BPMN: An introduction to the standard

Abstract

The Business Process Model and Notation (BPMN) is the de-facto standard for representing in a very expressive graphical way the processes occurring in virtually every kind of organization one can think of, from cuisine recipes to the Nobel Prize assignment process, incident management, e-mail voting systems, travel booking procedures, to name a few. In this work, we give an overview of BPMN and we present what are the links with other well-known machineries such as BPEL and XPDL. We give an assessment of how the OMG's BPMN standard is perceived and used by practitioners in everyday business process modeling chores.

Introduction

In the last few years a clear need for a modeling language for business processes which could be expressive and formal enough but easily understandable also by final users and not only by domain experts faced out. At the present, the state-of-the-art in the field is represented by BPMN (Business Process Model and Notation) [1], the leading standard in the frame of business processes and workflow modeling languages. It is possible to identify three different application domains for modeling languages: pure description, simulation and execution of processes. In this work we will present an introduction to BPMN assessing its position with respect to the cited application fields and comparing it with other proposals.

Use cases descriptions and documentation of complex procedures are often very difficult to understand and error prone. Since a (more or less) clear picture depicting either a workflow or a business process is in most cases self-explaining, many users aim to enrich descriptions of processes with diagrams in order to convey the intended meaning associated to the process. Moreover, examining a graphical description of a process allows users to easily discover inconsistencies and/or differences in names or acronyms, infinite loops, non-terminating conditions and so forth. Using a formal graphical notation is the de-facto standard choice to express a representation of a process which should be syntactically valid (thus assuring the consistence with the represented process) and having the same meaning as the (usually, natural language based) textual description of the process.

Domain experts are also interested in finding ways of studying properties of processes as represented through some graphical notation in order to verify the correctness of their representation, thus checking – for example – the absence of interrupting conditions, deadlocks, infinite loops. Analysts, on the other side, are more interested in collecting data – especially about resources spent during the execution of a process – so that it is possible to check whether a process could be refined and optimized.

Finally, execution capabilities of a modeling language are of great interest of business process experts and developers. Purposes for translating a diagram into some machine readable standard language can be grouped into two main categories: sharing and execution. To share a diagram (keeping it editable from different users) across multiple domains and using many different technologies and softwares is seen as big challenge nowadays. Furthermore, the need to (semi-)automatically execute a process, often in distributed environment (like in the case of web services) is becoming more and more important.

Summarizing the argumentations we will develop in the following, it can be safely affirmed that BPMN can be used for all of these purposes, sometimes in conjunction with other languages. For descriptive purposes only, BPMN itself is probably the best choice, at the present time. For simulation purposes, BPMN can be put at work in conjunction with XPDL [2], while WS-BPEL [3] can be considered the right choice to translate BPMN diagrams into directly executable code. Clearly, one has to be aware what are the features of the cited languages and, in particular, what are the differences with BPMN, which will be highlighted in the following.

WS-BPEL defines a model and a grammar for describing the behavior of a business process based on interactions between the process and its partners. WS-BPEL also introduces rather extensive mechanisms for dealing with business exceptions and process faults. Moreover, WS-BPEL introduces a mechanism to define how individual or composite activities within a unit of work are to be compensated in cases where exceptions occur or a partner requests reversal [3]. The aims of WS-BPEL, along with the choice made by the Business Process Management Initiative during the early steps of BPMN definition to adopt WS-BPEL as the preferred serialization format for BPMN diagrams (even if WS-BPEL had many differences from BPMN 1.x and it is less expressive [4], [5], [6], [7], [8]), made the choice of WS-BPEL the best suitable one as execution language for BPMN diagrams, even if the situation changed with the advent of the second major release of BPMN in January 2011. WS-BPEL had too many limitations to be considered the final choice to serialize BPMN diagrams. For instance, it is not possible to represent with a unique WS-BPEL process one BPMN collaboration diagram. The richness of BPMN had no complete correspondence in WS-BPEL. Many graphical elements, in fact, cannot be represented in WS-BPEL and almost all attributes and properties linked to them (needed to give a precise meaning to the diagram elements – and thus to the diagram itself) had no representation in WS-BPEL. On the other side, the XML Process Definition Language (XPDL) is an XML format standardized by the Workflow Management Coalition (WfMC) aimed at the interchange business process definitions between different workflow products, i.e. between different modeling tools and management suites. XPDL defines an XML schema for specifying the declarative part of workflow/business process. XPDL is designed to exchange the process definition, both the graphics and the semantics of a workflow business process. XPDL is currently one of the best file format for exchange of BPMN diagrams; it has been designed specifically to store all aspects of a BPMN diagram. [2] This distinguishes XPDL from WS-BPEL which focuses exclusively on the executable aspects of the process. WS-BPEL, in fact, does not contain elements to represent the graphical aspects of a process diagram. Thus, XPDL has been widely adopted as a common standard interchange format for BPMN diagrams. Also in this case the scenario is going to change due to the publication of BPMN 2.0. Although XPDL is an XML-based language, it has been mostly used in two application domains: to check the correctness of a BPMN diagram and to simulate process execution. Nowadays there are some tools exploiting the complete BPMN support (of both the graphical representation and the associated properties) offered by XPDL to provide verification mechanisms for BPMN processes. Among others, the web-based solutions provided by Potsdam University¹ and the BPI Community² are the best free-to-use applications supporting this kind of validation. The validation is based on the XML serialization of BPMN diagram created using XPDL, while simulation is mostly limited to a sequence of steps in which one or more tokens pass through the process elements. For XPDL compliance/support check also [9] where a list of more than 70 current implementation is published and regularly updated. As we just mentioned, in January 2011 the second major release of BPMN was published. The additions introduced encompass – among other things – a native XML serialization for BPMN, making it completely independent from other languages, such as WS-BPELand XPDL. Nowadays they are still widely adopted and used and we can foresee they will still be used. In fact, the advent of the new version of BPMN forces many software vendors to change their tools to become compliant with BPMN 2.0 and this is a time-consuming and expensive activity. When the migration process will be finalized, we expect the spotlight will be quite completely turned on BPMN 2.0.

The XPDL 2.2 (published in 2010) effort is focused on only covering a subset of the Process Modeling Conformance class of the BPMN 2.0 specification. Thus the XPDL 2.2 schema will offer a serialization and interchange transport for BPMN 2.0: Process Diagrams, and Collaboration Diagrams\(Without Conversation) (i.e. what was possible in BPMN 1.2). The XPDL 2.2 schema is to be backward compatible with the XPDL 2.0 and XPDL 2.1 schemas. This backward compatibility also extends to XPDL 2.1 schema's capability to serialize BPMN 1.2 drawings.

The XPDL 3.0 (published in 2011) effort is focused on covering the complete BPMN 2.0 specification. Thus the XPDL 3.0 schema will offer a serialization and interchange transport for BPMN 2.0: Process Diagrams, Collaboration Diagrams, Conversation Diagrams, Choreography Diagrams (and all attributes).