University of Utah SoC CS6961 - Lecture 12 1

Dynamic Models in Design

CS6961 – Lecture 12

Nathan Dykman

University of Utah SoC CS6961 - Lecture 12 2

Dynamic Models

• To review, there are two major type of models in UML

• Static models– Use Case, Class, Component, Deployment

• Composite structure (mix of static/dynamic)

• Dynamic models– Sequence, State, Activity

University of Utah SoC CS6961 - Lecture 12 3

Dynamic Models

• Dynamic modeling is often overlooked in the design process– Why is this, do you think?

• Using dynamic models comes down to balance:– Too little: Critical dynamic details may be

missed– Too much: Overwhelming detail

University of Utah SoC CS6961 - Lecture 12 4

Dynamic Modeling

• I think dynamic models are best used in two ways:– Exploratory: Does this design do what we

want, how could it do it?– Descriptive: This is a tricky aspect of the

system, this model (should) help explain what is going on

University of Utah SoC CS6961 - Lecture 12 5

Dynamic Modeling Traps

• Unfortunately, dynamic models are easy to overuse– For example, requiring a state diagram for

every class• Not surprising, most of these diagrams aren’t

helpful

– Or activity diagram, or sequence diagram, etc.

• Therefore, some avoid them completely– Missing out where they are useful.

University of Utah SoC CS6961 - Lecture 12 6

Dynamic Modeling Domains

• However, in some software domains, these kinds of models are critical:– Embedded Systems: Must have predictable,

reactive behavior– Telecommunications: Strict protocol

requirements– Service Oriented Applications: Conversations,

Transactions very important.– And so on.

University of Utah SoC CS6961 - Lecture 12 7

sd Example

StoreFront Cart Inv entory

loop AddItem

ReserveItem

Checkout

ProcessOrder

ConfirmOrder

PlaceItemsInOrder

UML 2.0 Example

University of Utah SoC CS6961 - Lecture 12 8

Sequence Diagram

• Hopefully, all of you recognized the preceding diagram as a sequence diagram– If not, review chapter 4 of UML Distilled.

• In design, the messages can be actual method calls– Because the instances on the sequence

diagram are class instances

University of Utah SoC CS6961 - Lecture 12 9

Sequence Diagrams

• Are probably the most popular means of showing interaction between objects in UML– Communication Diagrams are still preferred

by some (not me)

• Remember, UML 2.0 added interaction frames– Which added to the expressiveness of

sequence models.

University of Utah SoC CS6961 - Lecture 12 10

Sequence Diagrams

• It is often useful to take select use cases and build a sequence diagram in the design that reflects the use case– Insures that the design does cover use case

functionality– Doesn’t need to be perfect, but enough detail

to verify that the use case can be performed as required.

University of Utah SoC CS6961 - Lecture 12 11

Sequence Diagrams

• Complex systems often contain quite a bit of messaging– For example, most web systems pass

between multiple classes (and layers) to process/render web pages

• Sequence diagrams can show this interaction fairly well– Examples show how the flow of messages is

structured.

University of Utah SoC CS6961 - Lecture 12 12

UML 2.0 Examplead Example

Start

Start Car

Get Gas

Drive To Work

Park Car

Drink Coffee Walk To Office

Work

[GasLight Is On]

Action/ Activity

Decision Point

Fork/Join

Activity with Sub-activity model

University of Utah SoC CS6961 - Lecture 12 13

Activity Diagrams

• This is an activity diagram.– We will cover activity models in more detail

• For now:– Easy modeling of parallelism– Activities can nest (have sub-structure)– We will cover how to link activity models to

classes• As well as execution in general.

University of Utah SoC CS6961 - Lecture 12 14

Activity Diagrams

• I find activity diagrams best used to model large scale workflow in the system– Workflow is a topic in of itself, of course

• Workflows are more common in systems with longer-standing transactions– For example, maintaining a relationship with a

shipping office, a supplier of goods, a long-term customer, etc.

University of Utah SoC CS6961 - Lecture 12 15

Activity Diagrams

• Again, activity diagrams should feel forced– They should pop out naturally to you

• Also, remember that activity diagrams display parallel/concurrent behaviors– Sequence diagrams don’t do this nearly as

well.

• Activities can be linked to use cases– For example, a use case can become an

activity (or activities) in a system.

University of Utah SoC CS6961 - Lecture 12 16

State Examplesm State Example

Wait For Card

Do Transaction

Card Inserted

Reject Card Finished

Read Pin Choose Transaction

Return Card

Card Returned

No More Transactions

Pin Accepted

Card Rejected

University of Utah SoC CS6961 - Lecture 12 17

State Diagram

• Again, more details to come. But for now, note the following:– We have states. States can nest, or have their

own state machine– We have start states (final states as well)– Substates (or complex states) can have

multiple entry/exit states• Later, we will learn that these, like start/final states,

are called pseudostates.

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State Machines

• In some domains, state machines are rare– In embedded systems, they can be the most

commonly used diagram• Executable UML often centers around state

diagrams

– The state model in UML is quite complex

• Again, don’t force it– But sometimes, it is a very useful abstraction

to use. (So, don’t ignore it completely)

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Pushing the Edges

• For those interested in how dynamic models are used in real-time systems, check out:– Real Time UML (3rd Edition), by Bruce Powell

Douglas– You can get access to real-time tools from

Telelogic, iLogix, IBM/Rational if you liked• All require the use of a license server.

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Dynamic Models

• Tools like RSA tools do very little with dynamic models– You need a special modeling tool to do code

generation from state machines, etc.• These are very complex tools

– Some tools are starting to do forward and reverse engineering of sequence diagrams

• RSA is supposed to do this for Java, no clue if it really does

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Summary

• Next Time, we go into more details– This may include exercises!– Be warned.

• Reading: Chap. 4, 10, 11 in UML Distilled.