22 research outputs found
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Serializability test by pebbling
An execution of a database system is considered correct if its net effect is as if transactions were executed serially, and such an execution is called serializable. In this short paper we show that the serializability test problem can be reduced to a simple pebbling problem. Our characterization is intuitive and can handle uniformly both multi-versions and replicated data. A polynomial-time version of our method is also discussed.Key Words and Phrases: database system, concurrency control, serializability test, pebbling, NP-complete proble
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Multi-level concurrency control of a database system
A typical database system maintains target data, which contain information useful for users, and access path data, which facilitate faster accesses to target data. Further, most large database systems support concurrent processing of multiple transactions. For a static database system model, where units of concurrency control are not dynamically created or deleted, various concurrency control methods are known. Also, many methods that allow concurrent accesses to indexing structures without invalidating their integrity are known. However, a straightforward integration of these two kinds of concurrency control methods fails because of the phantom problem. In this paper, we introduce group locks in order to solve this problem and discuss their implementation. As one side benefit of introducing group locks, we show that if the lowest-level access path data as well as the target data are two-phase locked by transactions, consistency of the logical data will be preserved
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Implementation of a distributed commit/termination protocol by communicating Moore machines
In this short paper, we present an implementation method for a distributed commit/ termination protocol for a distributed database system. The protocol, which handles both commit and termination processing of distributed transactions, is represented by communicating Moore machines. Several advantages of our approach are discussed
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User interface system based on active objects
An active-object user interface system (AOUIS) is a user interface system implemented as an active object system (AOS). An AOS is a transition-based object-oriented system suitable for the design of various concurrent systems. In an AOUIS, user interface objects, which are sometimes called "widgets", are represented as active user interface objects (AUIOs). The behavior of an AUIO is defined by the transition rules, the equational assignment statements, and the event routines provided in its class definition. Furthermore, an AUIO can be constructed from its component AUIOs through structural composition as ifit were a hardware object. Thus, AUIOs are better encapsulated and provide more flexible communication protocols than ordinary user interface objects. In addition, declarative descriptions of multiple views can be provided for each AUIO.Key Words and Phrases : user interface management system, active object system, production system, structural composition, software IC, subject/view
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The structural active-object system approach to AGV system editor implementation
The Structural Active-Object System (SAOS) approach allows rapid development of various concurrent systems by structural and hierarchical object composition (SHOC). We implemented, as a SAOS program, a graphical editing and simulation program for automatic guided vehicle (AGV) systems, which carry around parts in automated factories. It is difficult to predict the performance of an AGV system by analytical calculations, and hence a software tool that allows rapid construction and modification of a simulated AGV system is very useful. We can create an AGV system simply by placing track segments, stations, and AGVs on the graphical editor canvas. Simulation is automatically started when the first AGV is placed on the AGV track. Furthermore , the track layout can be dynamically modified during simulation, and the effects of the modifications can be immediately observed by the user.Key Words and Phrases: object-oriented programming, active object system, manufacturing control, AGV, FMS, graphical editor, structural composition, hierarchical composition
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Capability-based resiliency control of distributed database systems
It is highly desirable for a distributed database system to achieve logically continuous operation even if some sites or message links fail. In this paper, we describe a scheme that can automatically reconfigure a fully-replicated distributed database system upon subsystem failures. The scheme can tolerate total failures of some sites. That is, some sites may lose their data completely including backup data.
In order to handle this problem, we divide the execution of the system into generations. Each generation starts with a consistent database state and ends with a consistent database state. Serializability of the execution of each generation is enforced by concurrency control based on conditional capabilities. Conditional capabilities are valid only if they are being endorsed by a majority of sites in the system. Thus, any majority set of sites can invalidate lost or stray capabilities of old generations, and they can newly create capabilities for a new generation. Therefore, as long as there exist a majority of working sites that belong to the same partition, the system can continue its operation. The scheme allows a system to be reconfigured even if some transactions are being processed and even if other system reconfiguration attempts are underway.Key Words and Phrases: distributed database system, resiliency control, consistent database state, replicated data, conditional capabilitie
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A deadlock prevention method for a sequence controller for manufacturing control
In a manufacturing system, machines, robots and storage areas are used as serially-reusable resources. If the usage of these resources is not properly controlled, deadlocks may occur. In this paper, we present a simple deadlock prevention method for a linear manufacturing line. Our deadlock prevention method can be implemented simply by adding some dummy resources to a Petri net-based sequence controller. Heuristic strategies for the algorithms that assist us to design such sequence controllers are also discussed.Key Words and Phrases: manufacturing line, sequence controller, Petri net, deadlock preventio
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Active object system
An active object system is a transition-based object-oriented system suitable for the design of various concurrent systems. An AOS consists of a collection of interacting objects, where the behavior of each object is determined by the transition statements provided for the class of that object. A transition statement is a condition-action pair, an equational assignment statement, or an event routine. The transition statements provided for each object can access, besides the state of that object, the states of the other objects known to it through its interlace variables. Interface variables are bound to objects when objects are instantiated so that desired connections among objects are established. The major benefit of the AOS approach is that an active system can be hierarchically composed from its active software components as if it were a hardware system.Key Words and Phrases: object-oriented concurrent programming, software IC, production system, hierarchical composition, behavioral composition
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Effect of update merging on reliable storage performance
Performance of a reliable storage subsystem for a centralized database system was studied by simulation. The reliable storage subsystem studied consists of three redundant disk units that are updated one at a time from a consistent database state to another consistent database state, Thus, even if a central processor and/or one disk unit fail simultaneously, at least one disk unit will contain a consistent database state.
Redundant multiple disk units allow simultaneous processing of multiple read operations. Our simulation result shows that when all transactions are read-only transactions, the throughput of a three-unit system is 2.5 times higher than that of a single-unit system. On the other hand, redundant disk units slows down write operations, ,since each disk unit must be updated for each virtual page updated. The major result in this paper shows that this adverse effect can be mitigated if updates of multiple transactions are merged and if an efficient disk scheduling policy (e.g., CSCAN) is employed. When the ratio of write operations to read operations is between 10 to 30%, the throughput of the three-unit system is still 89 to 144% higher than that of the single-unit system, which is not reliable. Th.is result is significant when we consider that the performance of a reliable storage subsystem based on logging cannot exceed that of a single-unit storage subsystem