A system container (also known as operating system container) is the oldest container type. It is an operating system (OS) centric solution that behaves like a standalone system, which does not require specialized software or custom images such as Docker. System containers are similar to virtual machines (VMs), but with much lower overhead and easier management.
System containers run full-featured init systems (systemd, SysVinit, Upstart, OpenRC, etc.), which allow spawning multiple processes (e.g., OpenSSH, crond, or syslogd) inside a single container under the same OS. System containers are most suitable for traditional or legacy monolithic applications, as they allow reusing architectures, tools, and configurations implemented for VMs.
The specifics of the system containers provide some benefits and peculiarities:
- It can be considered a stateful solution. System containers support live migration (across host nodes, data centers, or even Clouds) and do not lose data/state after reboot. Such data persistence is perfect for running long-lived stateful applications and services (including SQL, NoSQL, and in-memory database instances).
- System containers gracefully coexist with the Java ecosystem and do not require any special adjustments to make the Java process aware that it is running inside a container.
- System containers support existing solutions for implementing hot redeployment without restarting the container or the Java runtime. Also, they simplify and speed up the clustering of Java EE / Jakarta EE application servers.
There are multiple different implementations of system containers: BSD jails, Linux vServer, Solaris Zones, OpenVZ/Virtuozzo, LXC/LXD. For the platform, the Virtuozzo solution is used, which helps run thousands of system containers on the same host node and makes them easy to manage, just like a bunch of processes.
