Read-only Transactions Research Articles

Concurrency control in mobile distributed real-time database systems

This paper presents DMVOCC-2PLV (Distributed Multiversion Optimistic Concurrency Control with Two-Phase Local Validation) protocol for processing mobile distributed real-time transactions in mobile broadcast environments. In DMVOCC-2PLV protocol, transaction validation is performed at two levels: local validation and global validation. Local validation of transactions is performed in two phases: local pre-validation and local final validation. At the MHs (Mobile Hosts), all mobile transactions perform local pre-validation of transactions by using a backward validation mechanism. The local pre-validation process is carried out against committed transactions at the server in the last broadcast cycle. Such an early data conflict detection feature can save processing and communication resources. Transactions that survive in local pre-validation must be submitted to the server for local final validation. In global validation distributed update transactions have to check to ensure distributed serializability in all participants. The protocol can eliminate conflicts between mobile read-only and mobile update transactions, and resolve data conflicts flexibly using multiversion dynamic adjustment of serialization order to avoid unnecessary restarts of transactions. Mobile read-only transactions can be committed with no-blocking, and respond time of mobile read-only transactions is greatly reduced. The tolerance of mobile transactions of disconnections from the broadcast channel is increased. The results of simulation experiment show that the new protocol proposed offers better performance in terms of miss rate, restart rate, commit rate and throughput.

An energy-efficient mobile transaction processing method using random back-off in wireless broadcast environments

Broadcast is widely accepted as an efficient technique for disseminating data to a large number of mobile clients over a single or multiple channels. Due to the limited uplink bandwidth from mobile clients to server, conventional concurrency control methods cannot be directly applied. There has been many researches on concurrency control methods for wireless broadcast environments. However, they are mostly for read-only transactions or do not consider exploiting cache. They also suffer from the repetitive aborts and restarts of mobile transactions when the access patterns of mobile transactions are skewed. In this paper, we propose a new optimistic concurrency control method suitable for mobile broadcast environments. To prevent the repetitive aborts and restarts of mobile transactions, we propose a random back-off technique. To exploit the cache on mobile clients, our method keeps the read data set of mobile transactions and prefetches those data items when the mobile transactions are restarted. As other existing optimistic concurrency control methods for mobile broadcast environments does, it works for both read-only and update transactions. Read-only transactions are validated and locally committed without using any uplink bandwidth. Update transactions are validated with forward and backward validation, and committed after final validation consuming a small amount of uplink bandwidth. Our performance analysis shows that it significantly decreases uplink and downlink bandwidth usage compared to other existing methods.

A concurrency control scheme for mobile transactions in broadcast disk environments

Broadcast disk technique has been often used to disseminate frequently requested data efficiently to a large volume of mobile clients over wireless channels. In broadcast disk environments, a server often broadcasts different data items with differing frequencies to reflect the skewed data access patterns of mobile clients. Previously proposed concurrency control methods for mobile transactions in wireless broadcast environments are focused on the mobile transactions with uniform data access patterns. These protocols perform poorly in broadcast disk environments where the data access patterns of mobile transactions are skewed. In broadcast disk environments, the time length of a broadcast cycle usually becomes large to reflect the skewed data access patterns. This will often cause read-only transactions to access old data items rather than the latest data items. Furthermore, updating mobile transactions will be frequently aborted and restarted in the final validation stage due to the update conflict of the same data items with high access frequencies. This problem will increase the average response time of the update mobile transactions and waste the uplink communication bandwidth. In this paper, we extend the existing FBOCC concurrency control method to efficiently handle mobile transactions with skewed data access patterns in broadcast disk environments. Our method allows read-only transactions to access the more updated data, and reduces the average response time of updating transactions through early aborts and restarts. Our method also reduces the amount of uplink communication bandwidth for the final validation of the update transactions. We present an in-depth experimental analysis of our method by comparing with existing concurrency control protocols. Our performance analysis shows that it significantly decreases the average response time and the amount of uplink bandwidths over existing methods.

Scheduling transactions in mobile distributed real-time database systems

A DMVOCC-MVDA (distributed multiversion optimistic concurrency control with multiversion dynamic adjustment) protocol was presented to process mobile distributed real-time transaction in mobile broadcast environments. At the mobile hosts, all transactions perform local pre-validation. The local pre-validation process is carried out against the committed transactions at the server in the last broadcast cycle. Transactions that survive in local pre-validation must be submitted to the server for local final validation. The new protocol eliminates conflicts between mobile read-only and mobile update transactions, and resolves data conflicts flexibly by using multiversion dynamic adjustment of serialization order to avoid unnecessary restarts of transactions. Mobile read-only transactions can be committed with no-blocking, and respond time of mobile read-only transactions is greatly shortened. The tolerance of mobile transactions of disconnections from the broadcast channel is increased. In global validation mobile distributed transactions have to do check to ensure distributed serializability in all participants. The simulation results show that the new concurrency control protocol proposed offers better performance than other protocols in terms of miss rate, restart rate, commit rate. Under high work load (think time is 1s) the miss rate of DMVOCC-MVDA is only 14.6%, is significantly lower than that of other protocols. The restart rate of DMVOCC-MVDA is only 32.3%, showing that DMVOCC-MVDA can effectively reduce the restart rate of mobile transactions. And the commit rate of DMVOCC-MVDA is up to 61.2%, which is obviously higher than that of other protocols.

Can-Follow Concurrency Control

Can-follow concurrency control permits a transactionto read (write) an item write-locked (read-locked) by anothertransaction with almost no delays. By combining the merits of2PL and 2V2PL, this approach mitigates the lock contention notonly between update and read-only transactions, but also betweenupdate and update transactions.

Global Timestamp Serialization in Multi-Level Multi-Version DBMS

The concurrency control mechanism in the multi-level DBMS is required to promise the serializability of transactions and the multi-level security properties, avoid possible covert channels and the starving problem of high-level transactions. The multi-level multi-version timestamp ordering mechanisms satisfy these requirements but may cause transactions read old version data, and the scheduler is required to be a trusted process. This paper presents a multi-level multi-version global timestamp ordering (MLS-MVGTO) mechanism and the basic global timestamps generation steps based on the transaction’s snapshot. This paper also presents two improvements according to the pre-knowledge of the read-only transactions. In addition it can be implemented as a set of untrusted schedulers. Given the pre-knowledge about transactions’ operations, transactions are able to read more recent version.

Versioned boxes as the basis for memory transactions

In this paper, we propose the use of Versioned Boxes, which keep a history of values, as the basis for language-level memory transactions. Unlike previous work on software transactional memory, in our proposal read-only transactions never conflict with any other concurrent transaction. This may improve significantly the concurrency on applications which have longer transactions and a high read/write ratio. Furthermore, we discuss how we can reduce transaction conflicts by delaying computations and re-executing only parts of a transaction in case of a conflict. We propose two language-level abstractions to support these strategies: the per-transaction boxes and the restartable transactions. Finally, we lay out the basis for a more generic model, which better supports fine-grained restartable transactions. The goal of this new model is to generalize the previous two abstractions to reduce conflicts.

A read-only transaction anomaly under snapshot isolation

Snapshot Isolation (SI), is a multi-version concurrency control algorithm introduced in [BBGMOO95] and later implemented by Oracle. SI avoids many concurrency errors, and it never delays read-only transactions. However it does not guarantee serializability. It has been widely assumed that, under SI, read-only transactions always execute serializably provided the concurrent update transactions are serializable. The reason for this is that all SI reads return values from a single instant of time when all committed transactions have completed their writes and no writes of non-committed transactions are visible. This seems to imply that read-only transactions will not read anomalous results so long as the update transactions with which they execute do not write such results. In the current note, however, we exhibit an example contradicting these assumptions: it is possible for an SI history to be non-serializable while the sub-history containing all update transactions is serializable.

Fine-Grained Lazy Replication with Strict Freshness and Correctness Guarantees

Eager replication management is known to generate unacceptable performance as soon as the update rate or the number of replicas increases. Lazy replication protocols tackle this problem by decoupling transaction execution from the propagation of new values to replica sites while guaranteeing a correct and more efficient transaction processing and replica maintenance. However, they impose several restrictions on transaction models that are often not valid in practical database settings, e.g., they require that each transaction executes at its initiation site and/or are restricted to full replication schemes. Also, the protocols cannot guarantee that the transactions will always see the freshest available replicas. This paper presents a new lazy replication protocol called PDBREP that is free of these restrictions while ensuring one copy serializable executions. The protocol exploits the distinction between read-only and general transactions and works with arbitrary physical data organizations such as partitioning and striping as well as different replica granularities. It does not require that each read-only transaction executes entirely at its initiation site. Hence, each read-only site need not contain a fully replicated database. PDBREP furthermore generalizes the notion of freshness to finer data granules than entire databases. Beside its architectural advantages, experiments revealed that PDBREP outperforms related lazy replication techniques.

이동 컴퓨팅 환경에서 적응적 요청 메시지를 이용한 갱신 트랜잭션 스케줄링 기법

많은 수의 클라이언트를 갖는 이동 컴퓨팅 환경에서 방송 기술은 낮은 대역폭의 문제를 해결할 수 있다. 그러나 기존에 제안한 대부분의 동시성 제어 기법들은 이동 클라이어언트에서 발생하는 거래를 질의 거래로 제한하였다. 본 논문에서는 캐슁과 방송기술을 이용하고 이동 클라이언트에서 갱신 거래를 허락하는 UTSM-ARM(An Update Transaction scheduling Method Using an Adaptive Request Message)을 제안한다. UTSM-ARM은 이동 클라이언트의 캐쉬 데이터에 대한 유효성을 데이터의 동적인 갱신 패턴 비율을 기준으로 판단할 수 있다. 또한 이동 클라이언트에서 접근하는 캐쉬 데이터와 이동 트랜잭션에 대한 타임스템프는 이동 트랜잭션의 직렬가능한 수행을 보장한다. 그 결과 UTSM-ARM은 방송환경의 비대칭적인 대역폭을 효율적으로 사용할 수 있으며 이동 트랜잭션의 수행 성능을 향상시킬 수 있다. A broadcast method in mobile computing environments which have very large client populations solves the problem of low bandwidth. But most of previous proposed concurrency control protocols are restricted to read-only transactions from mobile client. In this paper, we propose the UTSM-ARM method which uses caching and broadcast method, and allows update transactions in mobile client. The UTSM-ARM decides the validation of cache data consistency with the dynamic update pattern ratio of accessed cached data. Also, the timestamps of accessed cached data and transaction in mobile client guarantee the serializable execution of mobile transactions. As a result, UTSM-ARM makes efficiently use of the asymmetric bandwidth of broadcast environment and can Increase the transaction throughput.

Using predeclaration for efficient read-only transaction processing in wireless data broadcast

Wireless data broadcast allows a large number of users to retrieve data simultaneously in mobile databases, resulting in an efficient way of using the scarce wireless bandwidth. However, the efficiency of data access methods is limited by an inherent property that data can only be accessed strictly sequentially by users. To properly cope with the inherent property, this paper presents three predeclaration-based transaction processing methods that yield a significant performance improvement in wireless data broadcast.

Efficient validation of mobile transactions in wireless environments

In broadcast environments, the limited bandwidth of the upstream communication channel from the mobile clients to the server bars the application of conventional concurrency control protocols. In this paper, we propose a new variant of the optimistic concurrency control (OCC) protocol that is suitable for broadcast environments. At the server, forward validation of a transaction is done against currently running transactions, including mobile transactions and server transactions. At the mobile clients, partial backward validation of a transaction is done against committed transactions at the beginning of every broadcast cycle. Upon completion of execution, read-only mobile transactions can be validated and committed locally and update mobile transactions are sent to the server for final validation. These update transactions have a better chance of commitment because they have gone through the partial backward validation. In addition to the nice properties of conventional OCC protocols, this protocol provides autonomy between the mobile clients and the server with minimum upstream communication, which is a desirable feature to the scalability of applications running in broadcast environments. This protocol is able to process both update transactions and read-only transactions at the mobile clients at low space and processing overheads.

이동 트랜잭션을 위한 새로운 낙관적 동시성 제어 방법

많은 수의 클라이언트들에게 데이터를 방송하는 환경에서 가장 큰 제약은 서버와 통신하는데 이용가능한 대역폭이 낮다는 점이다. 이동 컴퓨팅 환경에서 많은 응용들이 개발되고 있으나, 무선망의 낮은 대역폭으로 인하여 전통적인 동시성 제어 방법을 그대로 적용하기 어렵다. 이 논문에서는 이동 트랜잭션을 위한 새로운 낙관적 동시성 제어 방법을 제안한다. 제안하는 방법에서 판독 전용 트랜잭션은 서버와 부가적인 메시지 교환없이 지역적으로 완료가 가능하며, 이동 갱신 트랜잭션은 서버로 보내져 전역적 검증을 수행한다. 또한 충돌 정보를 이용한 낙관적 방법에서 발생하는 트랜잭션의 불필요한 철회를 줄이고, 데이터 테이블에 유지된 정보를 이용하여 직렬가능하지 않은 수행을 찾아 직렬성 위배를 해결할 수 있다. A crucial limitation in environments where data is broadcast to very large client populations is the low bandwidth available for clients to communicate with servers. Many advanced applications are developed in mobile computing environments, but conventional concurrency controls are not suitable because of the low bandwidth of wireless network. In this paper, we propose a new optimistic concurrency control protocol for mobile transactions. In this protocol, mobile read-only transactions can be completed locally at the clients without additional communication, only mobile update transactions are sent to the server for global validation. Our protocol reduces unnecessary aborts occurred in the previous study using only conflict information. In addition to, our algorithm can detect and resolve non-serializable execution using by data table maintained in a server.

이동 컴퓨팅 환경에서 읽기-전용 트랜잭션을 지원하기 위한 비동기적 캐쉬 무효화 기법

이동 클라이언트/서버 데이터베이스 시스템에서 클라이언트 캐쉬의 상태 정보가 없이 비동기적 캐쉬 무효화 기법을 사용하여, 읽기-전용 트랜잭션을 지역에서 처리한다면 비동기적 무효화 리포트는 완료를 요청한 이동 트랜잭션의 대기시간에 대해 보장을 해주지 않는 문제가 발생한다. 이 문제를 해결하기 위해, 본 논문에서는 두 가지 종류의 메시지 사용을 제안한다. 트랜잭션의 처리 지연을 줄이기 위한 비동기적 무효화 메시지와 완료를 요청한 트랜잭션의 대기 시간을 보장해 주기 위한 안내 메시지이다. 비동기 무효화 리포트는 자신을 나타내기 위한 일련번호를 가지며, 안내 메시지는 가장 최근에 브로드캐스트 된 무효화 메시지의 일련번호를 가진다. 이동 클라이언트는 이 메시지들의 일련번호를 사용하여 자신의 캐쉬에 대한 유효여부를 점검함으로 이동 트랜잭션의 대기시간에 대해 보장해준다. In stateless server, if an asynchronous cache invalidation scheme attempts to support local processing of read-only transaction in mobile client/sever database systems, a critical problem may occur ; the asynchronous invalidation reports provide no guarantees of waiting time for mobile transactions requesting commit. To solve this problem, the server in our algorithm broadcasts two kind of messages, asynchronous invalidation report to reduce transaction latency and periodic guide message to avoid the uncertainty of waiting time for the next invalidation report. The asynchronous invalidation report has its own sequence number and the periodic guide message has the sequence number of the most recently broadcast asynchronous invalidation report. A mobile client checks its cache validity by using the sequence numbers of these messages.

Two-version based concurrency control and recovery in real-time client/server databases

While there has been a significant amount of research in real-time concurrency control, little work has been done in logging and recovery for real-time databases. This paper proposes a two-version approach which considers both real-time concurrency control and recovery. We propose a network-server-based architecture and algorithms which can not only reduce the blocking time of higher-priority transactions and improve the response time of client-side read-only transactions, but also provide a diskless runtime logging mechanism and an efficient and predictable recovery procedure. The performance of the algorithms was verified by a series of simulation experiments by comparing the algorithms with the well-known Priority Ceiling Protocol (PCP), the Read/Write PCP, the New PCP, and the 2-version two-phase locking protocol, for which we have very encouraging results. The schedulability of higher-priority transactions and the response time of client-side read-only transactions were all greatly improved.

Processing read-only transactions in hybrid data delivery environments with consistency and currency guarantees

Different isolation levels are required to ensure various degrees of data consistency and currency to read-only transactions. Current definitions of isolation levels such as Conflict Serializability, Update Serializability or External Consistency/Update Consistency are not appropriate for processing read-only transactions since they lack any currency guarantees. To resolve this problem, we propose four new isolation levels which incorporate data consistency and currency guarantees. Further, we present efficient implementations of the proposed isolation levels. Our concurrency control protocols are envisaged to be used in a hybrid mobile data delivery environment in which broadcast push technology is utilized to disseminate database objects to a large number of mobile clients and conventional point-to-point technology is applied to satisfy on-demand requests. The paper also presents the results of a simulation study conducted to evaluate the performance of our protocols. According to the simulation results the costs imposed by the MVCC-SFBS protocol, which ensures serializability to read-only transactions are moderate relative to those imposed by the MVCC-SFBUS and MVCC-SFBVC protocols, which provide weaker consistency guarantees. A comparison study reveals that the MVCC-SFBVC scheme outperforms all other investigated concurrency control schemes suitable for mobile database systems.

Using reordering technique for mobile transaction management in broadcast environments

As computer hardware and wireless network technologies are developed to a high degree, there are many research efforts which intend to utilize data broadcasting to a large population of mobile clients through wireless channels. In recent years, different models of data delivery have been explored, particularly the periodic push model where the server repetitively disseminates information without any explicit request. In this paper, we devise new transaction processing algorithms, O-Post for update transactions and O-Pre for read-only transactions, in broadcast environments. Basically, each client executes its transactions in an optimistic manner and does some consistency checks based on periodic invalidation reports. When any kind of conflicts is found, the conflict order is determined according to the notion of reordering and the remaining operations are executed to hold the decision. We also develop a cache algorithm to cope with frequent restarts due to the optimistic execution. Experimental results are given to show the benefits of the proposed algorithms.

On transaction processing with partial validation and timestamp ordering in mobile broadcast environments

Conventional concurrency control protocols are inapplicable in mobile broadcast environments due to a number of constraints of wireless communications. Previous studies are focused on efficient processing of read-only transactions at the mobile clients, neglecting update transactions. In this paper, we design a new protocol for processing both read-only and update mobile transactions. The protocol can detect data conflicts at an early stage at the mobile clients and resolve data conflicts flexibly using dynamic adjustment of timestamp ordering. Early data conflict detection saves processing and communication resources, while dynamic adjustment of timestamp ordering allows more schedules of transaction executions such that unnecessary transaction aborts can be avoided. We performed extensive simulation studies to evaluate the effectiveness of these two features for the performance of the new protocol. The analysis of simulation results showed that both features are effective and contribute differently to the satisfactory performance of the protocol.

Relaxing consistency requirement for read-only transactions

In this paper, we investigate the approach to relaxing consistency requirement of serializability to improve the performance of real-time database systems. We make use of the property that a read-only transaction (ROT) only reads, but does not update any data item. Since there is a significant proportion of ROTs in several real-time systems, it is important to investigate how to process ROTs effectively. We define a weaker form of consistency, view consistency, which allows ROTs to perceive different serialization orders of update transactions. Although view consistency permits non-serializablility, ROTs are still ensured to see consistent data. We devise a separate algorithm to process ROTs from update transactions for view consistency. For comparison, we devise another separate algorithm to process ROTs for strong consistency that is equivalent to serializability. Our simulation results show that the two separate algorithms outperform the locking and optimistic protocols significantly. Using separate algorithms reduces the interference between ROTs and update transactions. This reduced interference alleviates the impact of concurrency control on real-time priority-driven scheduling and improves the timeliness of the system. To our great surprise, the two separate algorithms perform almost the same. In our investigation, we found that most data conflicts between an ROT and update transactions are independent data conflicts. These independent data conflicts can be easily resolved by using a separate algorithm even under serializability. Thus, this observation indicates that using a separate algorithm to process ROTs from update transactions is more effective than relaxing consistency requirement.

CONCURRENCY CONTROL FOR MOBILE SYSTEMS WITH DATA BROADCAST

Although data broadcast has been shown to be an efficient method for disseminating data items in a mobile computing system with large number of clients, the issue on how to ensure the data consistency observed by mobile transactions, which are generated by mobile clients, has been largely ignored by researchers in the area. While data items are being broadcast, update transactions may install new values for the data items. If the executions of update and broadcast of data items are interleaved without any control, the mobile transactions may observe inconsistent data values. In this paper, we propose a serialization checking method (SCM) for concurrency control between read-only mobile transactions and update transactions. SCM is based on the framework of an earlier algorithm, Update First with Ordering (UFO), but improves on that algorithm by reducing re-broadcast overhead when the probability of data conflict between updates and data broadcast is high. Simulation experiments have been performed to investigate the performance characteristics of the proposed method.