Economical Fault-Tolerant Networks
The “Modified Bully Election” algorithm is our modification of the basic Bully Election algorithm. Assume a stable network in which an array of servers is available. One of these machines is acting as the master machine and providing services to the network. The master server also multicasts a “heartbeat” signal to all slaves, apprising them of its existence. All the machines are assigned a performance index number. This number depends on various parameters such as processor speed, availability of latest data, available resources, etc. It may be dynamically assigned at each election, or in the simplest case, is an IP address. The higher the sequence number of a machine, the more it is suitable to act as the master server.
Now, assume a fault occurs, causing the master to crash. The rest of the machines detect its failure from the absence of the heartbeat and move into the election state. In this state, a machine sends election calls to all those with a higher index number than it has. If a higher-indexed machine is available, it will reply to this election call. On receiving an election reply, the machine moves into a “bind-wait” state. If a machine in election state receives no reply within a specified amount of time, it concludes that it has the highest index number. It then moves into “init” state. In this state, it starts the interrupted network services and resumes the heartbeat signal. It also sends a bind signal to other machines, moving them into slave state. Finally, it moves itself into master state, and the network becomes stable again.
After receiving election replies, if the lower-numbered machines are unable to reach the slave state within a specified time, the election is timed out and restarted. Hence, this distributed process will elect a master under any sequence of failure. If the failed master is now fixed and brought up again, it will not initiate an election. In fact, it starts by listening to the heartbeat. If a master is found, it simply moves to slave state until the next failure occurs.
Our modification of the original Bully Election algorithm is that the computer with the highest index number is not necessarily the master at all times. In the original algorithm, a higher-index-number machine will always call for re-election and take control whenever it appears, regardless of the state of the network. In our case, this bullying procedure was found to be impractical for actual implementation. We implemented a variation, such that a stable network with a master will not be disturbed when a higher-indexed computer revives. It detects the presence of a master during startup, and on finding one, moves into slave state. Only if a new election is initiated will the highest-indexed machine take over. The current master is always the winner of the most recent election, and thus of the highest-indexed machine alive at that time but not necessarily at the current time.
A direct consequence of not including the bullying part in the election algorithm is the possibility of the existence of two masters. As we are relying on the master to take control of a key IP address as virtual server, two masters are simply unacceptable. Such a problem may occur when a master is isolated due to network congestion, the other machines conclude it dead and elect another master. As the congestion resolves and the original master reappears, there will be two masters. A very simple technique to overcome this is that all machines monitor the source of the heartbeat signal. If it is not found to be unique, a re-election is done, resulting in a single master once again.
By following this algorithm, we are able to facilitate the infallible presence of a working master server on the network. Since it is a distributed implementation, failure of any single component does not affect the election. Hence, various services can be continuously provided to the network.
The election process elects a master, but we are still far from making this change of masters transparent at the client end. In order to achieve transparency, we choose an arbitrary IP address henceforth referred to as the virtual server address. This address is not that of a machine in the cluster, but a separate one. All the clients are configured to use the virtual server address for requesting services. Whenever a machine becomes master, it takes over the virtual server address and continues with its original. IP aliasing is used to achieve this, i.e., assigning more than one IP address to a single network adapter. The new master then starts providing the network services previously provided by the failed master. Thus, the virtual server is always available, powered by any of the machines taking part in the election process. Since the clients always communicate with the virtual server, the proceedings of the fault-tolerance algorithm remain invisible to them.
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