An activity in Unified Modeling Language

Unified Modeling Language

Unified Modeling Language is a standardized general-purpose modeling language in the field of object-oriented software engineering. The standard is managed, and was created, by the Object Management Group...

 (UML) is a major task that must take place in order to fulfill an operation contract. Activities can be represented in activity diagram

Activity diagram

Activity diagrams are graphical representations of workflows of stepwise activities and actions with support for choice, iteration and concurrency. In the Unified Modeling Language, activity diagrams can be used to describe the business and operational step-by-step workflows of components in a system...

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An activity can represent:

• the invocation of an operation,
• a step in a business process
  Business process
  A business process or business method is a collection of related, structured activities or tasks that produce a specific service or product for a particular customer or customers...
  
  , or
• an entire business process.

Activities can be decomposed into subactivities, until at the bottom where we find atomic actions.

The underlying conception of an activity has changed between UML 1.5 and UML 2.0. In UML 2.0 an activity is no longer based on the state-chart rather it is based on a Petri net

Petri net

A Petri net is one of several mathematical modeling languages for the description of distributed systems. A Petri net is a directed bipartite graph, in which the nodes represent transitions and places...

like coordination mechanism. There the activity represents user-defined behavior coordinating actions. Action in turn are pre-defined (UML offers for this a series of actions).