Measurements of Behavior of Site Failure/Network Partitioning • How did partition occur? • How many objects become out of date? • How many transactions are blocked or aborted? • Availability. • Effects of creating more copies when failures occur. Site Future
Replication Control enables a distributed database system to operate effectively despite periods of failures and communication breakdowns. At Purdue University, we have developed the Mini-RAID system for conducting experiments in replication control. In the following slides, an example based on the system is presented. The configuration for our example is: • 3 database sites (S 0, S 1, S 2) • a fully replicated database of 4 objects (a, b, c, d) Site Future
Dynamic Majority for Network Partitioning • Majority of majority continues processing. • Declare a tie when too few sites in majority follow optimistic approach and continue processing without commit. • Full availability to read-only transactions (view serializability). * • Arbitrary merges of partition to form majority once again. * To appear in Journal of Information Science, Feb. 1988 Site Future
New Protocols and Algorithms (continued) • Multiple Site Failure and Recovery* - Session numbers. Read one write all available (ROWAA). Fail-locks. Database available as long as a single copy is up. Operation site do little work for failed site(s). Failed site recovers on demand or automatically via updates on open sites. * Journal of Management Information Systems, Vol. 4, No. 2, 1987. Site Future
![Examples: 1. Ta = Ra[X] Wa[Y], Tb = Rb[Y] Wb[X] X = {x 1,](https://present5.com/presentation/62a5bb0d4d8c35e359bb29c606d0594d/image-5.jpg "Examples: 1. Ta = Ra[X] Wa[Y], Tb = Rb[Y] Wb[X] X = {x 1,")
Examples: 1. Ta = Ra[X] Wa[Y], Tb = Rb[Y] Wb[X] X = {x 1, x 2}, Y = {y 1, y 2} x 1 and y 1 are written by copier transactions Tc and Td H = ra[x 1] rb[y 1] (site 1 fails) wa[y 2] wb[x 2] (site 1 recovers). . . rc[x 2] wc[x 1] rd[y 2] wd[y 1] H is serializable, but has the same effect as an incorrect history ra[x 1] rb[y 1] wa[y 2] wb[x 1] wa[y 1] 2. T 2. a = Ra[X] Wa[X], Tb = Rb[Y] wb[X] X = {x 1, x 2}, Y = {y 1} Copier transaction Tc writes to x 2 H = (site 2 recovers) ra[x 1] rb[y 1] wa[x 1] (site 2 recovers) rc[x 1] wc[x 2] wb[x 1] Missing update – x 2 reflects Wa[X} but not Wb[X] Site Future
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