Economical Fault-Tolerant Networks
If unchecked, any alien machine can generate an election call and the servers will move into an unwanted election state. For this reason, security and integrity of the signals exchanged between servers is highly stressed. We have implemented three levels of security for safe operation.
Level 1: a list of all servers taking part in the election is maintained at all servers. Only signals from these machines are accepted; all other messages related to the election process are discarded.
Level 2: the servers are state-oriented. That is, they acquire certain states, e.g., election state, master state, etc. In every state, some signals are anticipated and only these are accepted. Any other signal received, even originating from listed servers, is discarded.
Level 3: an encryption scheme is used to encrypt all the signals. A random key is created for encryption and is valid for only one message. Therefore, even if a signal is intercepted and cracked, the encryption key will not be valid at any other time.
For our implementation, we empirically found that weekly synchronization of large data suffices, while the password databases are replicated on every election. Other scenarios may require a more frequent synchronization.
The level of reliability and fault tolerance of the cluster increases in proportion to the size of the cluster. Increasing the number of servers increases the maintenance time for synchronization and unduly lengthens the election process. In our situation, we determined from experimenting that three to four servers are enough to guarantee a practical working solution.
On average, the slave servers are negligibly loaded, while the master servers are not loaded for more than 0.1% of the CPU usage. Network load is also very low, except during heavy synchronization, which is therefore run as a scheduled process.
Our test bed for this implementation is the Digital Computer Laboratory, UET, Lahore, Pakistan. The lab consists of 10 Pentium-based servers and 60 diskless workstations connected by 10MBps Ethernet.
There is a need for development and implementation of techniques that provide for an immediate synchronization. One method could be that whenever a file is updated, all the servers update their versions of the file. In this way, all data on all servers at all times is perfectly synchronized, thus eliminating heavy network/server loads during scheduled synchronization.
This technique is a practical and feasible implementation of fault tolerance for low-budget LANs running open-source operating systems, such as in developing countries and resource-scarce academic institutions where expensive commercial solutions are just that—expensive.
Jahangir Hasan is a final-year student of EE-Communication Systems at UET, Lahore, Pakistan.
Kamran Khalid is a final-year student of EE-Communication Systems at UET, Lahore, Pakistan.
Farhan-ud-din Mirza is a final-year student of EE-Communication Systems at UET, Lahore, Pakistan.
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