How to model parallel threads in a UML sequence diagram

Question

How can I model two parallel threads that perform operations on an object in a sequence diagram?

Answer 1

In a sequence diagram, a lifeline represents an individual participant in the interaction. So your object that is shared between the threads should appear once and only once in the diagram.

You would also represent with a lifeline each threaded object that interact with your shared object. It could be thread instantiations directly, or it could be several objects that are created in the context of the two threads.

But this is not sufficient. In principle, the sequence of the interactions with your object is indicated by the vertical order of the messages. So how to show that interactions may happen in parallel ?

For representing parallelism, you would use a combined fragment introduced by the operator par. Graphically (see link), the combined fragment is represented as a boxed region in your sequence diagram, parallel sequences being separated by horizontal dashed lines (each of the slices would then correspond to one thread of execution.

P.S: this example shows 2 threads, but you can add more threads by adding more horizontal dashed lines.