Docker Alternatives

What is Docker?

You probably heard of the statement ‘Write once, run anywhere’, a catchphrase that SUN Microsystems came out with to capture Java’s ubiquitous nature. This is a great paradigm except that, if you have a java application, for example, in order to run it anywhere you need platform-specific implementations of the Java Virtual Machine. On the other end of the ‘run anywhere’ spectrum, we have Virtual Machines. This approach, while versatile, comes at the cost of large image sizes, high IO overhead, and maintenance costs.
What if there is something that is light in terms of storage, abstracted enough to be run anywhere, and independent of the language used for development?

This is where Docker comes in! Docker is a technology that allows you to incorporate and store your code and its dependencies into a neat little package – an image. This image can then be used to spawn an instance of your application – a container. The fundamental difference between containers and Virtual Machines is that containers don’t contain a hardware hypervisor.

This approach takes care of several issues:

No platform specific, IDE, or programming language restrictions.
Small image sizes, making it easier to ship and store.
No compatibility issues relating to the dependencies/versions/setup.
Quick and easy application instance deployment.
Applications and their resources are isolated, leading to better modularity and security.

Docker Architecture

To allow for an application to be self-contained the Docker approach moves up the abstraction of resources from the hardware level to the Operating System level.

To further understand Docker, let us look at its architecture. It uses a client-server model and comprises of the following components:

Docker daemon: The daemon is responsible for all container related actions and receives commands via the CLI or the REST API.
Docker Client: A Docker client is how users interact with Docker. The Docker client can reside on the same host as the daemon or a remote host.
Docker Objects: Objects are used to assemble an application. Apart from networks, volumes, services, and other objects the two main requisite objects are:
Images: The read-only template used to build containers. Images are used to store and ship applications.
Containers: Containers are encapsulated environments in which applications are run. A container is defined by the image and configuration options. At a lower level, you have containerd, which is a core container runtime that initiates and supervises container performance.
Docker Registries: Registries are locations from where we store and download (or “pull”) images.

Docker Alternatives

While Docker is the most widely used and recognized container technology, there are other technologies that either preceded Docker, emerged side-by-side with Docker, or have been introduced more recently. All follow a similar concept of images and containers, but have some technical difference worth understanding:

rkt (pronounced ‘rocket’) from the Linux distributor, CoreOS

CoreOS released rkt in 2014, with a production-ready release in February 2016, as a more secure alternative to Docker. It is the most worthy alternative to Docker as it has the most real-world adoption, has a fairly big open source community, and is part of the CNCF. It was first released in February 2016.

Additional reading: Product page, GitHub page

LXD (pronounced “lexdi”) from Canonical Ltd., the company behind Ubuntu

Canonical launched its own Docker alternative, LXD, in November 2014, with the focus of offering full system containers. Basically, LXD acts like a container hypervisor and is more Operating System centric rather than Application Centric.

Additional reading: Product page, GitHub page

Linux VServer

Like OpenVZ, Linux VServer provides operating system-level virtualization capabilities via a patched Linux kernel. The first stable version was released in 2008.

Additional reading: GitHub page

Windows Containers

Microsoft has also introduced the Windows Containers feature with the release of the Windows Server 2016, in September 2016. There are currently two Windows Container types:

Windows Containers: Similar to Docker containers Windows containers use namespaces and resource limits to isolate processes from one another. These containers share a common kernel, unlike a VM which has its own kernel.
Hyper-V Containers: Hyper-V containers are fully isolated highly optimized virtual machines that contain a copy of the Windows Kernel. Unlike Docker containers that isolate processes and share the same kernel, Hyper-V containers each have their own kernel.

See the Microsoft Documentation ›

How Do They Differ from Docker?

Let us take a quick look at how each of these alternatives differs from Docker:

	Rkt	LXD	OpenVZ	LinuxVServer	Windows Containers
Compared to Docker	Focuses on compatibility, hence it supports multiple container formats, including Docker images and its own format. Like Docker, it is optimized for application containers, not full-system containers and has fewer third-party integrations available.	Emulates the experience of operating Virtual Machines but in terms of containers and does so without the overhead of emulating hardware resources. While the LXD daemon requires a Linux kernel it can be configured for access by a Windows or macOS client.	An extension of the Linux kernel, which provides tools for virtualization to the user. It uses Virtual Environments to host Guest systems, which means it uses containers for entire operating systems, not individual applications and processes.	Uses a patched kernel to provide operating system-level virtualization features. Each of the Virtual Private Servers is run as an isolated process on the same host system and there is a high efficiency as no emulation is required. However, it is archaic in terms of releases, as there have been none since 2007.	The Docker Engine for Windows Server 2016 directly accesses the windows kernel. Hence native Docker containers cannot be run on Windows Containers. Instead, a different container format, WSC (Windows Server Container), is to be used.
Use Cases	Public cloud portability, Stateful app migration, and Rapid Deployment.	Bare-metal hardware access for VPS, multiple Linux distributions on the same host.	CI/CD and DevOps, Containers and big data, Hosting Isolated Set of User Applications, Server consolidation.	Multiple VPS Hosting and Administration, and Legacy support.
Adoption	Moderate.	Low.	Low.	Low – Moderate (Mostly Legacy Hosting).
Used By	CA Technologies, Verizon, Viacom, Salesforce.com , DigitalOcean, BlaBlaCar, Xoom.	Walmart PayPal, Box.	FastVPS, Parallels, Pixar Animation Studios, Yandex.	DreamHost, Amoebasoft, OpenHosting Inc., Lycos, France, Mosaix Communications, Inc.

As the various Docker alternatives mentioned move ahead with rooting out bugs, increasing security, extending functionality, and adding new features, we will also have service providers offering hosting options, even more, agile development options, distributed applications, and microservices.

One of the directions that these platforms would be heading towards is better support, tools, and strategies for containerizing legacy applications. We also have a consortium of companies like the Cloud Native Computing Foundation that is driving innovation across such projects as well as incubating new projects centered around these platforms and services. Among the various container runtimes discussed above, rkt and containerd have been accepted as ongoing projects by the Cloud Native Computing Foundation.

As you can see, there may be many container runtimes available. This is a good thing, as it promotes innovation and gives developers a range of tools to suit their specific needs, while still having consolidation around Docker itself. Hopefully, this article has you thinking about what container format you would use for your next application.

The Road Ahead for Docker Alternatives

As the various Docker alternatives mentioned move ahead with rooting out bugs, increasing security, extending functionality, and adding new features, we will also have service providers offering hosting options, even more, agile development options, distributed applications, and microservices.

One of the directions that these platforms would be heading towards is better support, tools, and strategies for containerizing legacy applications. We also have a consortium of companies like the Cloud Native Computing Foundation that is driving innovation across such projects as well as incubating new projects centered around these platforms and services. Among the various container runtimes discussed above, rkt and containerd have been accepted as ongoing projects by the Cloud Native Computing Foundation.

As you can see, there may be many container runtimes available. This is a good thing, as it promotes innovation and gives developers a range of tools to suit their specific needs, while still having consolidation around Docker itself. Hopefully, this article has you thinking about what container format you would use for your next application.