SYSML2_ — Flow connections are incorrectly both structure and behavior

According to the KerML specification, "Structures and behaviors do not overlap". Consistent with this, the approved resolution to KERML-43 added validations to disallow a structure from specializing a behavior and vice versa.

ConnectionDefinitions in SysML are kinds of PartDefinitions, which are (indirectly) kinds of KerML Structures. But FlowConnectionDefinitions are both ConnectionDefinitions and ActionDefinitions, and ActionDefinitions are kinds of KerML Behaviors. This means that FlowConnectionDefinitions are both Structures and Behaviors, violating the restrictions of KerML.

As a result, the specializations in the declarations of MessageConnection and FlowConnection in the Systems Library model Connections now violate the new validateStructureSpecialization and validateBehaviorSpecialization constraints added by the resolution to KERML-43.

Separate flow connection definitions and usages from other connection definitions and usages

This resolution proposes the following:

1. Revise the syntax for flow connections so that FlowConnectionDefinition is no longer a subclass of ConnectionDefinition and FlowConnectionUsage is no longer a subclass of ConnectionUsage. This allows ConnectionDefinition and ConnectionUsage to remain structural, while FlowConnectionDefinition and FlowConnectionUsage become primarily behavioral as specializations of ActionDefinition and ActionUsage, respectively (though they also still indirectly specialize Association and Connector, respectively, from KerML).
2. Separate the description of flow connections from that of other connections in the language description, concrete syntax (textual and graphical notations), abstract syntax (creating a new FlowConnections package), and semantics.
3. Move the base types for flow connections from the library model Connections to a separate library model FlowConnections.

It also resolves the following related issues:

1. SYSML2_-27 Missing graphical notation for Flows Compartment
   • The new graphical BNF subclause for flow connections includes notation for a flows compartment.
2. SYSML2_-405 Messages cannot have explicit flow connection definitions
   • MessageConnection in the new FlowConnections library model specializes Action and Link but not Transfer. The new flow connection definition MessageTransferConnection then specializes both MessageConnection and Transfer, and messageConnections specializes MessageTransferConnection instead of MessageConnection.
   • Abstract flow connections with less than two ends are only required to specialize MessageConnection, and a message can then be validly typed by abstract flow connection definitions with no ends.
   • Since messages always subset messageConnections, which is typed by MessageTransferConnection, the values of messages are still transfers, with the same semantics as previously.
3. SYSML2_-407 Language description of messages is incorrect
   • Corrected in the new language description subclause for flow connections.