Sequence Diagram, its Significance and limitation.

A Sequence Diagram by definition depicts, as parallel vertical lines, different processes or objects that live simultaneously, and, as horizontal arrows, the messages exchanged between them, in the order in which they occur. This allows the specification of simple runtime scenarios in a graphical manner. I.e it tells how the different participants (or in UML terms actors), participate to make the overall functionality.

In OO Terms, the class diagram already depicts the different methods that achieve the overall functionality. The sequence diagrams gives how these methods participate to achieve the overall functionality i.e in the diagram, the overall restaurant experience. As you would have noticed, the diagram is called sequence diagram, as all computers work sequentially and all behavior has to be sequenced like this to achieve a computer functionality.

The line in the diagram (for e.g Order food, etc) is the method call to the object Bob the waiter. All these objects (in this case, Fred, Bob, Hank,) are sequenced by message or method calls.

i.e. The overall functionality, which could be a real world with parallel activities, have to be sequenced, so that this can be modelled in a sequential computer. This modeling constraint of sequentiality is the reason why the whole SDLC is very expensive. As the world which is parallel needs to be sequenced and modeled so that it can be executed by a sequential computer.

As you can see in this model, say tomorrow, Bob the waiter gets capability to also become the cook. But this model now have to reflect this, as you can see this change of roles for Bob is very difficult to acheive in this seuquential model. As the whole sequential algorithm has to be re-sequenced to give the right model. Simply a very expensive and laborious process .This is the problem that is faced by all enterprises. As when their business model changes. the computer model is not able to keep up with the business change, due to the sequential modeling employed.

Can we break from this paradigm of sequentiality? If this paradigm is shifted from sequential to parallel, this would help us model systems better. I.e. we might be able to express parallel systems directly, significantly improving productivity and agility of our IT systems.

In further posts, i will depict how these modelling can be changed so that we can model businesses directly.