Key differences between BPMN diagrams and Workflow Engine process schemes

Element specialization

• In BPMN, there are a lot of elements that often have narrow specializations and are categorized into Tasks, Gateways, and Events, although other distinct elements exist as well. The Sequence flow is particularly significant and stands out.
• In Workflow Engine, there are only two types of elements in the scheme — Activities and Transitions. By combining the configurations of these two elements, you can create Workflow Engine schemes that function similarly to the corresponding BPMN elements.

Activity is not a Task

At first glance, an Activity appears as a rectangle, similar to a Task, and it may seem like these elements are alike. However, they're actually quite different. An Activity can represent a Service Task, a Gateway, or even an Event. The main function of an Activity is to execute your code, but the overall behavior of the scheme (i.e., the direction and manner in which the process flows next) depends on the settings of the outgoing Transitions from the Activity.

For example, if an Activity has Transitions triggered by Commands, it acts similarly to an Event-based Gateway; however, unlike a Gateway, the Activity can execute some code. If the Transitions from an Activity are configured to start parallel processes, then that Activity will behave like a Parallel Gateway. And so on.

Transition is not a Sequence Flow

At first glance, Transition and Sequence Flow may seem like similar elements, as both represent arrows between elements on a diagram. However, they're actually quite different.

In Workflow Engine, a Transition defines how and where the process execution will transition. For example, a process can move to the next state based on a Timer or a Command. What is set by events in BPMN is managed through transitions in Workflow Engine. Additionally, Transitions can be conditional, and access to them can be restricted to specific users or roles. Transitions also determine parallelism, which in BPMN is either set through Gateways or implicitly defined.

In BPMN, however, a Sequence Flow only defines the direction of the transition. This element has almost no settings, except for flows coming out of exclusive gateways.

Parallelism difference

Parallelism in BPMN and Workflow Engine operates on different, yet non-contradictory principles.

• In BPMN, parallel execution is defined either explicitly, using a Parallel Gateway, or implicitly, for example, when multiple Sequence Flows emerge from a single Service Task or Event. In this case, it is considered that the process remains singular, but several tokens are introduced. This can lead to a situation where, if you don't introduce a merging element or remove these tokens, multiple tokens will continue during sequential process execution.
• In Workflow Engine, you can assume that there is always only one token within a single process, and parallelism is achieved by creating dependent processes (called subprocesses in Workflow Engine). These subprocesses depend on the process that spawned them, and you can retrieve a list of Activities where the parent process and all its child subprocesses are located, as well as manage the parent process and its subprocesses collectively. The creation and merging of processes are handled by transition settings. However, if this approach is unfamiliar, you can use the Parallel Gateway, which replicates the behavior of its BPMN counterpart.

Subprocesses in BPMN and Workflow Engine are completely different

• In BPMN, a Subprocess is merely a nested part of the main diagram that can be collapsed. It can't be reused and is primarily for aesthetic purposes. Workflow Engine has a similar feature called Inline Activity. This is a special activity that embeds another scheme into the current one. The embedded scheme can be used in multiple other schemes, allowing for reusability.
• In Workflow Engine, a subprocess is a dependent process, as mentioned earlier.

Workflow Engine provides real access control to Commands

In Workflow Engine, Actors are part of the process, and you can set restrictions on command execution (similar to a Message Catch Event) by specific users directly within the scheme. BPMN doesn't offer such functionality explicitly.

Execution ready approach

A valid Workflow Engine scheme is always ready for execution and can be run on the Workflow Engine. In contrast, a valid BPMN diagram might just be an illustration, and no engine will necessarily be able to execute it.

Ability to set the Process to any state

In Workflow Engine, a running process can always be forcibly set to any of its states. BPMN doesn't offer such a capability.

Armed with this knowledge, let's move on to exploring which BPMN elements are supported in Workflow Engine and how they’re implemented.