BPMN21 — Number of Participant bands and Messages in a Choreography Task

i) Chapter/Section 7.2.2. Page 32 . Table 7.2. Row “Choreography Task”. It says:
“A Choreography Task is an atomic Activity in a Choreography (see page 333). It represents a set of one (1) or more Message exchanges. Each Choreography Task involves two (2) Participants. The name of the Choreography Task and each of the Participants are all displayed in the different bands that make up the shape’s graphical notation. There are two (2) or more Participant Bands and one Task Name Band.”
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ii) Chapter/Section 11.4. Page 332. Figure 11.6.
Model association from “ChoreographyTask” to “MessageFlow” (role name “messageFlowRefs”) has multiplicity [1..2]
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iii) Chapter/Section 11.4. Page 332. Table 11.1. Row “participantRefs”. It says:
“participantRefs: Participant [2..*]”
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iv) Chapter/Section 11.4.1. Page 333. It says:
“A Choreography Task is an atomic Activity in a Choreography Process. It represents an Interaction, which is one (1) or two (2) Message exchanges between two (2) Participants. Using a Collaboration diagram to view these elements (see page 109 for more information on Collaboration), we would see the two (2) Pools representing the two (2) Participants of the Interaction …”
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v) Chapter/Section 11.4.1. Page 333. It says:
“There are two (2) or more Participant Bands and one Task Name Band (see Figure 11.8).”
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vi) Chapter/Section 11.4.1. Page 335. It says:
“The three (3) bands in the Choreography Task shape provide the distinction between this type of Task and an Orchestration Task (in a traditional BPMN diagram).”
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vii) Chapter/Section 11.4.1. Page 338. Table 11.2. Row “messageFlowRef”. It says:
“messageFlowRef: Message Flow [1..*]”
“Although not graphical represented, Choreography Task contain one (1) or more Message Flows that represent the interaction(s) between the Participants referenced by the Choreography Task.”
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COMMENTS:

There are inconsistencies concerning the number of Messages, Participants and Participant Bands in a Choreography Task.

In
Messages: 1 or more.
Participants: 2
Participant bands: 2 or more

In (ii)
Messages: 1 or 2.

In (iii)
Participants: 2 or more.
(Because “ChoreographyTask” inherits “participantRefs” from “ChoreographyActivity”.)

In (iv)
Messages: 1 or 2.
Participants: 2

In (v)
Participant bands: 2 or more

In (vi)
Participant bands: 2.
(There are 3 bands, one of them is de Task Name band.)

In (vii)
Messages: 1 or more.

It seems that the correct numbers should be:
Messages: 1 or 2
Participants: 2
Task name band: 1
Participants bands: 2
Total bands: 3 (= 1 + 2)

SUGGESTIONS:

In
“one (1) or more Message exchanges” should be replaced by “one (1) or two (2) Message exchanges”
“two (2) or more Participant Bands” should be replaced by “two (2) Participant Bands”

In (ii) nothing

In (iii)
A new model association should be drawn in Figure 11.6 from “ChoreographyTask” to “Participant”, which actually would not be a new model association but a redefinition of “participantsRefs” form “ChoreographyActivity” to “Participant”.

It should be annotated: [2..2] +participantRefs

(See OMG Unified Modeling LanguageTM (OMG UML), Infrastructure. formal/2009-02-04. Pages 113, 150)

In (iv) nothing

In (v)
“two (2) or more Participant Bands” should be replaced by “two (2) Participant Bands”

In (vi) nothing

In (vii)
“messageFlowRef: Message Flow [1..*]” should be replaced by “messageFlowRefs: Message Flow [1..2]”

“one (1) or more Message Flows” should be replaced by “one (1) or two (2) Message Flows”

A new row should be added to Table 11.2
“participantRefs: Participant [2..2]”
“A Choreography Task has two (2) Participants (see page 115 for more information on Participants). This is a redefinition of participantRefs of ChoreographyActivity (see Table 11.1).”