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I'm modeling shipment creation sequence diagram.

User creates shipment request -> system checks container/voyage candidates (reference to generate candidates diagram).

(1)If candidates exist -> user selects or declines.

(2)If no candidates -> shipment marked as pending, freight forwarder notified.

Now 3 possible paths:

(a) User cancels the shipment.

(b) User waits, and eventually freight forwarder may create a voyage that can transport user shipment, when creating the voyage (voyage creating sequence diagram) it checks all pending shipments to assign possible candidates (also reference the generate candidates diagram), if a possible candidate found for our shipment notifies user and back to (1).

(c) Freight forwarder does nothing or creates a voyage schedule that does not align with our shipment's route, and the shipment request remains pending until timeout.

The key challenge is how to represent the possibility of (b), the voyage being created later, which is not a direct result of the initial interaction, it may or may not happen at some point in the future.

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    It is not possible to answer this question without seeing at least the draft diagram. By the way, sequence diagrams are meant to illustrate a scenario, not to document a process. For the latter, activity diagrams may be better suited. Commented May 17 at 10:01
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    @Christophe oh I needed a long time to write an answer and during it you put that comment. I hesitated a long time before to replace several comments by an answer because I am thinking like you, and then I am not sure I was right to do. If you have the opportunity to come back please do not hesitate to may be say me it is better to remove my answer ;-) Commented May 17 at 11:03

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Modeling Asynchronous Interactions in shipment Lifecycle

Just looking at the title of your question one can imagine the solution is to use an asynchronous message, but it's body indicates this is not the way to do because things are more complicated.

b isn't done by the system itself managing lately an asynchronous message associated to a pending request created in 2 , but because a human manager looks at the voyages and may be creates a new one etc

The key challenge is how to represent the possibility of (b), the voyage being created later, which is not a direct result of the initial interaction, it may or may not happen at some point in the future.

b, and not only it, is just decoupled with 1 and 2. 1 and 2 are two alternatives in the same tree of message, whose root is the shipment request done by a user, a b and c aren't in it, that's all. Of course for 1 and 2 use a combined fragment alt.

As I understand a is done lately by the user, so it is also separated with the other management, I mean without a common root message. But it is surely part of a more complex interaction, like the a user access to his shipment requests and can cancel some of them etc.

One can imagine to use a combined fragment par to separate 1+2 with a and b and c, but that just complicates the things without adding a real plus value, it is better to use separated sequence diagrams. Notice also some parts must be done in critical section, but to show that with the corresponding combined fragment is useless for me.

The separation between b and c seems a little artificial, at least you cannot place a b and c at the same level because there are two different actors initiating the managements. Very probably in the same way a pending request is done in 2, an other one is done in a and will be taken into account lately by the manager among what you name b/c.

... back to (1).

Not possible, because 1 and b are decoupled

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from what I understood and after reading UML 2.5.1 specifications on sequence diagrams, I was trying to do too much in a single diagram as Christophe said "sequence diagrams are meant to illustrate a scenario", and as @bruno concluded what I want to do is simply is not possible. instead I decoupled the problem into several sequence diagrams to module the scenario of doing X thing, and I found that state diagrams are better to module the life cycle of the shipments and containers, and the intersection of both of them
@NouiraFiras it is absolutely possible to put all in a single sequence diagram, I just wanted to say in your case it is more readable to use several to have them easy to read. Christophe is right, and in a lot of cases people use a sequence diagrams while an activity diagram is more appropriate. Do not hesitate to publish on S.O. your states diagrams if you have questions about them.

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