Using SysML ItemFlow on an Association to show compatibility with a physical flow

Apart from showing typical flows (packets, messages, requests) on "communication" Associations between Blocks in BDDs (which Associations may then be used to Type Connectors in IBDs), one use for a SysML ItemFlow on an Association is to indicate compatibility with FlowProperty items on the associated blocks.

This example is for physical flows, but the same principle works for representing exchange of requests, messages, protocol-like signals etc. if flow properties are used for them.

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The example happens to use ValueTypes as the conveyed Classifiers, we could also in SysMLv1 have used a Block or a Signal.

The ItemFlow on the Association uses Water, which is compatible with type Liquid of each the FlowProperty defined on the Associated blocks.

However, is it is usually better to use an ItemFlow on a Connector in a context IBD for this purpose of restricting the type of flow through to a more specific sub-type, even without an 'itemProperty', in which case one might have the more general Liquid ItemFlow on an Association in a general BDD and a more specific Water ItemFlow on a Connector in a particular context IBD.

Note that an an 'itemProperty' is optional and may only be used for an ItemFlow on a Connector in an IBD, not on an Association in a BDD:

SysML-1.6: 'ItemFlow::itemProperty : Property [0..1] An optional property that relates the flowing item to the instances of the connectors enclosing block. This property is applicable only for item flows realized by connectors. The itemProperty attribute has no ...'

SysML-1.6: 'The itemProperty attribute has no values if the item flow is realized by an Association.'

The main power of ItemFlow comes when one indeed uses an itemProperty as shown next

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ItemFlow without and with itemProperty - and compatible ItemFlows

Notes

[MODELLING, TIP]{SUGGESTED} Webel: SysMLv1: TIP: Consider using high level "communication" Associations between Blocks in BDDs with ItemFlows and corresponding context IBDs WITHOUT any Ports, with Connectors typed by the Associations (as well, in parallel with your port-based IBDs).

Snippets (quotes/extracts)

[SysML-1.6] An ItemFlow describes the flow of items across a connector or an association. It may constrain the item exchange between blocks, block usages, or ports as specified by their flow properties.

[SysML-1.6] One can label an ItemFlow with the classifiers of the items that may be conveyed. For example: a label Water would imply that instances of Water might be transmitted over this ItemFlow.

[SysML-1.6] The itemProperty attribute has no values if the item flow is realized by an Association.