to try to understand the difference between Docker and a full VM. How does it manage to provide a full filesystem, isolated networking environment, etc. without being as heavy?
Why is deploying software to a Docker image (if that's the right term) easier than simply deploying to a consistent production environment?
Answers
Docker and full virtual machines (VMs) serve similar purposes but achieve them through different mechanisms, resulting in different levels of resource utilization and operational overhead. Here's how Docker manages to provide features like a full filesystem and isolated networking environment without being as heavy as full VMs:
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Containerization vs. Virtualization:
- Docker uses containerization technology, while full VMs use virtualization.
- Containerization allows Docker to share the host OS kernel among containers, reducing overhead compared to running multiple independent guest OS kernels in full VMs.
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Filesystem Management:
- Docker containers use a layered filesystem called UnionFS (or OverlayFS) to provide a full filesystem within containers.
- UnionFS allows layers of filesystem changes to be stacked on top of each other, enabling efficient sharing of files between containers and reducing the storage overhead.
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Isolated Networking:
- Docker provides isolated networking environments for containers using Linux kernel features like namespaces and cgroups.
- Each container gets its own network namespace, allowing it to have its own network interfaces, IP addresses, routing tables, and firewall rules, while still sharing the host system's network stack.
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Resource Utilization:
- Docker containers share the host system's resources (CPU, memory, disk, and network) more efficiently compared to full VMs.
- Containers are lightweight, with minimal overhead, as they don't require a separate OS kernel for each instance.
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Deployment Efficiency:
- Docker simplifies the deployment process by encapsulating applications and their dependencies into portable containers.
- Docker images contain everything needed to run an application, including the application code, runtime, libraries, and dependencies.
- Docker's declarative approach to defining infrastructure in Dockerfiles makes it easier to maintain consistent development, testing, and production environments.
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Immutable Infrastructure:
- Docker promotes the concept of immutable infrastructure, where applications and their environments are packaged into immutable Docker images.
- Deploying software to Docker containers ensures consistency across different environments, reducing the risk of configuration drift and dependency issues.
In summary, Docker provides a lightweight, efficient alternative to full VMs by leveraging containerization technology and Linux kernel features. Deploying software to Docker containers simplifies the deployment process and ensures consistency across different environments, making it easier to manage and scale applications in modern IT environments.
