To understand Docker's approach to application isolation, you must first know how virtual machines (VMs) work.

• Virtual machines are software-based emulations of physical computers;
• Each VM runs its own complete operating system (OS), along with all necessary binaries, libraries, and application code;
• The hypervisor (such as VMware ESXi or Microsoft Hyper-V) sits between the hardware and the VMs;
• The hypervisor allocates hardware resources—such as CPU, memory, and storage—to each VM, allowing multiple VMs to run on a single physical server;
• This design provides strong isolation between applications, making it possible to run different operating systems and software stacks on the same hardware;
• However, VMs are resource-intensive because each one requires a full OS, leading to higher memory and storage usage.

Typical use cases for VMs:

• Running legacy applications that require specific operating systems;
• Hosting multiple operating systems on the same hardware;
• Providing strong security boundaries in enterprise environments.

Containers: Lightweight Isolation and Shared OS Kernel

Containers use a different approach to application isolation:

• No full machine emulation: Instead of emulating entire machines, containers package an application with its dependencies;
• Shared operating system kernel: Containers share the host system's OS kernel, reducing resource overhead;
• Process isolation: Each container runs as an isolated process in user space, using OS features like namespaces and control groups for separation;
• Minimal resource usage: Containers do not require a full OS for each instance, making them significantly more lightweight than virtual machines;
• Fast startup and high density: Containers start almost instantly, consume less memory and disk space, and allow you to run thousands of instances on the same hardware where only a few VMs would fit;
• Ideal for modern workflows: This lightweight isolation is perfect for microservices, continuous integration/continuous deployment (CI/CD) pipelines, and environments that demand rapid scaling and high portability;
• Consistency across environments: By sharing the OS kernel, containers make it easier to maintain consistency between development, testing, and production environments.

Containers vs. Virtual Machines: Side-by-Side Comparison

Comparing containers and virtual machines (VMs) side by side highlights their key differences and strengths:

Performance

• Containers avoid the overhead of running multiple operating systems;
• Containers offer faster startup times and lower resource consumption;
• VMs require a full OS for each instance, leading to higher resource usage.

Portability

• Containers excel in portability; container images move easily between environments;
• Applications in containers behave the same way everywhere;
• VMs are less portable due to reliance on specific hypervisors and larger image sizes.

Scalability

• Containers allow you to run many more instances on the same hardware;
• Containers scale applications up or down rapidly based on demand;
• VMs are less efficient for rapid scaling.

Isolation and Security

• VMs provide stronger isolation, making them preferred for running different operating systems or applications with strict security requirements;
• Containers offer lightweight isolation, suitable for most modern application scenarios.

Understanding these differences will help you choose the right tool for your needs as you work with Docker.