A World Of Concurrency

As hardware capabilities advance, software has become increasingly richer in features and  thus harder to develop and understand by developers. Yet, the complexity associated with writing software must be hidden from the end user. This has been an ongoing trend between software complexity and user-friendliness. To satisfy both ends of the equation, software systems must be modeled to give an exact representation of the real world problems that they solve. Hence, concurrency becomes an issue since many real world problems are inherently concurrent. The world around us is highly concurrent, and so should software systems be if they are to solve any one of them. Furthermore, with the current trend towards distributed computing (i.e. the Internet),  more software systems are being developed with support for concurrency to accommodate the fact that computers nowadays control a wide range of devices and machines.

A concurrent system typically has many parts (processes) running at the same time. The opposite of concurrency is where the execution of a program flows in a pre-determined path from start till finish, better known as sequentiality. The degree of concurrency in a system is determined based on a granularity factor that constitutes a scale of concurrency. The lower end of the scale is represented by fine grained concurrency, while its upper end represents coarse grained concurrency. A fine grained concurrent program may be one performing mathematical calculations and executing several of its operations in parallel. A concurrent program with a coarser granularity, on the other hand, would be a program which has two or more processes running in parallel, each executing several distinct operations and each of which has its own life thread throughout the execution time of the program. At the upper end of the scale, one would find systems with several networks of computers run and communicate in parallel. In general, concurrency as a technical term includes the terms parallel execution, parallel programming, parallel processing, distributed computing, etc.

One may ask, what are the merits and demerits of concurrency? And why it is needed? The answer is, partially, a simple one; mainly due to the fact that developing concurrent solutions is a harder task than reasoning about it. Concurrency provides a higher level of problem understanding, more flexibility and supposedly higher performance than that found in sequential software systems.

Moreover, some software systems can’t be represented by sequential ones. For instance, an air conditioning system controls the temperature of a building by monitoring a series of thermostats throughout the building during its normal operation. Another example would be an air traffic controller who must deal with hundreds of airplanes simultaneously. In such environments, concurrency is not a convenience or a better tool for understanding. Rather, it is a must for a correct solution. A downfall of concurrency is that it adds more complexity during several phases of software development; and in many cases, concurrent software requires a capable hardware platform in order to reach viable levels of performance.

The requirements phase includes the analysis of the software problem and drafting it in a either a formal or a pictorial representation. The requirements phase usually concludes with a complete specification of the desired external and the initial internal behaviors of the system. In most cases, Object Oriented Analysis (OOA) is the method of choice used for understanding and modeling requirements; also it is well situated for modeling  concurrency. The object model, which is the first part of OOA, gives a highly concurrent representation of the software environment. Each class in the object model represents a real life entity that has its own life thread.

To minimize the complexity associated with building concurrent software systems, analysts and designers have resorted to the use of formal methods to eliminate ambiguities and reach a precise description of the software.  Formal methods are specification and design methods that are mathematically based and thus maintain a much needed level of precision and rigor for concurrent systems. Among the many formal methods available, three are highly distinguishable: Calculus of Communicating Systems (CCS), Communicating Sequential Processes (CSP), and Language of Temporal Orderings (LOTOS), all of which are formal specification methods that are concerned with modeling process communication and ordering of events. LOTOS, however, which is based on the roots of CCS and CSP, provides an extended mechanism that allows for abstract data types. The language has become an ISO standard in 1989 and is widely used in the industrial and research communities.

Regardless of the difficulties that concurrency may present, it has received wide attention in recent years. Concurrent software systems have proven to be essential for a wide range of applications such as real-time systems, embedded software systems, safety critical systems, and distributed Systems. Mapping the parallel behavior found in the real world onto software models increases problem-space understandability and paves the road for reasonable future maintenance effort.