The heartbeat running on an alternate host can detect the running state of the primary server over an Ethernet connection and automatically take over the resources of the primary server once it cannot detect the heartbeat of the primary server. Typically, the heartbeat connection between the primary and standby servers is a separate physical connection, which can be a serial cable, an Ethernet connection implemented by a crossover line. Heartbeat can even detect the working state of the primary server through multiple physical connections, and the primary server is considered to be in a normal state as long as it can receive information about the active state of the primary server through one of the connections. From a practical point of view, it is recommended to configure multiple independent physical connections for heartbeat to avoid a single point of failure in the heartbeat communication line itself.
1. Serial Cable: It is considered to be a slightly more secure connection than an Ethernet connection, because hacker cannot run a program such as Telnet, ssh, or rsh through a serial connection, thereby reducing its chances of re-entering the backup server through the hijacked server. However, the serial cable is limited by the available length, so the primary and standby servers must be very short distances.
2, Ethernet Connection: Use this method to eliminate the length of the serial cable limitations, and this connection can be used to synchronize the file system between the primary and standby servers, thereby reducing the bandwidth from the normal communication connection.
Based on redundancy, the heartbeat control information should be transmitted using two physical connections on the primary and standby servers, thus avoiding the contention of resources when a network or cable failure causes two nodes to consider themselves to be the only active server. This contention resource scenario is called "Brain Fissure" (split-brain) or "partitioned cluster". In the case of two nodes sharing the same physical device resources, brain fissures can have quite dire consequences.
To avoid brain fissures, the following precautions can be taken:
1, as mentioned above, a redundant and reliable physical connection is established between the main and the standby nodes to transmit the control information simultaneously;
2, in the event of a brain fissure, the use of additional equipment to forcibly close one of the nodes;
The second way is known as "the other node ' headshot ' (Shoot the other nodes in the head), referred to as Stonith. Heartbeat can implement configurable stonith based on the ability to shut down a particular hardware device through a software directive. However, when the primary and standby servers are communicating over the WAN, it is difficult to avoid the emergence of a "brain fissure" scenario. Therefore, when constructing the "disaster tolerant" application, we should try to avoid the sharing of physical resources between the main and the standby nodes.
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Brain fissure problems in high availability (split-brain problem in HA)
