Ensuring relaxed atomicity for flexible transactions in multidatabase systems
TL;DR: Global transaction management requires cooperation from local sites to ensure the consistent and reliable execution of global transactions in a distributed database system.
Abstract: Global transaction management requires cooperation from local sites to ensure the consistent and reliable execution of global transactions in a distributed database system. In a heterogeneous distr...
Summary (3 min read)
- A flurry of research activity has been devoted to the problems of enhancing transaction management by using extended transaction models.
- In particular, some flexible transaction models proposed for the MDBS environment, such as Flex Transactions  and S-transaction  , increase the failure resiliency of global transactions by allowing alternative subtransactions to be executed when a local database fails or a subtransaction aborts.
- Transaction to adhere to a weaker form of atomicity, which the authors term semi-atomicity, while still maintaining its correct execution in the multidatabase.
- The following example is illustrative: Ift 2 fails, ta may replace t2.
- Most of this work has assumed the availability of visible prepare-to-commit states in local database systems.
1.1 Proposed Research
- The authors present an approach which preserves semi-atomicity in an MDBS environment in which the local database systems are requ.ired only to ensure serializability and recoverability  .
- Each partial order provides one alternative for the successful execution of the flexible transaction.
- This methodology differs from previous approaches in that no specific application semantics are involved.
- Therefore, a theoretical basis for flexible transaction management can be built.
- As the compensation and retry approaches [15) are unified and employed as flexible transaction failure recovery techniques, local prepare-to-commit states are no longer required.
1.3 Structure of the Paper
- Section 2 introduces the fundamental flexible transaction model and Section 3 defines the property of semi-atomicity.
- In Section 4, the authors define those flexible transac tions that can be executed in the error-prone MDBS environment without requiring local prepare-tocommit states.
- In Section 5, the authors present the flexible transaction recovery protocol and demonstrate its effectiveness in preserving the semi-atomicity of flexible transactions.
- Concluding remarks are offered in Section 6.
4.1 Commit Dependencies
- Flexible transaction may cause its commitment to result in a deadlock situation, in which each subtransaction has to wait for another subtransa.ction to commit before its commitment.
- In addition, the possession of two or more pivot subtransactions in a -<-partial order may render it difficult to determine a commit order among them which ensures that the -<-partial order can move either forward to the commitment of its sub transactions or backward to the removal of any partial effects of the committed subtransactions.
4.2 Well-formed Flexible Transactions
- By the critical point determination rules, t2 is the critical point for all three partial orders, because there is altemative if the pivot 8ubtransaetion i3 fails.
- Thus, ta. is, t6, t7 are abnormal subtransactions and among them, ls. ts. and t6 are the blocking points.
- Thus, in a well-formed flexible transaction t for any -<-partial order (Tit -<) which contains an abnormal subtransaction t, there is at least one alternative -<-partial order (Tj, -<) which shares a prefix with (TiI-<) such that the aborting of t will lead the execution of T from (Tit -<) to (Tj,-<) without resulting in any database inconsistency.
Observation 1 If a flexible transaction is well-formed. it has at least one -<-partial order such
- That it has no abnormal subtransaction and no other -<-partial order has lower priority than it.
- The only blocking point t4 constitutes a switching set.
- In Example 3, the flexible transaction 7 has one -<-partial order 1'3 that contains no abnormalsubtransaction.
- Since the blocking point t3 constitutes the switching set and the blocking points t s and ta also constitute the switching set, T is also well-formed.
S is commit dependency preserving and F-recoverable, then the semi-atomicity of T is preservable.
- Without loss of generality, the authors assume that each -<-partial order of T contains at least one pivot subtransaction.
- Then, at least one pivot subtransaction of (7i, -<) has committed.
- Since T is well-formed, by Definition 3, the execution of (Ti, -<) can be changed to another -<-partial order (Tj, -<) without resulting in any database inconsistency.
- Because abnormal subtransactions are permitted, the concept of a well-formed flexible transaction greatly extends the scope of global transactions that can be specified in the MDBS environment beyond that of the basic global transaction model proposed in [15}.
5 The Flexible Transaction Commit Protocol
- As each subtransaction begins to execute, the type of the sub transaction is sent to the local database server with the begin command.
- The operations belonging to the sub transaction are submitted to an individual local data.
- The completion of each submitted operation, as well as the begin and commit operations, are individually acknowledged by the local database server to the GTM.
5.2 The Commit Protocol
- When all subtransactions in the currently executing -<-partial order of the flexible transaction enter the committed state, the flexible transaction commits.
- All its subtransactions which are not in the committed -<-partial order should be in either inactive, aborted, or committed-reversed states.
- Otherwise, there is no committed -<-partial order, the flexible transaction aborts.
- All its subtransactions should be in either inactive, aborted, or committed-reversed state.
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