A computer cluster is a set of loosely or tightly connected computers that work together so that, in many respects, they can be viewed as a single system.
High-availability clusters (also known as HA clusters or fail-over clusters) are groups of computers that support server applications that can be reliably utilized with a minimum amount of down-time.
Pacemaker is an open-source high availability resource manager software used on computer clusters since 2004.
Proxmox Virtual Environment (Proxmox VE) is an open-source server virtualization environment.
SIOS Protection Suite for Linux (formerly known as SteelEye LifeKeeper) is a high-availability cluster software, for Linux computer systems.
Veritas Cluster Server (rebranded as Veritas Infoscale Availabilty and also known as VCS and also sold bundled in the SFHA product) is a high-availability cluster software for Unix, Linux and Microsoft Windows computer systems, created by Veritas Technologies.