2repdb - CSE-5120-Fall-2009 To guarantee conict...

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CSE-5120-Fall-2009 REPLICATED DATA Multiple copies of some data items are stored at multiple sites. One copy serializability : Multiple copies of an object must appear as a single logical object to the transactions. Replicated Data History logical operations are mapped to physical op- erations: R i [ X ] R i [ X a ] W i [ X ] W i [ X a ] , W i [ X b ] , W i [ X c ] C D B A Xa Xb Yc Yd W0[ Y ] C0 T1 = R1[X] W1[X] C1 T2 = W2[X] R2[X] W2[Y] C2 T3 = W0[ X ] T0 = R3[ X ] R3[ Y ] C3 W0[Xa] H= W0[Xb] W0[Yc] W0[Yd] C0 R1[Xb] W2[Xb] R2[Xb] W1[Xb] W1[Xa] C1 R3[Xa] W2[Yc] C2 R3[Yd] C3 W2[Yd] Equivalent to: T0 T2 T1 T3 To guarantee conFict serializability: 2-Phase Locking Quorum Consensus Read Quorum: A replicated object X may be read by access- ing a set of copies - read quorum Write Quorum: A replicated object X may be written by ac- cessing a set of copies - write quorum Must guarantee: nonempty intersection of read and write quorums nonempty intersection of write and write quorums Version Number Write operations: write with a version num- ber greater than all the current version num- bers in a write quorum. Read operations: read the copy with the high- est version number. Problem : de±ne the quorums to achieve low cost and high availability Majority protocol Read one Write all protocol Primary copy protocol 11
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The Generalized Tree Quorum Protocol D. Agrawal, A. El Abbadi The logical structure is a tree. Assume one data copy per node in the tree. First consider complete trees where the degree for each non-leaf node is d , and each leaf node is at the same depth. COMPLETE TREES A tree quorum of length l and width w , denoted by the pair h l, w i : Selecting the root and w children of the root, and for each selected child, w of its children and so on, for depth l . If successful this forms a tree quorum of height l and degree w . However, if some node is inaccessible at dis- tance h 0 from the root while constructing the tree quorum, then the subtree rooted at this node is re- placed recursively by w tree quorums of length l - h 0 starting from the children of the inaccessi- ble node. The algorithm fails if the length l - h 0 of the quorum exceeds the height of the remaining subtree. For a complete tree with height 3 and degree 3, some tree quorums of dimensions h l, w i = h 1 , 2 i (length = 1, width = 2) are shown be- low, namely { 1 } , { 2,3 } , { 2,9,10 } : 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13 Read and Write Quorums Read Quorums : h l r , w r i Write Quorums :
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This note was uploaded on 04/23/2010 for the course CSC CSC5120 taught by Professor Adafu during the Fall '09 term at CUHK.

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2repdb - CSE-5120-Fall-2009 To guarantee conict...

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