The 0.x and 1.x series of Heartbeat implemented serial, UDP, and PPP/UDP heartbeats together with a group based resource model.
We estimate that we currently have around thirty thousand installations up in mission-critical uses in the real world since it became suitable for mission critical production in 1999.
However resource monitoring required the use of third-party applications like Mon and resource management was limited to 2 nodes. For these reasons (and others) VersionTwo was been developed and released in July/August 2005.
See Also: GettingStartedWithHeartbeat