Victor Fay Wolfe

Real-time CORBA

This paper describes the requirements for real-time extensions to the CORBA standard, which are being developed by the object management groups special Interest Group on Real-Time CORBA. The paper also surveys efforts that are developing Real-Time CORBA systems. It provides a more detailed description of the dynamic real-time CORBA system being developed at the US Navys NRaD facilities and at the University of Rhode Island.

Object-based semantic real-time concurrency control

This paper presents a technique that is capable of supporting two major requirements for concurrency control in real-time databases; data temporal consistency, and data logical consistency, as well as tradeoffs between these requirements. Our technique is based upon a real-time object-oriented database model in which each object has its own unique compatibility function that expresses the conditional compatibility of any two potential concurrent operations on the object. The conditions use the semantics of the object, such as allowable imprecision, along with current system state, such as time and the active operations on the object. Our concurrency control technique enforces that allowable concurrency expressed by the compatibility function by using semantic locking controlled by each individual object. The real-time object-oriented database model and process of evaluating the compatibility function to grant semantic locks are described.<<ETX>>

RTC: language support for real-time concurrency

This paper presents a model and language constructs for expressing timing and concurrency requirements in distributed real-time programs. Our approach combines an abstract data type paradigm for the specification of shared resources and a distributed transaction-based paradigm for the specification of application processes. Resources provide abstract views of shared system entities, such as devices and data structures. Each resource has a state and defines a set ofactions that can be invoked by processes to examine or change its state. A resource also specifies scheduling constraints on the execution of its actions to ensure its consistency. Processes access resources by invoking actions and by expressing precedence, execution and timing constraints on action invocations. The implementation of our language constructs and the use of this system to control the simulation of a distributed robotics application is also described.

RTSORAC: A Real-Time Object-Oriented Database Model

A real-time database is a database in which both the data and the operations upon the data may have timing constraints. We have integrated real-time, object-oriented, semantic and active database approaches to develop a formal model called RTSORAC for real-time databases. This paper describes the components of the RTSORAC model including objects, relationships, constraints, updates, and transactions.

Expressing and Enforcing Timing Constraints in a DynamicReal-Time CORBA System

Distributed real-time applications have presented the need to extend the Object Management Groups Common Object Request Broker Architecture (CORBA) standard to support real-time. This paper describes a Dynamic Real-Time CORBA system, which supports the expression and enforcement of end-to-end timing constraints as an extension to a commercial CORBA system. The paper also describes performance tests that demonstrate the systems ability to enforce expressed timing constraints.

Timed atomic commitment

Timed atomic commitment is defined, protocols to implement it in a realistic operating environment are devised, and its usefulness is shown through an example. In a large class of hard-real-time control applications, components execute concurrently on distributed nodes and must coordinate, under timing constraints, to perform the control task. As such, they perform a type of atomic commitment. Traditional atomic commitment differs, however, because there are no timing constraints; agreement is eventual. The authors define timed atomic commitment (TAC), which requires the processes to be functionally consistent, but allows the outcome to include an exceptional state, indicating that timing constraints have been violated. Centralized and decentralized protocols to implement TAC are presented. Programming constructs for TAC are introduced, and their use is illustrated in a coordinating robots example. >

Scheduling and Priority Mapping for Static Real-Time Middleware

This paperpresents a middleware real-time scheduling technique for static,distributed, real-time applications. The technique uses globaldeadline monotonic priority assignment to clients and the DistributedPriority Ceiling protocol to provide concurrency control andpriorities for server execution. The paper presents a new algorithmfor mapping the potentially large number of unique global prioritiesrequired by this scheduling technique to the restricted set ofpriorities provided by commercial real-time operating systems.This algorithm is called Lowest Overlap First Priority Mapping;we prove that it is optimal among direct priority mapping algorithms.This paper also presents the implementation of these real-timemiddleware scheduling techniques in a Scheduling Service thatmeets the interface proposed for such a service in the Real-TimeCORBA 1.0 standard. Our prototype Scheduling Service is integratedwith the commercial PERTS tool that provides schedulability analysisand automated generation of global and local priorities for clientsand servers.

The design of real-time extensions to the Open Object Oriented Database system

The paper describes real time extensions to the Open Object Oriented Database system using the RTSORAC data model. This model combines an object oriented data model, real time requirements, flexible transactions, semantic relationships among objects, and active database features. Several extensions to the Open Object Oriented Database system, including development of interfaces for real time objects and real time transactions, use of a real time operating system, incorporation of real time object management, and incorporation of real time transaction management, are also described.

Object-based semantic real-time concurrency control with bounded imprecision

The paper describes a concurrency control technique for real-time object-oriented databases that supports logical consistency and temporal consistency, as well as bounded imprecision that results from their trade-offs. The concurrency control technique uses a semantic locking mechanism within each object and user-defined conditional compatibility over the methods of the object. The semantics can specify when to sacrifice precise logical consistency to meet temporal consistency requirements. It can also specify accumulation and bounding of any resulting logical imprecision. The authors show that this technique, under certain general restrictions, can preserve global correctness and bound imprecision by proving it can guarantee a form of epsilon serializability specialized for object-oriented databases.

A protocol for timed atomic commitment

A model and correctness criteria for timed atomic commitment (TAC) are presented which require the processes to be functionally consistent, but allow the outcome to include an exceptional state, indicating that timing constraints have been violated. Correct TAC behavior is defined by presenting an abstract description of the processes involved in the commitment and minimal correctness criteria for their behavior. The correctness criteria capture the intuitive notion that an exception outcome should only occur in the presence of faults, and an aborted outcome should only occur if faults occur or some process votes no. A centralized two-phase commit protocol was modified to meet the correctness criteria by introducing deadlines on the various stages the participants go through (voting and performing), and on the decision phase for the coordinator. The deadlines are derived using several system parameters: maximum message delay, clock drift, and execution time. The protocol is then shown to be correct.<<ETX>>