However, this scheme is not reliable because failure any one of the servers
will cause failure of the write operation, and finally failure of data accessing.
The quorum consensus method aims at reducing the number of servers taking
part in a data operation. A quorum is a subgroup of servers whose size
gives it the right to make decisions. For example, if having a majority is the
criterion, a quorum must consist of at least n/2 servers cooperating to carry
out operations, where n is the total number of servers in the system.
The quorum method was used early on in [1]. Gi?®ord in his paper developed
a file replication scheme in which a number of ???votes??™ is assigned to each
physical copy at a replica manager of a single logical file. In each transaction,
a read quorum of R votes must be collected to proceed with a read opera-
235
Dan-Dan Liu and Xiao-Hua Jia
tion, and a write quorum of W votes must be collected to perform a write
operation. The R and W are set as:
W > half the total votes,
R+W > total number of votes for the group.
This ensures that there is at least one common copy involved in an operation
between any pair of a read quorum and a write quorum or two write
quorums. There is always a subset of the representatives of a file whose votes
total to W that are current. Thus, any read quorum is guaranteed to get a
current copy, which is determined by the version numbers.
The quorum method has also been used in preventing transactions in different
partitions of networks from producing inconsistent results [2].
Pages:
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384