Docker has emerged over the last 18 months as a much-hyped container technology for application virtualization, but it's not without its detractors. CoreOS was one of the leading backers of Docker until Dec. 1, when it announced a competitive container virtualization technology called Rocket.
Rocket is completely new and isn't a fork of Docker, Alex Polvi, CEO of CoreOS, told eWEEK. CoreOS is an open-source operating system vendor that uses Docker containers as a basic building block to deploy clustered applications. The first stable CoreOS release debuted on July 25.
CoreOS started the Rocket effort because of a number of perceived deficiencies in Docker.
"We went about this because Docker is no longer a standard composable unit; Docker is a platform which now includes clustering, cloud server provisioning and more," Polvi said. "The community needs a simple composable unit that can be used in a variety systems—this is why we introduced Rocket and the App Container specification."
The App Container Image (ACI) is intended to be a similar concept to what the Amazon Machine Image (AMI) is for the public cloud. An ACI is an image format for bundling a container application, while Rocket is the command line tool for ACI.
At the core of Docker, the open-source Linux Containers (LXC) technology has long served as a foundational component. With Rocket, Polvi said, the system is designed to be pluggable to use any number of different virtualization tools.
"We are using a tool called nspawn for doing the actual containerization in Rocket, but that is just an implementation detail," he said. "We could use any isolator, such as LXC, or even KVM [Kernel-based Virtual Machine]."
Although CoreOS is developing its own competitive alternative to Docker, it isn't actually abandoning Docker for its existing CoreOS Linux distribution users.
"CoreOS will still support Docker," Polvi said. "We will not be moving our users off of Docker or leaving anyone out to hang while we develop Rocket."
Going a step further, Polvi said that CoreOS' intention is to work on Docker compatibility, but the project is too early to give a firm date on when that will happen. Docker has been embraced by many of the world's leading IT vendors, including Red Hat, Microsoft, VMware and Amazon.
As a company, Docker Inc.—the lead commercial sponsor behind the open-source Docker platform—announced its Series C round of funding in September, bringing in $40 million. CoreOS is also venture-backed and received $8 million in a Series A round of funding in June.
Docker creator Solomon Hykes responded to the CoreOS Rocket announcement in a series of comments that were publicly posted on Hacker News and on Twitter.
"Competition is always good," Hykes wrote.
Hykes noted that LXC itself is competitive with other Linux container efforts, including openvz and vserver, and added that now Rocket will bring competition to Docker.
"In response Docker is forced to up its game and earn its right to be the dominant tool," he wrote. "This is a good thing."
Sean Michael Kerner is a senior editor at eWEEK and InternetNews.com. Follow him on Twitter @TechJournalist.