The Concurrency Viewpoint

Historically, information systems were designed to operate with little or no concurrency, running via batch mode on large central computers. A number of factors (including distributed systems, increasing workloads, and cheap multiprocessor hardware) have combined so that today’s information systems often have little or no batch mode operations.

In contrast, control systems have always been inherently concurrent and event-driven, given their need to react to external events in order to perform control operations. It is natural, then, that as information systems become more concurrent and event-driven, they start to take on a number of characteristics traditionally associated with control systems. Similarly, it is unsurprising that, in order to deal with this concurrency, the information systems community has adopted and adapted proven techniques from the control systems community. Many of these techniques form the basis of the Concurrency viewpoint.

The Concurrency view is used to describe the system’s concurrency and state-related structure and constraints. This involves defining the parts of the system that can run at the same time and how this is controlled (e.g., defining how the system’s functional elements are packaged into operating system processes and how the processes coordinate their execution). To do this, you need to create a process model and a state model: The process model shows the planned process, thread, and interprocess communication structure; the state model describes the set of states that runtime elements can be in and the valid transitions between those states.

Definition	Describes the concurrency structure of the system, mapping functional elements to concurrency units to clearly identify the parts of the system that can execute concurrently, and shows how this is coordinated and controlled
Concerns	task structure mapping of functional elements to tasks interprocess communication state management synchronization and integrity supporting scalability startup and shutdown task failure reentrancy
Models	system-level concurrency models state models
Pitfalls	modeling the wrong concurrency modeling the concurrency wrongly excessive complexity resource contention deadlock race conditions
Stakeholders	Communicators, developers, testers, and some administrators
Applicability	All information systems with a number of concurrent threads of execution