Container Orchestration: Docker vs. Kubernetes

Kubernetes and Docker are both foundational tools in the container ecosystem. Although both are critical to running container-based applications, they can sometimes present a confusing picture for organizations making the jump from legacy infrastructure to more modern distributed system patterns. It’s important to understand the use cases for each, and where they offer overlapping, complementary, or competing features.

In fact, there’s a bit of a misnomer involved. While “Docker” represents a suite of tools, the core functionality has always centered on providing a container runtime; software for hosting and running containers. In contrast, Kubernetes is a container orchestration platform that enables users to select from several different container runtimes that are installed on the cluster to actually run the containers.

The goal is for organizations to have a better understanding of container orchestration and to be able to choose the tools and platforms that best fit the needs of their software applications.

What are Docker’s Container Orchestration Tools?

Docker is an open-source platform that has revolutionized the way applications are developed, shipped, and deployed. It simplifies the process by using containerization, which allows developers to package their applications, along with all dependencies, into lightweight, portable units called containers. These containers can run on any system with Docker installed, ensuring consistency across different environments.

A Docker image is a self-contained executable software package that includes everything needed to run a piece of software, including the application code, runtime, and dependencies. Docker images are built from a set of instructions specified in a Dockerfile. The Docker images are stored in a registry, which is a centralized place for sharing and distributing images. Docker Hub is a popular public registry, but organizations can also create private registries for their images.

Docker Swarm: Concepts and Components

For organizations that want to take a simplified approach to container orchestration, Docker Swarm offers a good choice. Swarm is Docker's native container orchestration tool, which Docker describes as a “...mode for natively managing a cluster of Docker Engines called a swarm.” One of the primary advantages of Docker Swarm is that it is built into Docker Engine; if an organization is already building applications with Docker, they have access to Docker Swarm.

Key components of Docker Swarm include:

• Nodes: Nodes are virtual machines or servers that run containers. There are two types of nodes: manager nodes and worker nodes. Manager nodes maintain the swarm's state and handle tasks, while worker nodes execute these tasks.

• Services: A service is a high-level configuration object that is an abstract representation of your application. It defines the desired state of the application, such as the number of replicas, the Docker image to use, and the desired ports to expose.

• Tasks: Tasks are the individual units of work that Docker Swarm assigns to nodes. A task is a combination of a Docker container and the command to run inside the container.

• Load Balancing: Load balancing for a Docker Swarm cluster is handled in two parts: ingress load balancing for handling external requests for services within the swarm, and internal load balancing for ensuring cluster services maintain consistent resource usage.

Users can administer a Docker Swarm cluster entirely from the CLI, including provisioning and deploying services. These can also be defined in yaml files, which is similar to how Kubernetes workloads are provisioned. We’ll cover this in more detail in a later section.

Benefits of Docker Swarm

• Simplicity: Docker Swarm's straightforward architecture and design make it easier to understand and implement, especially for users who are already familiar with Docker. This simplicity results in a shorter learning curve and faster deployment of containerized applications.

• Seamless Integration with Docker: Docker Swarm is natively integrated with Docker, allowing users to leverage the same Docker CLI commands they are accustomed to when working with containers. This means less overhead in learning new commands and tools, streamlining the transition to container orchestration.

• Lower Complexity: Docker Swarm's reduced complexity compared to Kubernetes makes it easier to manage and maintain, which can be a significant advantage for organizations with limited resources or smaller-scale applications.

• Lightweight Resource Usage: Docker Swarm is less resource-intensive compared to Kubernetes, making it an attractive option for organizations looking to optimize resource utilization and minimize overhead.

Docker Swarm Limitations

• Limited Scalability: Docker Swarm has limitations when it comes to scaling large-scale, complex applications, as it lacks some advanced features available in Kubernetes, such as horizontal pod auto-scaling and custom resource definitions. This makes it less suitable for organizations with rapidly growing applications or large-scale deployments.

• Smaller Community and Ecosystem: Docker Swarm has a smaller community and ecosystem compared to Kubernetes. This can result in fewer resources, third-party tools, and available support, making it potentially more challenging to troubleshoot issues or customize the platform.

• Reduced Flexibility: Docker Swarm is tightly integrated with Docker and does not support other container runtimes out-of-the-box. This limitation can be restrictive for organizations looking to use alternative container runtimes or adopt a more flexible container orchestration platform.

• Less Feature-Rich: Docker Swarm lacks some advanced features that Kubernetes offers, such as advanced load balancing, rolling updates, and self-healing capabilities. This may make it less appealing for organizations requiring these features to manage their containerized applications effectively.

• Limited Multi-Cloud and Hybrid Cloud Support: Docker Swarm's support for multi-cloud and hybrid cloud deployments is less comprehensive than Kubernetes. This can be a disadvantage for organizations seeking a container orchestration platform that can seamlessly integrate with various cloud providers and on-premises infrastructure.

A Note on Docker Compose

It’s difficult to discuss container orchestration without at least mentioning Docker Compose. Docker Compose is definitely a valid container orchestrator for defining and running multi-container applications. However, while it simplifies the process of managing multiple containers and their configurations, it doesn't merit further exploration as a full-fledged container orchestration solution because it's limited to single hosts. This constraint makes it less suitable for large-scale, distributed applications that require load balancing, fault tolerance, and other advanced features found in platforms like Kubernetes or Docker Swarm.

What is Kubernetes?

Kubernetes, also known as K8s, is an open-source container orchestration platform that automates deploying, scaling, and managing containerized applications. Initially developed by Google, Kubernetes was inspired by Google's internal orchestration system, Borg, and was later donated to the Cloud Native Computing Foundation (CNCF). Today, Kubernetes is widely regarded as the industry standard for container orchestration, thanks to its robust feature set, flexibility, and extensive community support.

Kubernetes Architecture: Clusters, Nodes, Pods, and Services

The fundamental unit of Kubernetes is the cluster, which is a set of machines (physical or virtual) that work together to run containerized applications. A cluster consists of the following components:

• Nodes: These are the virtual machines or servers that run containers. Nodes can be either physical or virtual machines and are managed by a central control plane. There are two types of nodes: control plane nodes, which manage the overall state of the cluster, and worker nodes, which run containerized applications.

• Pods: The most granular, basic unit in Kubernetes besides the containers themselves, a pod represents a single instance of a running application. A pod can contain one or multiple containers that are tightly coupled and share the same network namespace, which allows them to communicate with each other using localhost. Pods are ephemeral and can be created, destroyed, or replaced depending on the needs of the application.

• Services: A Kubernetes service is an abstraction that defines a logical set of pods and a policy by which to access them. Services provide fixed URIs like IP addresses and DNS names, making it easier to manage and discover pods as they are created and destroyed.

Benefits of Kubernetes

Kubernetes offers several benefits that make it a popular choice for container orchestration:

• Flexibility: Kubernetes supports a wide range of container runtimes, not just Docker, and allows for custom resource definitions and extensible APIs. This flexibility enables you to tailor the platform to your specific needs.

• Scalability: Kubernetes is designed to handle large-scale, distributed applications, offering advanced features like horizontal pod auto-scaling and custom resource definitions. This makes it suitable for organizations with large-scale applications or plans for rapid growth.

• Powerful Features: Kubernetes comes with a comprehensive set of features for container orchestration, such as load balancing, auto-scaling, rolling updates, and self-healing capabilities. These features make it easier to manage large-scale, complex applications.

• Extensive Community Support: As one of the most successful and popular projects in the CNCF, Kubernetes has an extensive and active community that provides a wealth of resources, such as documentation, tutorials, and third-party tools. Additionally, Kubernetes has a large ecosystem of supporting projects available in the CNCF. An organization could build an application hosting platform based on Kubernetes and utilize projects chosen almost exclusively from the CNCF to fill needs such as load balancing, observability, and configuration.

• Managed Service Availability: Every major cloud provider offers some form of hosted Kubernetes infrastructure, making it easy to migrate container workloads from on-premise or self-hosted clusters into scalable, managed services.

Kubernetes Limitations

• Steep Learning Curve: Kubernetes has a complex architecture and numerous components. The cluster networking and DNS can be difficult to manage for larger clusters with multiple applications. Additionally, deploying Kubernetes to a cloud platform requires integrating with that platform’s IAM service, demanding additional cloud expertise.

• Complexity: With its extensive feature set and flexibility, Kubernetes can be complex to set up, configure, and manage. This complexity can be daunting, especially for smaller organizations or those with limited resources. A small team of developers is unlikely to have the bandwidth or collective experience to managed a fully-fledged cluster. 

• Cost: Kubernetes clusters deployed at scale can quickly become very expensive, particularly if an organization is utilizing managed K8s services.

Kubernetes has become the standard for distributed systems and large-scale container orchestration. However, its power and flexibility also represent its downside: increased complexity and expense that can potentially require a team of experienced operations and platform engineers to effectively manage.

Making the Right Choice for Container Orchestration

For application workloads, both Docker Swarm and Kubernetes offer attractive benefits and potential downsides. Docker Swarm offers simplicity, ease of use, and tight integration with Docker, making it an attractive option for smaller organizations or those already familiar with the Docker ecosystem. On the other hand, Kubernetes provides a more powerful and flexible platform, with advanced features and a vast community to support large-scale, complex applications.

When choosing between these two platforms, it's essential to evaluate your organization's size, existing infrastructure, and required feature set. Ultimately, the choice between Docker Swarm and Kubernetes will depend on your unique requirements and the resources available to your organization. Take the time to explore both platforms, experiment with their features, and engage with their communities to find the solution that best meets your needs.