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Applications
All important data and applications that form the outer face of your systems must
be arranged in such a way that they will not prevent a restart. If an application does
not release its lock files after a crash, this prevents the relevant process from
restarting. This means that the application is not suitable for a high availability
environment. Ideally, the "health" of certain applications, operating system process-
es, and network connections should be monitored with a suitable monitoring tool.
Data
After a system fails, all key data must be available to the failover system complete
and intact. This type of high availability is achieved by distributing stored data
over several systems or hard disks. For this, the contents of a disk are regularly
mirrored on another disk (or several disks), which can take over with the intact
data record if a failure occurs. Use a journaling file system to ensure that a file
system restarts in a consistent state after a system crash.
Network
All network infrastructure should be configured for redundancy, from the router
and switch infrastructure down to the simple network cable.
12.4 The Software Side of High
Availability
The key software aspects of high availability solutions are described below.
12.4.1 heartbeat
heartbeat is a package that is used to monitor all the nodes used in the cluster. heartbeat
exchanges "heartbeats" on the network interfaces of the members of the cluster to find
out which nodes in the cluster are active. If a node fails, it does not emit a signal. In
this case, heartbeat ensures that another node takes over the relevant tasks and identity
and makes the failover known within the network. This means that the cluster remains
consistent. See also
Chapter 13, Installing a Heartbeat 2 Cluster Using YaST
(page 279).
High Availability under Linux
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Troubleshooting
