Linux-HA [offsite] is an open source project to provide flexible a High Availability framework. Its core component is Heartbeat [offsite] which implements a heartbeat-protocol. That is, messages are sent at regular intervals between machines and if a message is not received from a particular machine then the the machine is assumed to have failed and some form of evasive action is taken. Heartbeat can send heartbeat messages over both serial links and ethernet interfaces. Heartbeat is also able to use qourum devices to help determine which nodes should be active,this is done using the ipfail plugin.
When heartbeat is configured, a master node is selected. When heartbeat starts up this node sets up an interface for a virtual IP address, that will be accessed by external end users. If this node fails then another node in the heartbeat cluster will start up an interface for this IP address and use gratuitous ARP to ensure that all traffic bound for this address is received by this machine. This method of fail-over is called IP Address Takeover. Unless the auto_failback directive is set to off in the ha.cf file, once the master node becomes available again resources will fail-over again so they are once again owned by the master node.
Each virtual IP address is considered to be a resource. Resources are encapsuated as programes that work similarly to unix init scripts, that is the resource can be started and stopped, and its can be asked if it is running or not. In this way ~eartbeat is able to start and stop resources depending on the status of the other node that it is communicating with using the heartbeat-protocol. As the resources are encapsulated as programmes, arbitary resources may be used.
Ultra Monkey makes use of heartbeat on linux-directors to mangage IP addresses and monitor real-servers using the IPaddr2 resource and ldirectord running as a resource respectively.
Information on how to configure heartbeat is included in the discussion of sample topologies. Some additional techincal notes on running heartbeat are also provided.
Ultra Monkey 3 makes use of heartbeat 1.2.3 with
selected patches from the developers CVS tree.
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