Many organizations are adopting containers to develop and manage stable applications. Docker is one of the most feature-rich and widely used tools in this space, with millions of applications already using it. Docker is a Linux-based open-source containerization technology that is used to build, run, inspect, and manage container images for developing applications.
Although Docker is a robust standalone ecosystem of its own and provides an extensive tool kit for managing the containerization process, there are other alternatives to Docker that offer unique use cases and features. This article delves into alternatives to Docker, which includes a range of comprehensive platforms like Docker as well as individual tools that could serve as an alternative to a component of the Docker ecosystem.
List of Docker Alternatives
In the container technology landscape, Podman is a popular container engine. A container engine is an all-in-one software that manages user requests, loads and verifies container images from a registry server, monitors, allocates, isolates system resources, and runs containers using a bundled container runtime. It allows users to handle and use containers by providing a user interface that abstracts the complexities involved in dealing with system security rules and policies like Seccomp and SELinux.
Podman is a daemon-less, open-source, Linux-native container engine developed by RedHat, that is used to build, run and manage Linux OCI containers and container images. Although Podman provides a command-line interface similar to Docker's, it operates differently.
One significant difference between Docker and Podman is that the former runs a persistent, self-sufficient runtime that manages its objects or daemon called <terminal inline>dockerd<terminal inline>. On the other hand, Podman does not rely on a daemon to work. Instead, Podman starts containers as child processes. It also interacts directly with the registry and with the Linux Kernel using a runtime process. It is for this reason that Podman is called a daemon-less container technology.
The absence of a daemon improves Podman's flexibility as a container engine because it removes the dependency on a single process that could act as a point of failure that propagates to child processes causing them to fail or be orphaned.
Podman is also significantly different from Docker in that it does not require root access. This feature provides an additional security buffer that restricts potentially dangerous processes that can manipulate crucial system settings and make the container and the contained application vulnerable.
Additionally, Podman can run pods - collections containing one or more containers managed as a single entity and utilize a shared pool of resources. As a result of this additional ability, Podman users can move their workloads to Kubernetes.
LXD is another open-source container engine that is designed specifically for LXC Linux Containers. LXC enables users to run applications in isolated containers or virtual environments similar to virtual machines without the technical burden of managing individual kernels. LXD provides an interface used to connect to the LXC software library, while creating a daemon responsible for handling networking, data storage, and managing multiple LXC containers.
Although LXC can run as a standalone tool, it possesses a limited subset of features. LXD provides those additional features and thus relies on LXC to work. However, they both operate in a small subsection of the container technology ecosphere and have a small number of users. They are also more suitable for use cases that require long-term persistent environments for running virtual applications as opposed to those that use short-lived containers.
Unlike Docker that recommends a single process per container design pattern, the containers in LXC/LXD can run multiple processes. Additionally, docker containers are more portable because Docker efficiently abstracts resources in comparison to LXD. Lastly, Docker can run on Windows and OS X hosting environments, but LXD only supports Linux.
Containerd is a high-level container runtime that runs <terminal inline>runc<terminal inline> under the hood to provide an interface between the OS and container engines. Runc is a daemon with Windows and Linux support that abstracts OS-specific functionality and makes it easier to run and supervise containers and manage image transfer and storage.
This level of abstraction provided by Containerd eliminates the complexity involved in making several low-level system calls. This enables container portability since these system calls can differ for different operating systems.
Unlike Docker, Containerd however, does not handle the building of images or the creation of volumes. Interestingly, <terminal inline>containerd<terminal inline>is the default runtime for Docker, which is now an independent tool just like <terminal inline>runc<terminal inline>. This makes Containerd a handy orchestrator tool just like Kubernetes, and as a result, is one of the most popular Docker alternatives.
Buildah is an OCI image-building tool developed by The Red Hat Foundation for containerization systems. It is a tool that provides functionality that is similar to running `docker build` in Docker. It is a complementary tool often used together with Podman. In fact, Podman uses a subset of the Buildah functionality under the hood to implement its build process.
It can build images from a Dockerfile or Containerfile and produces images that operate the same way as those created with Docker as the images are OCI compliant.
Additionally, it offers fine-grained control over image layers that allows multiple changes commits into a single layer. It also provides the ability to build images from scratch, that is, images that contain nothing, which gives users the freedom to add only the packages needed to run the application. Lastly, in Buildah, users can only see images they built because it is user-specific, unlike Docker.
BuildKit is a second-generation image-building Moby project also provided as an experimental feature in newer Docker versions. Like Docker, it runs using a daemon. However, one of the key differences between the standard Docker build and BuildKit is that while the former builds each layer one layer at a time, the latter provides parallel build processing. This added feature improves performance and results in faster builds.
It also allows for the skipping of unused stages, improves incremental builds, and permits rootless builds. Additionally, it uses a cache to reduce the need to rebuild every layer of an image.
Kaniko is a Google image-building tool that can build images from Dockerfiles. It is daemon-less like Buildah but focuses more on building images in Kubernetes. Kaniko is, however, not very convenient for local development instances as it is usually run as an image with a container orchestrator like Kubernetes. However, for continuous integration and delivery pipelines in a Kubernetes cluster, Kaniko can be a practical utility.
RunC, formerly a module embedded into the Docker architecture, was released in 2015 as a standalone tool. It has since then become a widely used, standardized, interoperable container runtime DevOps teams can use as part of Docker or other custom container engines.
RunC belongs to the container runtime section of the containerization ecosystem. A container runtime is a lower-level component used in a container engine that handles the running of containers.
Although Docker offers a comprehensive toolkit for every aspect of what organizations require in a containerization process, certain DevOps functions may require various reasons to explore other alternatives. It is, however, essential to keep in mind the host OS such alternatives operate and their use cases when choosing any of such options.