CoreOS, is a highly streamlined Linux distribution designed with containers, massive server deployments, and distributed systems/applications in mind.
CoreOS is built around a number of key concepts/technologies:
The
OS is updated as a whole, not package-by-package. CoreOS uses the
Omaha protocol–initially engineered by Google for updating things like the Chrome browser and Chrome OS—to stay up-to-date with new versions. CoreOS also employs an active/passive dual root partition scheme. This dual root partition scheme allows CoreOS to run off one root partition while updating the other; the system then reboots onto the updated partition once an update is complete. If the system fails to boot from the updated partition, then reboot it again and it will revert to the known-good installation on the first partition.
All applications run in containers. CoreOS provides out-of-the-box support for
Docker containers. In fact, all applications on CoreOS run in containers. This enables separation of applications from the underlying
OS and further streamlines the CoreOS update process (because applications are essentially self-contained).
CoreOS leverages systemd.
systemd is not unique to CoreOS; it is the new standard system and service manager for Linux. (Debian has elected to use systemd; Ubuntu will adopt systemd with 14.10, if I understand correctly; and Red Hat and related distributions already use systemd.) In CoreOS, systemd unit files are used not only for system services, but also for running Docker containers.
CoreOS has a distributed key-value data store called etcd. The etcd distributed key-value data store can be used for shared configuration and service discovery. etcd uses a simple REST
API (HTTP+JSON) and leverages the
Raft consensus protocol. Docker containers on CoreOS are able to access etcd via the loopback interface, and thus can use etcd to do dynamic service registration or discovery, for example. etcd is also configurable via cloud-init, which means it’s friendly to deployment on many cloud platforms including
OpenStack. More information on etcd is available via the
etcd GitHub site.
CoreOS supports deploying containers across a cluster using fleet.
Fleet is another open source project that leverages etcd to deploy Docker containers (written as systemd unit files) across a cluster of CoreOS systems. Fleet leverages both etcd and systemd to support the deployment of containers across a cluster of systems. See
this page for more information on clustering with CoreOS and fleet.
Taken individually—the use of a minimal Linux distribution, systemd support, the distributed key-value data store, Docker support, dual root partition w/ recoverable system updates, fleet—these technologies are interesting, but not all that revolutionary. Put them all together, however, and you have (in my opinion) a very interesting solution.