H. Garcia-Molina and G. Wiederhold. Read-only transactions in a distributed database. ACM Transactions on Database Systems, 7(2): pp 209-234, June 1982.  Home/Search   Document Not in Database   Summary   Related Articles   Check

....multiversion broadcast imposes an additional increase in latency, since long running read only transactions wait for old versions to appear at the end of the bcast. For quantitative results refer to [14] Currency Read only transactions can be classified based on their currency requirements [10]. Currency requirements specify what update transactions are reflected by the data read by read only transactions. Table 2 summarizes the currency properties of read only transaction in each of the methods. Disconnections. The techniques presented differ on whether they require active clients to ....

H. Garcia-Molina and G. Wiederhold. Read-Only Transactions in a Distributed Database. ACM TODS, 7(2), 1982.

.... From the trader s perspective, the inability of database management system to maintain the serializability may have important financial consequences [3] For instance, if the users who submitted multiple read only transactions communicate and compare their query results, they may be confused [5]. 2.2 Control Information To guarantee serializability, additional control information has to be broadcast by the server to the mobile clients. The volume and the complexity of the control information affect the performance of the system. First of all, sending control information consumes ....

Garcia-Molina, H., and Wiederhold, G., Read-Only Transactions in a Distributed Database, ACM Transactions on Database Systems, vol. 7, no. 2, pp. 209-234, 1982.

....inconsistency of data, which may not be acceptable to some real time applications. On the contrary, the protocols we propose in this paper ensure the consistency of data. In fact, the semantics of ROTs has been exploited extensively to improve system efficiency in non real time database systems [3, 4]. For example, multiversion two phase locking [3] is an extension of two phase locking (2PL) with versioning to avoid the data contention problem of 2PL. In these studies, multiversions of data items in the databases are maintained to allow ROTs to run against past transaction consistent database ....

Garcia-Molina, H. and G. Wiederhold, "Read-Only Transactions in a Distributed Database", ACM Transactions on Database Systems, 7(2):209-234, 1982.

.... data (such as stock price) dissemination is likely to be a major mode of information transfer in wireless environments [2] 6] 13] Some important applications in broadcasting environments are information dispersal systems and information retrieval systems where readonly transactions [5] are dominant. For example, in stock trading applications, it is expected that millions of brokers might read the prices of multiple stocks for the computation of composite index before they decide to buy any stock. Broadcast disks [1] are a form of data dissemination systems that are well suited ....

Garcia-Molina, H., and Wiederhold, G., Read-Only Transactions in a Distributed Database, ACM Transactions on Database Systems, vol. 7, no. 2, pp. 209-234, 1982.

....to the database systems. Transaction correctness is then defined as meeting its timing constraints and using data that is absolutely and relatively timing consistent [23] Many applications in mobile broadcast environments have read only transactions. Read only transactions do not modify any data [7]. Examples include information dispersal systems for temporal or time sensitive information such as stock prices, traffic condition, weather information and electronic auctions. Specifically, in stock markets and electronic auctions, it is expected that the number of stock buyers or bidders is ....

....issue in these applications. Although read only transactions can be processed with conventional transaction processing algorithms, in many cases it is more efficient to process read only transactions with special algorithms which take advantage of the knowledge that the transaction only reads [7]. Therefore, our proposed protocols exploit the semantics of the read only transactions from the mobile clients such that they are processed locally without communicating with the server. We assume that serializability is the notion of correctness in transaction processing [4] Concurrency ....

[Article contains additional citation context not shown here]

Garcia-Molina, H., and Wiederhold, G., "Read-Only Transactions in a Distributed Database," ACM Transactions on Database Systems, vol. 7, no. 2, pp. 209-234, 1982.

....and Reuter 1993] Serializability has been used as the correctness criterion for transaction execution [Gray and Reuter 1993; Bernstein et al. 1987] to ensure both consistency of database and isolation of transaction execution. In many applications, however, strict serializability is too strong. Garcia Molina and Wiederhold [1982] discussed access consistency for read only transactions. A weaker notion of consistency, Fig. 3. Multiview access algorithm for master sites. Multiview Access Protocols . 171 ACM Transactions on Database Systems, Vol. 23, No. 2, June 1998. termed insular consistency, has been proposed. An ....

....if it remains small in size for a linear solution to be efficiently applicable. In comparison with the relaxed and combined strict relaxed consistency solutions, our approach provides a combined strict insular relaxed consistency spectrum. Insular consistency is described in the work of [GarciaMolina and Wiederhold 1982]. It is integrated into our approach to provide special support for read only transactions, which are the dominant transaction type in large scale systems. Insular consistency provides much higher performance and availability than strict consistency, and for read only transactions, conducts ....

GARCIA-MOLINA,H .AND WIEDERHOLD, G. 1982. Read-only transactions in a distributed database. ACM Trans. Database Syst. 7, 2 (June), 209--234.

....consistency criteria weaker than serializability. These include database systems where performance of query transactions is improved by allowing them to access objects even when such accesses are not serializable. Examples of systems that allow weak consistency include read only transactions [10], epsilon serializability [21] and many others. In these systems, update transactions must still be serializable. In contrast, causal consistency and causal serializability criteria allow concurrent update transactions to be ordered differently in the views of different processes. Weaker ....

H. Garcia-Molina and G. Wiederhold. Read-only Transactions in a Distributed Database. ACM Trans. on Database Systems, 7(2):209--234, June 1982.

....values may then be too old to be useful. An early example of read only redundancy is timestamped versions [4] Queries that are serialized in the past do not block, and immutable versions can be replicated freely. Another early proposal using the idea of readonly redundancy is weak consistency [13] defined over the class of read only transactions. A readonly transaction satisfies weak consistency if it is locally consistent, but may cause non SR results in the global log formed by the union of local logs. ESR differs in two major ways from weakconsistency: first, it allows more interleaving ....

H. Garcia-Molina and G. Wiederhold. Read-only transactions in a distributed database. ACM Transactions on Database Systems, 7(2):209--234, June 1982.

....according to the relationships among read and write operations, but is not bounded. This categorization is considered as a qualitative approach. ESR, on the other hand, is a quantitative approach, where each conflict that causes inconsistency will be 13 weighted and bounded within a limit. In [35], Garcia Molina and Wiederhold introduced the notion of weak consistency. Weak consistency requires the execution of update transactions to be serializable, but allows each read only transaction to have its own consistent view. Another paper that allows weaker forms of consistency in a way similar ....

....If the fuzziness of two transactions, q and r, in H are calculated by comparing to two different serializable histories H SR q and H SR r , it is possible for both transactions to have zero fuzziness while H is not serializable. This point can be easily shown by an example of weak consistency [35]. Assume that update transaction u x transfers money from checking account C x to saving account S x for customer x and u y transfers money from checking account C y to saving account S y for customer y. Also, a query transaction, q c , reads checking accounts and q s reads saving accounts. ....

[Article contains additional citation context not shown here]

H. Garcia-Molina and G. Wiederhold. Read-only transactions in a distributed database. ACM Transactions on Database Systems, 7(2):209--234, June 1982.

....record will never be included in any message and thus it will not be needed for any purpose and can be deleted. 3. 4 Read Only Transactions There has been a lot of work on increasing concurrency and fault tolerance for read only transactions by reducing the number of sites they need to access [13, 10, 32, 7, 30]. In our protocol, read only transactions can also be executed locally without the need global synchronization. To see that this is possible consider all of the committed update transactions in the system and a single read only transaction t. All update transactions must be serializable, and t can ....

H. Garcia-Molina and G. Weiderhold. Readonly transactions in a distributed database. ACM Transactions on Database Systems, 7(2):209--234, June 1982.

.... for these two new levels, we also present implementationindependent specifications of some existing intermediate levels that have been described in the database literature or are being used in commercial databases, e.g. Snapshot Isolation, Read Consistency [Ora95] Update Serializability [GW82, HP86] and Cursor Stability [Dat90] Our specifications are of interest since they are supported in commercial systems, and previous specifications are either given in English (e.g. Snapshot Isolation) or presented in terms of locking (e.g. Cursor Stability) We define these levels based on ....

....needs to observe a consistent state of the database, PL 2 will be sufficient. In this section, we present an isolation level, PL 3U, for read only transactions that is stronger than PL 2 but weaker than serializability. This consistency guarantee has been presented earlier in the literature [GW82, HP86] we present it using our terminology and describe an optimistic scheme in Section 5.4 that provides this isolation level. Isolation level PL 3U ensures that a transaction is serializable with all the committed update transactions. The following condition is sufficient to ensure PL 3U for a ....

[Article contains additional citation context not shown here]

H. Garcia and G. Weiderhold. Read-Only Transactions in a Distributed Database. ACM Transactions Database Systems, 7(2):209--234, June 1982.

....In the introduction, we argued that performance advantages may be gained by relaxing the level of consistency provided to queries. In this section, we review four forms of consistency which all guarantee that queries see a transaction consistent database: strict consistency, strong consistency [Garc82], weak consistency [Garc82] and update consistency. In the next section, we review the MV2PL algorithm, which provides only strict consistency, and we then present the MVQL algorithm as a generalization of MV2PL that can provide any of these levels of consistency. An underlying requirement that ....

....argued that performance advantages may be gained by relaxing the level of consistency provided to queries. In this section, we review four forms of consistency which all guarantee that queries see a transaction consistent database: strict consistency, strong consistency [Garc82] weak consistency [Garc82], and update consistency. In the next section, we review the MV2PL algorithm, which provides only strict consistency, and we then present the MVQL algorithm as a generalization of MV2PL that can provide any of these levels of consistency. An underlying requirement that we impose is that the ....

[Article contains additional citation context not shown here]

Garcia-Molina, H. and G. Wiederhold, "Read-Only Transactions in a Distributed Database," ACM Transactions on Database Systems, 7(2), June 1982.

.... system by a Poisson process with parameter l global because Poisson streams have been found to be a good approximation for a large number of users submitting jobs independently [6,20,21,24] Although read only transactions (queries) are easier to process for a database system than updates [18], many models consider updates only [10,11,28,37] We model the percentage of queries in the overall workload by the parameter a q [0;1] This means that l global is split up into two separate Poisson streams with rates l l q global q global a = and l u global = 1 a q global l ....

H. Garcia-Molina, G. Wiederhold: "Read-Only Transactions in a Distributed Database", ACM TODS, Vol. 7, No. 2, June 1982, pages 209-234.

....state, thus their execution cannot lead to inconsistent database states. In our scheme, read only transactions with weaker consistency requirements are considered a special case of weak transactions that have no write operations. Two requirements for read only transactions were introduced in [8]: consistency and currency requirements. Consistency requirements specify the degree of consistency needed by a read only transaction. In this framework, a read only transaction may have: a) no consistency requirements; b) weak consistency requirements, if it requires a consistent view (that is, ....

H. Garcia-Molina and G. Wiederhold. Read-Only Transactions in a Distributed Database. ACM Transactions on Database Systems, 7(2):209--234, June 1982.

....other for query processing, are proposed. Periodically, a time step is advanced to synchronize the database state for query processing and that for transaction access. 5. 4 One version There are proposals that do not use multiple versions to support more efficient processing of readonly actions [2, 27]. These schemes intrinsically have very severe data contention between transactions and queries, if serializability is to be observed. Pu has proposed an algorithm to read the entire single version database concurrently with the normal transaction processing [2] His objective is to use a special ....

....normal transaction processing [2] His objective is to use a special locking protocol to realize an on the fly, incremental, and consistent reading of the entire database. Garcia Molina and Wielderhold have considered using the semantically read only information to process queries efficiently [27]. Read only queries are classified into different types, and their correctness criteria are defined accordingly. Their emphasis, however, is on systems where the state of an individual data item is replicated, and their goal is primarily to locally process a given read only action without making ....

H. Garcia-Molina and G. Wiederhold, "Read-only transactions in a distributed database," ACM Trans. on Database Systems, vol. 7, no. 2, pp. 209--234, June 1982.

....: set initially fg ensures if 9e 2 spre : e 2 = yieldedpre then yieldedpost Gamma yieldedpre = feg e 2 reachable(spre) suspends else returns Fig. 6. Growing and Shrinking Set, Optimistic Failure Handling 4 Related Work Our analysis was greatly influenced by Garcia Molina and Wiederhold s [4] taxonomy of queries. They use two dimensions for classification, and ignore communication failures. Consistency is the degree to which application constraints on data can be satisfied while currency is concerned with the version of the data returned by the query. In our terminology, set ....

H. Garcia-Molina and G. Wiederhold. Read-only transactions in a distributed database. ACM Transactions on Database Systems, 7(2), June 1982.

....values of a data object that are slightly different in age or in precision are oftenacceptable as read data for transactions. This observation underlies the concept of similarity among data values. Another paper that allows weaker forms of consistency in a way similar to Read Only transactions [6] is a recent algorithm by Bober and Carey [2] However, their queries still see a transactionconsistent view of the database, possibly with some older version that does not contain all the updates. In addition to the demarcation protocol [1] mentioned in Section 3.3, there are other papers ....

H. Garcia-Molina and G. Wiederhold. Read-only transactions in a distributed database. ACM Transactions on Database Systems, 7(2):209--234, June 1982.

....the standard serializability based correctness criteria[10] to exploit the concept of similarity. Three correctness criteria defined in [3] are final state, view and conflict Delta serializability. Other different correctness criteria have been proposed for different purposes and application areas[2, 8, 11]. Because of space limitation, only the definition of view Delta serializability is included in this paper. The transaction view of a transaction instance is a vector of data object values such that the i th component is the value read by the i th read event of the transaction instance[10] ....

H. Garcia-Molina and G. Wiederhold, "Read-Only Transactions in a Distributed Database," ACM Transactions on Database Systems, Vol. 7, No. 2, June 1982.

....is current, that is, reflect the most recent updates to the data. Whereas we independently discovered the above correctness criterion when we began investigating appropriate correctness criteria for broadcast environments, our correctness criterion bears some similarities to those proposed in [15] and is similar to the update consistency correctness proposed in [6] These correctness criteria, however, were proposed in the context of different system architectures and so the mechanisms proposed for implementing them did not take into account the nature of the broadcast medium ( 15] and ....

.... in [15] and is similar to the update consistency correctness proposed in [6] These correctness criteria, however, were proposed in the context of different system architectures and so the mechanisms proposed for implementing them did not take into account the nature of the broadcast medium ([15] and [6] propose locking based protocols which, as outlined before, are infeasible in broadcast environments) Moreover, unlike our work, a formal characterization of the correctness criterion was not presented. We provide not only a formal analysis of the correctness criterion but also propose ....

H. Garcia-Molina and G. Wiederhold, "Read-Only Transactions in a Distributed Database," ACM Transactions on Database Systems, Vol. 7, No. 2, June 1982, pp. 209-234.

....data version. However it does so by sacrificing availability. Several consistency schemes that enhance availability have been proposed. These schemes trade availability against consistency. One such scheme, weak consistency, is similar to strong consistency except that it ignores read dependencies [GW82]. Thus weak consistency provides instantaneous data consistency with respect to writes only. Writes update the latest copy of data, but reads can return older copies. This increases read availability of data. However, writes can only occur in a single partition. If write availability is essential, ....

Hector Garcia-Molina and G. Wiederhold. "Read-Only Transactions in a Distributed Database." ACM Transactions on Database Systems, 7(2), June 1982.

....(right) Response time distribution for weak transactions. ing. Our model offers a hybrid approach. Strict transactions may be performed only if one copy serializability is ensured in a pessimistic manner. Weak transactions may be performed locally in an optimistic manner. Read only transactions [9] do not modify the database state, thus their execution cannot lead to inconsistent database states. In the proposed schema read only transactions with weaker consistency requirements are considered a special case of weak transactions. Epsilon serializability (ESR) 22] allows temporary and ....

H. Garcia-Molina and G. Wiederhold. Read-Only Transactions in a Distributed Database. ACM Transactions on Database Systems, 7(2):209--234, June 1982.

....of this chapter, we argued that performance advantages may be gained by relaxing the level of consistency provided to queries. In this section, we review four forms of consistency which all guarantee that queries see a transaction consistent database: strict consistency, strong consistency [Garc82], weak consistency [Garc82] and update consistency. An underlying requirement that we impose is that the execution of update transactions alone must be serializable; this is guaranteed in MV2PL and MVQL by having update transactions run using dynamic 2PL. This prohibits serialization graph cycles ....

....argued that performance advantages may be gained by relaxing the level of consistency provided to queries. In this section, we review four forms of consistency which all guarantee that queries see a transaction consistent database: strict consistency, strong consistency [Garc82] weak consistency [Garc82], and update consistency. An underlying requirement that we impose is that the execution of update transactions alone must be serializable; this is guaranteed in MV2PL and MVQL by having update transactions run using dynamic 2PL. This prohibits serialization graph cycles that contain only update ....

[Article contains additional citation context not shown here]

Garcia-Molina, H., and G. Wiederhold, "Read-Only Transactions in a Distributed Database," ACM Transactions on Database Systems, 7(2), June 1982.

....approach. Read only Transactions. Read only transactions do not modify the database state, thus their execution cannot lead to inconsistent database states. In our framework read only transactions with weaker consistency requirements are considered a special case of weak transactions. In [8] two requirements for read only transactions were introduced: consistency and currency requirements. Consistency requirements specify the degree of consistency needed by a read only transaction. In this framework, a read only transaction may have: a) no consistency requirements; b) weak ....

H. Garcia-Molina and G. Wiederhold. Read-Only Transactions in a Distributed Database. ACM Transactions on Database Systems, 7(2):209--234, June 1982.

....we do not require any fixed order between two non conflicting actions. Our definition is compatible with standard serializability theory. In fact, our consistency definition can be rephrased in terms of serializability theory, by treating the warehouse view evaluation as a read only transaction [GMW82] at the sources(See Appendix C) Although completeness is a nice property since it states that the view tracks the base data exactly, we believe it may be too strong a requirement and unnecessary in most practical warehousing scenarios. In some cases, convergence may be sufficient, i.e. ....

H. Garcia-Molina and G. Wiederhold. Read-only transactions in a distributed database. ACM Transaction on Database Systems, 7(2):209--234, June 1982.

No context found.

H. Garcia-Molina and G. Wiederhold. Read-only transactions in a distributed database. ACM Transactions on Database Systems, 7(2): pp 209-234, June 1982.

First 50 documents

Online articles have much greater impact   More about CiteSeer.IST   Add search form to your site   Submit documents   Feedback  

CiteSeer.IST - Copyright Penn State and NEC