Better linting for ad-hoc subprocess

[jarekdanielak]

Problem you would like to solve

There is no linting for ad-hoc subprocess except for checking for Camunda version >= 8.7.

Proposed solution

Add linting for ad-hoc subprocess.

Ensure BPMN specification constraints (bpmn-lint):

• Contains only: Activities, Sequence Flows, Gateways, Intermediate Events, Data Objects and Data Associations. *
• Intermediate Event must have an outgoing Sequence Flow.

* we also allow artifacts, following our logic for a regular subprocess (comment)

Ensure Camunda constraints (bpmnlint-plugin-camunda-compat):

• Must have at least one activity.

The following Camunda constraints are enforced by BPMN standard:

• Must not have start events or end events.
• Any intermediate catch event must have an outgoing sequence flow.

Additional context

• Ad-hoc subprocess BPMN specification
• Camunda ad-hoc subprocess docs

[nikku]

To consider: What is Camunda specific, what is core, and hence shall be validated via bpmnlint.

[barmac]

This is coming from BPMN:

An Ad-Hoc Sub-Process or Process contains a number of embedded inner Activities and is intended to be executed
with a more flexible ordering compared to the typical routing of Processes. Unlike regular Processes, it does not
contain a complete, structured BPMN diagram description—i.e., from Start Event to End Event. Instead the Ad-Hoc
Sub-Process contains only Activities, Sequence Flows, Gateways, and Intermediate Events. An Ad-Hoc
Sub-Process MAY also contain Data Objects and Data Associations. The Activities within the Ad-Hoc Sub-
Process are not REQUIRED to have incoming and outgoing Sequence Flows. However, it is possible to specify
Sequence Flows between some of the contained Activities. When used, Sequence Flows will provide the same
ordering constraints as in a regular Process. To have any meaning, Intermediate Events will have outgoing
Sequence Flows and they can be triggered multiple times while the Ad-Hoc Sub-Process is active.

Too bad the spec lacks MUST in this clause.

Source: https://www.omg.org/spec/BPMN/2.0/PDF#page=461

[jarekdanielak]

I updated the top comment with BPMN and Camunda constraints which should be implemented. @barmac could you confirm it's correct?

[nikku]

@jarekdanielak I can confirm that above thing is correct/a great start.

[barmac]

Contains only: Activities, Sequence Flows, Gateways, Intermediate Events, Data Objects and Data Associations.

From the context, I think we can assume that this is the limitation for the flow objects. I'd rather not disallow artifacts in an ad-hoc subprocess (text annotations, groups).

[barmac]

Interesting point brought today by @jarekdanielak: Should we disallow Data Store Reference in an ad-hoc subprocess? While the spec seems to be clear about this (allows explicitly data object and data associations but misses data storage), I'd rather not enforce it in the tooling. The only thing relevant for the execution in Camunda context is the flow objects set, and out of these the spec disallows start and end events only. The artifacts and item aware elements should be ignored.

[nikku]

We allow data stores outside of pools, too, for modeling convenience. Hence I'd argue for 👍, handle data stores in a forgiving way, allow them inside ad-hoc sub-processes, too.

[jarekdanielak]

So the linting rule for an ad-hoc subprocess will only extend the existing rule for a regular subprocess by disallowing start and end events.

Additionally, in Camunda linting we enforce it has at least one activity.