Skip to main content

BPMN engine

BPMN engine tries to adhere to the bpmn 2 specification and be compatible with Camunda 8. The engine can be used through the ZenBPM platform or as a standalone library.

The engine is fully instrumented via OpenTelemetry:

  • Tracing: Each process, token, and flow node has spans.
  • Metrics: Tracks counts of started, ended processes, worker executions etc.

Storage​

The engine uses a storage interface to interact with storage systems. ZenBPM platform implements a solution based on RqLite database. The codebase also provides a default implementation for in memory storage that is used by the engine's Unit tests.

Node processing in the engine​

Internally the engine uses "Tokens" as a pointers that traverse the diagram and represent execution state. Tokens are manipulated based on node logic (e.g., split for gateways, wait for messages/timers). One loop through the engine's main loop means processing the token's flow node and outgoing sequence flows.

Execution Tokens​

Tokens (ExecutionToken) track progress through flow nodes:

  • Key: Unique identifier.
  • ElementId: ID of the current BPMN element.
  • State: Running, Waiting, Completed, Failed.

Exclusive Gateway example​

The following example shows processing of an Exclusive Gateway.

Since Exclusive Gateway cannot create parallel flows the token gets reused and continues to flow through the diagram.

Parallel Gateway example​

The following example shows processing of a Parallel Gateway.

To keep track of the parallel executions the token that activated the gateway is completed and outgoing parallel flows get assigned a new tokens that each represents a parallel execution flow.

Interacting with the BPMN engine​

Workers​

Supported elements​

TODO: we should split these into separate pages with examples.

Start event​

Start event is fully supported.

End event​

End event is fully supported.

Service task​

Fully supported through external workers.

User task​

Fully supported through external workers.

Business rule task​

Future plans

Supported through internal dmn engine.

Call activity​

The subprocess for call activity is started on the same partition as the process that invoked it.

Parallel gateway​

Current implementation handles parallel flows correctly if there is only one overlapping flow in the process instance. Multiple recursive parallel flows have currently undefined behaviour.

Inclusive gateway​

Inclusive gateway is fully supported.

Exclusive gateway​

Exclusive gateway is fully supported.

Event based gateway​

Event based gateway currently supports message and timer events.

Message catch event​

Message catch event is fully supported.

Link intermediate throw event is fully supported.

Link intermediate catch event is fully supported.

Timer intermediate catch event​

Tiemr intermediate catch event is fully supported.

Future plans

Message throw event​

Error event​

Boundary event​

Activity multi instance​

Script task​

XML parser​

TODO: add information about how we are parsing xml definitions of processes