CS 448 Database Systems. Serializability Theory 2. Serializability Theorem

Transcription

1 CS 448 Database Systems Serializability Theory 2 Serializability Theorem Theorem: A history H is serializable iff SG(H) is acyclic. Proof: IF Suppose H is a history over T={T 1, T 2,, T n }. WLOG assume T 1, T 2,, T m (m < n) are all txns in T that are committed in H. Thus T 1, T 2,, T m are the nodes in SG(H). Since SG(H) is acyclic, it can be topologically sorted.

2 Serializability Theorem Let i 1, i 2,, i m, be a permutation of 1,2,,m such that T i1, T i2,, T im is a topological sort of SG(H). Let Hs be the serial history T i1, T i2,, T im. We claim that C(H) Hs. Let p i T i and q j T j, where T i, T j are committed in H. Suppose p i, q j conflict and p i < H q j. By the definition of SG(H), T i! T j is in SG(H). Serializability Theorem Therefore in any topological sort of SG(H), T i must appear before T j. Thus in H s all operations of T i must precede all operations of T j, and in particular, p i < Hs q j. Thus any two conflicting operations are ordered in the same way in C(H) as H s. Thus C(H) H s, which is serial, therefore H is SR.

3 Serializability Theorem ONLY IF: Suppose H is SR. Let H s be a serial history equivalent to C(H). Consider an edge T i! T j in SG(H). Thus there are two conflicting operations p i, q j of T i, T j (respectively), such that p i < H q j. Because C(H) H s, p i < Hs q j. Because H s is serial, and p i precedes q j, it implies that T i precedes T j in H s. Serializability Theorem Thus we see that if T i! T j is in SG(H), then T i precedes T j in H s. Suppose that there is a cycle in SG(H), say T 1!T 2!!T k!t 1 This implies that T 1 appears before itself in H s, which is absurd. Thus no cycle can exist in SG(H) if H is SR. QED

4 Recoverable Histories A txn T i reads x from T j in history H if w j [x] < r i [x]; NOT (a j < r i [x]) and If there is some w k [x] such that w j [x] < w k [x] < r i [x], then a k < r i [x]. A history is Recoverable (RC) if, whenever T i reads from T j (i j) in H, c i H, c j < c i. A history Avoids Cascading Aborts (ACA) if, whenever T i reads x from T j (i j) in H, c i < r i [x]. A history H is Strict (ST) if whenever w j [x] < o i [x] ( i j), either a j < o i [x] or c j < o i [x], where o i [x] is r i [x] or w i [x]. Examples T 1 =w 1 [x] w 1 [y] w 1 [z] c 1 T 2 =r 2 [u] w 2 [x] r 2 [y] w 2 [y] c 2 w 1 [x] w 1 [y] r 2 [u] w 2 [x] r 2 [y] w 2 [y] c 2 w 1 [z] c 1 Not RC w 1 [x] w 1 [y] r 2 [u] w 2 [x] r 2 [y] w 2 [y] w 1 [z] c 1 c 2 RC, not ACA w 1 [x] w 1 [y] r 2 [u] w 1 [z] w 2 [x] c 1 r 2 [y] w 2 [y] c 2 RC, ACA, not Strict

5 SR RC ACA ST Serial Prefix Commit-closed A property of a history is called prefix commitclosed if, whenever the property is true of history H, it is also true of history C(H ), for any prefix H of H. Since failures may occur when a prefix of an acceptable history has been processed, DBMS schedulers and recovery managers must satisfy prefix commit-closed properties for CC and recovery, i.e. every C(H ) must be acceptable too.

6 Theorem Serializability is a prefix commit-closed property. Proof: Since H is SR, SG(H) is acyclic. Consider SG (C(H )) where H is any prefix of H. If T i! T j is an edge of this graph, then we have two conflicting operations p i, q j belonging to T i, T j (respectively) with p i < C(H ) q j. But then clearly p i < H q j and thus T i!t j exists in SG (H). Therefore SG(C(H )) is a subgraph of SG(H). If SG(H) is acyclic, so must SG(C(H )), hence C(H ) is SR. Other Operations So far, we have limited ourselves to reads and writes. However, serializability does not limit us to these. We just need to redefine conflicting operations as any pair for which the result, in general, depends upon the order of their execution. Effect is: value returned, and final value of data. Thus we need only define the notion of conflict appropriately. For example, we could add Increment and Decrement as basic (atomic) operations. Assume they do not return a value.

7 Compatibility Matrix Read Write Increment Decrement Read Write Increment Decrement Y N N N N N N N N N Y Y N N Y Y View Equivalence So far, we have based equivalence of histories on the fact that the ordering or writes with respect to other operations on the same object should be the same. We can say that the effects are simply the values read and the final values of data objects. If these are the same in two histories, then they are declared to be view equivalent.

8 View Equivalence The final write of x in a history H is the operation w i [x] H, such that a i H and for any w j [x] H (j i) either w j [x] < w i [x] or a j H. Two histories H, H are view equivalent if they are over the same set of txns and have the same operations; For any T i,t j such that a i, a j H (hence a i, a j H ) and for any x, if T i reads x from T j in H then T i reads x from T j in H and For each x, if w i [x] is the final write of x in H then it is also the final write of x in H. View Serializability A history, H, is defined to be view serializable (VSR) if for any prefix H of H, C(H ) is view equivalent to some serial history. We need to ensure prefix commit closure w 1 [x] w 2 [x] w 2 [y] c 2 w 1 [y] c 1 w 3 [x] w 3 [y] c 3 The complete history is view equiv. to T 1 T 2 T 3. However, upto c 1 it is not view equiv. to either T 1 T 2 or T 2 T 1!

9 CSR vs. VSR Theorem: If H is conflict serializable then it is view serializable. The converse is not, generally, true. Proof. Suppose H is CSR. Let Hs be a serial history equivalent to C(H ). If T i reads x from T j in C(H ), then w j [x] < C(H ) r i [x] and there is no w k [x] such that w j [x] < C(H ) w k [x] < C(H ) r i [x]. Hs must order these in the same way i.e. w j [x] < Hs r i [x], and no intermediate w k [x]. Hence they have the same reads-from relationships. Similarly for final writes. VSR CSR w 1 [x] w 2 [x] w 2 [y] c 2 w 1 [y] w 3 [x] w 3 [y] c 3 w 1 [z] c 1 T1 T3 T2