Research issues in real-time database systems

Cataloged from PDF version of article.Today's real-time systems are characterized by managing large volumes of data. Efficient database management algorithms for accessing and manipulating data are required to satisfy timing constraints of supported applications. Real-time database systems involve a new research area investigating possible ways of applying database systems technology to real-time systems. Management of real-time information through a database system requires the integration of concepts from both real-time systems and database systems. Some new criteria need to be developed to involve timing constraints of real-time applications in many database systems design issues, such as transaction/query processing, data buffering, CPU, and IO scheduling. In this paper, a basic understanding of the issues in real-time database systems is provided and the research efforts in this area are introduced. Different approaches to various problems of real-time database systems are briefly described, and possible future research directions are discussed

Implementing PRISMA/DB in an OOPL

PRISMA/DB is implemented in a parallel object-oriented language to gain insight in the usage of parallelism. This environment allows us to experiment with parallelism by simply changing the allocation of objects to the processors of the PRISMA machine. These objects are obtained by a strictly modular design of PRISMA/DB. Communication between the objects is required to cooperatively handle the various tasks, but it limits the potential for parallelism. From this approach, we hope to gain a better understanding of parallelism, which can be used to enhance the performance of PRISMA/DB.\ud The work reported in this document was conducted as part of the PRISMA project, a joint effort with Philips Research Eindhoven, partially supported by the Dutch "Stimuleringsprojectteam Informaticaonderzoek (SPIN)

Concurrence control for transactions with priorities

Priority inversion occurs when a process is delayed by the actions of another process with less priority. With atomic transactions, the concurrency control mechanism can cause delays, and without taking priorities into account can be a source of priority inversion. Three traditional concurrency control algorithms are extended so that they are free from unbounded priority inversion

Isolation in XML Bases

The eXtensible Markup Language (XML) is well accepted in many different application areas. As a consequence, there is an increasing need for persistently storing XML documents. As soon as many users and applications work concurrently on the same collection of XML documents - i.e. an XML base - isolating accesses and modifications of different transactions becomes an important issue. We discuss six different core protocols for synchronizing access to and modifications of XML document collections. These core protocols synchronize structure traversals and modifications. They are meant to be integrated into a native XML base management System (XBMS). Four of the six core protocols are based on two phase locking, one uses time stamps, and the last one uses a novel dynamic commit-ordering approach. The latter two protocols achieve a higher degree of concurrency by a novel implicit representation of multiple versions. We also discuss extensions of these core protocols to full-fledged protocols. Further, we show how the two phase locking based protocols can achieve a higher degree of concurrency by exploiting the semantics expressed in Document Type Definitions (DTDs)

Analysis of Transaction Management Performance

There is currently much interest in incorporating transactions into both operating systems and general purpose programming languages. This paper provides a detailed examination of the design and performance of the“¢ transaction manager of the Camelot system. Camelot is a transaction facility that provides a rich model of transactions intended to support a wide variety of general-purpose applications. The transaction manager's principal function is to execute the protocols that ensure atomicity. The conclusions of this study are: a simple optimization to two-phase commit reduces logging activity of distributed transactions; non-blocking commit is practical for some applications; multithreaded design improves throughput provided that log batching is used; multi-casting reduces the variance of distributed commit protocols in a LAN environment; and the performance of transaction mechanisms such as Camelot depend heavily upon kernel performance

Rethinking serializable multiversion concurrency control

Multi-versioned database systems have the potential to significantly increase the amount of concurrency in transaction processing because they can avoid read-write conflicts. Unfortunately, the increase in concurrency usually comes at the cost of transaction serializability. If a database user requests full serializability, modern multi-versioned systems significantly constrain read-write concurrency among conflicting transactions and employ expensive synchronization patterns in their design. In main-memory multi-core settings, these additional constraints are so burdensome that multi-versioned systems are often significantly outperformed by single-version systems. We propose Bohm, a new concurrency control protocol for main-memory multi-versioned database systems. Bohm guarantees serializable execution while ensuring that reads never block writes. In addition, Bohm does not require reads to perform any book-keeping whatsoever, thereby avoiding the overhead of tracking reads via contended writes to shared memory. This leads to excellent scalability and performance in multi-core settings. Bohm has all the above characteristics without performing validation based concurrency control. Instead, it is pessimistic, and is therefore not prone to excessive aborts in the presence of contention. An experimental evaluation shows that Bohm performs well in both high contention and low contention settings, and is able to dramatically outperform state-of-the-art multi-versioned systems despite maintaining the full set of serializability guarantees

Mechanical verification of concurrency control and recovery protocols

The thesis concerns the formal specification and mechanized verification of concurrency control and recovery protocols for distributed databases. Such protocols are needed for many modern application such as banking and are often used in safety-critical applications. Therefore it is very important to guarantee their correctness. One method to increase the confidence in the correctness of a protocol is its formal verification. In this thesis a number of important concurrency control and recovery protocolshave been specified in the language of the verification system PVS. The interactive theorem prover of PVS has been used to verify their correctness. In the first part of the thesis, the notions of conflict and view serializability have been formalized. A method to verify conflict serializability has been formulated in PVS and proved to be sound and complete with the proof checker of PVS. The method has been used to verify a few basic protocols. Next we present a systematic way to extend these protocols with new actions and control information. We show that if such an extension satisfies a few simple correctness conditions, the new protocol is serializable by construction. In the existing literature, the protocols for concurrency control, single-site recovery and distributed recovery are often studied in isolation, making strong assumptions about each other. The problem of combining them in a formal way is largely ignored. To study the formal verification of combined protocols, we specify in the second part of the thesis a transaction processing system, integrating strict two-phase locking, undo/redo recovery and two-phase commit. In our method, the locking and undo/redo mechanism at distributed sites is defined by state machines, whereas the interaction between sites according to the two-phase commit protocol is specified by assertions. We proved with PVS that our system satisfies atomicity, durability and serializability properties. The final part of the thesis presents the formal verification of atomic commitment protocols for distributed recovery. In particular, we consider the non-blocking protocol of Babaoglu and Toueg, combined with our own termination protocol for recovered participants. A new method to specify such protocols has been developed. In this method, timed state machines are used to specify the processes, whereas the communication mechanism between processes is defined using assertions. All safety and liveness properties, including a new improved termination property, have been proved with the interactive proof checker of PVS.We also show that the original termination protocol of Babaoglu and Toueg has an error