Gifford's quorum consensus replication is in use at servers X, Y and Z which all hold replicas of data items A and B. The initial values of all replicas of A and B are 100 and the votes for A and B are 1 at each of X, Y and Z. Also R = W = 2 for both A and B. A client reads the value of A and then writes it to B.

(i) At the time the client performs these operations, a partition separates servers X and Y from server Z. Describe the quora obtained and the operations that take place if the client can access servers X and Y.
(ii) Describe the quora obtained and the operations that take place if the client can access only server Z.
(iii) The partition is repaired and then another partition occurs so that X and Z are separated from Y. Describe the quora obtained and the operations that take place if the client can access servers X and Z.

i) Partition separates X and Y from Z when all data items have version v0 say:





A client reads the value of A and then writes it to B:





read quorum = 1+1 for A and B - client Reads A from X or Y





write quorum = 1+1 for B client Writes B at X and Y





ii) Client can access only server Z: read quorum = 1, so client cannot read, write quorum = 1 so client cannot write, therefore neither operation takes place.





iii) After the partition is repaired, the values of A and B at Z may be out of date, due to clients having written new values at servers X and Y. e.g. versions v1:





Then another partition occurs so that X and Z are separated from Y.


The client Read request causes an attempt to obtain a read quorum from X and Z. This notes that the versions


(