Linux-HA

A Heartbeat (English for " heartbeat ") is a network connection between two (or more) hosts in a cluster to each other to inform each other about the fact that they are ready and their tasks can meet yet, so " alive " are.

If the notifications from another machine fail, a program goes to the " surviving " Computer assume that these partners counterpart is no longer available (eg due to a defect or a bug ) and that it should provide that these tasks are performed by a still-functioning computer.

He finds access to the network layer, usually over a null-modem cable, Ethernet or Fibre Channel instead.

Outside the clustering technique, the term was also used for used for error analysis function in the Ethernet cabling on Yellow Cable (10 Mbit / s). Heartbeat was turned on or off, for each transceiver.

Split-brain situations

Split Brain is a situation when the heartbeat connection between the computers is interrupted (eg via Ethernet or serial interface) and does not take place again within the required time. Although the computer each function properly for themselves, the control programs must assume that the failed each other on these computers.

Then no node knows the role he is to play up to date and makes itself automatically to the primary node. This results in Aktiv-/Passiv-Konfigurationen to failure of the cluster, the services offered and may result in the use of a shared data memory ( storage backends such as DRBD ) to the fact that both systems attempt to simultaneously write to the same memory.

Mutual exclusion

When two or more computers require the same resource to perform a task, for example, a network address, MAC address, or a file system, There may be a need to ensure that this resource is never used by more than one computer at a time. In the English literature, the term of this Node Fencing is in use, which means as much as computer - fencing.

STONITH is a possibility of this exclusion. If both computers are connected to a STONITH device (usually a serial port ), a computer can shut down the opposing computer in a split-brain situation. There are two ways to use the STONITH principle: In application or a hardware level. The latter has the advantage that it is not a software ( for example an SSH daemon ) dependent. In order to minimize the impact of hardware failures are heartbeat networks often built with redundant switches and each participating system with two or more network cards connected.