Concurrent Exception Research Articles

Formalization of an architectural model for exception handling coordination based on CA action concepts

Architectures based on Coordinated Atomic action (CA action) concepts have been used to build concurrent fault-tolerant systems. This conceptual model combines concurrent exception handling with action nesting to provide a general mechanism for both enclosing interactions among system components and coordinating forward error recovery measures. This article presents an architectural model to guide the formal specification of concurrent fault-tolerant systems. This architecture provides built-in Communicating Sequential Processes (CSPs) and predefined channels to coordinate exception handling of the user-defined components. Hence some safety properties concerning action scoping and concurrent exception handling can be proved by using the FDR (Failure Divergence Refinement) verification tool. As a result, a formal and general architecture supporting software fault tolerance is ready to be used and proved as users define components with normal and exceptional behaviors.

The guardian model and primitives for exception handling in distributed systems

This work presents an abstraction called guardian for exception handling in distributed and concurrent systems that use coordinated exception handling. This model addresses two fundamental problems with distributed exception handling in a group of asynchronous processes. The first is to perform recovery when multiple exceptions are concurrently signaled. The second is to determine the correct context in which a process should execute its exception handling actions. Several schemes have been proposed in the past to address these problems. These are based on structuring a distributed program as atomic actions based on conversations or transactions and resolving multiple concurrent exceptions into a single one. The guardian in a distributed program represents the abstraction of a global exception handler, which encapsulates rules for handling concurrent exceptions and directing each process to the semantically correct context for executing its recovery actions. Its programming primitives and the underlying distributed execution model are presented here. In contrast to the existing approaches, this model is more basic and can be used to implement or enhance the existing schemes. Using several examples we illustrate the capabilities of this model. Finally, its advantages and limitations are discussed in contrast to existing approaches.

A comparative study of exception handling mechanisms for building dependable object-oriented software

Modern object-oriented systems have to cope with an increasing number of exceptional conditions and incorporate fault tolerance into systems' activities in order to meet dependability-related requirements. An exception handling mechanism is one of the most important schemes for detecting and recovering errors, and for structuring fault-tolerant activities in a system. The mechanisms that were ill designed can make an application unreliable and difficult to understand, maintain and reuse in the presence of faults. This paper surveys various exception mechanisms implemented in different object-oriented languages, evaluates and compares different designs. A taxonomy is developed to help address 10 basic technical aspects for a given exception handling proposal, including exception representation, external exceptions in signatures, separation between internal and external exceptions, attachment of handlers, handler binding, propagation of exceptions, continuation of the control flow, clean-up actions, reliability checks, and concurrent exception handling. Practical issues and difficulties are summarized, major trends in actual languages are identified, and directions for future work are suggested.

Implementing transactions using Ada exceptions

Transactional Drago programming language is an Ada extension that provides transaction processing capabilities. Exceptions have been integrated with transactions in Transactional Drago; exceptions are used to notify transaction aborts and any unhandled exception aborts a transaction. Transactions can be multithreaded in Transactional Drago, and therefore, concurrent exceptions can be raised. In that case a single exception must be chosen to notify the transaction abort. Transactional Drago provides exception resolution within transactions to tackle such a situation. In this paper we describe the transaction model of Transactional Drago focusing on how the Ada exception model can be used to implement Transactional Drago semantics. In the paper we identify which features of Ada were useful, as well as some weaknesses we have found in the language. We point out some missing basic mechanisms for the Ada standard that can be of great help for developing applications with strong exception handling requirements like transactional frameworks and many other applications.

Concurrent exception handling and resolution in distributed object systems

We address the problem of how to handle exceptions in distributed object systems. In a distributed computing environment, exceptions may be raised simultaneously in different processing nodes and thus need to be treated in a coordinated manner. Mishandling concurrent exceptions can lead to catastrophic consequences. We take two kinds of concurrency into account: 1) Several objects are designed collectively and invoked concurrently to achieve a global goal and 2) multiple objects (or object groups) that are designed independently compete for the same system resources. We propose a new distributed algorithm for resolving concurrent exceptions and show that the algorithm works correctly even in complex nested situations, and is an improvement over previous proposals in that it requires only O(n/sub max/N/sup 2/) messages, thereby permitting quicker response to exceptions.

Extending conventional languages by distributed/concurrent exception resolution

The state of art in handling and resolving concurrent exceptions is discussed and a brief outline of all research in this area is given. Our intention is to demonstrate that exception resolution is a very useful concept which facilitates joint forward error recovery in concurrent and distributed systems. To do this, several new arguments are considered. We understand resolution as reaching an agreement among cooperating participants of an atomic action. It is provided by the underlying system to make it unified and less error prone, which is important for forward error recovery, complex by nature. We classify atomic action schemes into asynchronous and synchronous ones and discuss exception handling for schemes of both kinds. The paper also deals with introducing atomic action schemes based on exception resolution into existing concurrent and distributed languages, which usually have only local exceptions. We outline the basic approach and demonstrate its applicability by showing how exception resolution can be used in Ada 83, Ada 95 (for both concurrent and distributed systems) and Java. A discussion of ways to make this concept more object-oriented and, with the help of reflection, more flexible and useful, concludes the paper.

Practical exception handling and resolution in concurrent programs

The paper discusses how atomic actions based on forward error recovery in the form of concurrent exception handling and resolution can be programmed within standard conventional languages (Ada and Ada 95). We express the main characteristics of the general atomic action scheme in terms of these languages and discuss a set of templates and programmers' conventions which would allow the programming of atomic actions within Ada and Ada 95. We offer an approach to implementing a resolution procedure (function) and outline other approaches. The scheme is very flexible in that it gives an opportunity for programmers to use any sort of the resolution procedure. We introduce a general concept of self-checking programming, which allows the kind of failure assumption necessary for simplifying the atomic action support, and discuss how it can be applied (to Ada, in particular). It is shown how this approach helps to solve the deserter process problem. We outline the main improvements which can be made in the scheme when Ada 95 is used. We believe that this paper discusses important practical questions because it seems unlikely that an existing practical language will have concurrent exception handling of the level sufficient for supporting atomic actions based on forward error recovery.

Implementation of blocking coordinated atomic actions based on forward error recovery

The coordinated atomic action concept was proposed as a means for providing fault tolerance in complex objectoriented systems that incorporate both cooperative and competitive concurrency. This paper has two purposes: to discuss a particular implementation of this concept and to address a number of the implementation issues that are common to any experiments with this concept. Our implementation relies on a detailed set of programming conventions for the standard Ada 95 language and uses a scheme of forward error recovery incorporating concurrent exception handling and resolution. Ada 95 has a number of unique features which make it a particularly good choice for our experiments. We believe that our approach is practical and useful for many critical applications with high dependability requirements.