Could anyone explain me what is the difference between sequence diagram and communication diagram?
Both diagrams give the same information, but the sequence diagram emphasizes time in its layout and the communication diagram emphasizes the objects that are communicating in its layout.
Time is implicit in sequence diagrams (it is inferred by vertical position), while it is given explicitly in communication diagrams (via numbers).
To quote Scott Ambler
The main difference between communication diagrams and sequence diagrams is that sequence diagrams are good at showing sequential logic but not that good at giving you a “big picture view” whereas communication diagrams are the exact opposite.
emphasizing the time ordering of messages (using sequence diagrams)
emphasizing the structural relationships among the objects that interact (using communication diagrams).
reference :UML user manual
Sequence diagrams illustrate interactions in a kind of fence format, in which each new object is added to the right.
What might this represent in code? Probably, that class A has a method named doOne and an attribute of type B. Also, that class B has methods named doTwo and doThree. Perhaps the partial definition of class A is:
public class A
{
private B myB = new B();
public void doOne()
{
myB.doTwo();
myB.doThree();
}
// . .
}
Communication diagrams illustrate object interactions in a graph or network format, in which objects can be placed anywhere on the diagram (the essence of their wall sketching advantage).
What are the Strengths and Weaknesses of Sequence vs. Communication Diagrams?
Each diagram type has advantages, and modelers have idiosyncratic preference—there isn't an absolutely "correct" choice. However, UML tools usually emphasize sequence diagrams, because of their greater notational power.
Sequence diagrams have some advantages over communication diagrams. Perhaps first and foremost, the UML specification is more sequence diagram centric—more thought and effort has been put into the notation and semantics. Thus, tool support is better and more notation options are available. Also, it is easier to see the call-flow sequence with sequence diagrams—simply read top to bottom. With communication diagrams we must read the sequence numbers, such as "1:" and "2:". Hence, sequence diagrams are excellent for documentation or to easily read a reverse-engineered call-flow sequence, generated from source code with a UML tool.
But on the other hand, communication diagrams have advantages when applying "UML as sketch" to draw on walls (an Agile Modeling practice) because they are much more space-efficient. This is because the boxes can be easily placed or erased anywhere—horizontal or vertical. Consequently as well, modifying wall sketches is easier with communication diagrams—it is simple (during creative high-change OO design work) to erase a box at one location, draw a new one elsewhere, and sketch a line to it. In contrast, new objects in a sequence diagrams must always be added to the right edge, which is limiting as it quickly consumes and exhausts right-edge space on a page (or wall); free space in the vertical dimension is not efficiently used. Developers doing sequence diagrams on walls rapidly feel the drawing pain when contrasted with communication diagrams.
A fundamental rule in Object Oriented Design is to use different diagrams based on diverse purposes. In this regard, sequence diagrams are used to model sequential logic while to show the behavior of several objects collaborating together to fulfill a common purpose the communication diagrams, formerly known as collaboration diagrams in UML 1.x, can be used. - Read more >> here << and >> here <<
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I would like to know which diagram from those two should be created first when designing large IT system.
In every article I read about UML, the component diagram is mentioned as almost last.
I think that, when designing a larger system is better to divide it to smaller parts/modules on the component diagram and then for every component create a detailed class diagram. Is that correct?
How it look in practice from your experiences?
Do you know of any article/tutorial that shows from beginning to end, the design of system using UML?
There is no mandatory order for UML diagram design - in fact, you will often find yourself creating several simple diagrams of different types, and refining them later. The process is generally iterative but doesn't have a follow a particular order.
That being said, some UML diagrams provide a more abstract view than others, and it may sometimes make sense to start with the most abstract views and work your way down to the most specialised ones. Use case diagrams typically come early, while object diagrams or state diagrams often come later in the design process, once the original, abstract solution has been refined.
You will also want to take a look at consistency between your diagrams: indeed, with several views you will introduce redundant elements, and it is important to make sure that they do not contradict each other. For example, if you have a sequence diagram and a class diagram, any call message in the former should correspond to an operation in the latter. See Alexander Egyed et al's work for automated consistency management and correction in UML models.
As i read through UML specification superstructure that there is sequence diagram and there is a collaboration diagram
so,What is the differences between
sequence diagram and collaboration diagram?
Sequence and collaboration diagrams both aim at describing the dynamic interactions between objects.
The information you can describe are basically the same, but the two models have a different focus:
Sequence diagrams highlight more the temporal aspect, by showing invocation and responses along a (vertical) timeline and by explicitly showing the activation time of objects.
Sequence diagrams show how objects communicate with each other in terms of a temporal sequence of messages. The time flow is the most visible aspect in these diagrams, as messages are sequenced according to a vertical timeline and also the lifespan of objects associated to
those messages is reported. The figure below (taken from our book) shows an example of a sequence diagram describing 3 objects (instances of classes Shipment, Invoice and PartList) and the messages exchanged between each other. Interaction diagrams describe execution scenarios of the system.
Collaboration diagrams aim at showing the communications that happen between objects, by defining messages that flow between each other. They basically consist of superimposing the communication actions upon an object diagram. The temporal aspect can be shown here too, by numbering the interactions with sequential labels.
A collaboration diagram shows the interactions between objects or classes in terms of links (solid undirected lines connecting the elements that can interact) and messages that flow through the links. This describes at the same time some kind of static structure (links and nodes) and dynamic behavior (messages) of the system. An example is shown below.
UML1: These diagrams are the same, but present data from different points of view.
UML2: The Collaboration diagram is renamed to Communication diagram (there were misunderstanding since people thought that Collaboration diagram was used to show collaborations).
These diagrams are almost the same, but present the Sequence diagram has much more possibilities for visualization (for example, combined fragments, state invariants).
Sequence diagrams specify interaction in a time sequence manner which may be among objects and/or classes. These diagrams are created during early elaboration phase where each flow of the use case is defined in terms of sequences , i.e. after each step what is going to happen next. This kind of representation is very helpful to understand & discuss the use cases with the customer, where both can come out with all possible functional aspects.
On the other hand collaboration diagram provides a direct interaction among the object. These diagram seem to used more in the design phase of the development when you are designing the implementation of the relationship.
There is a difference between sequence diagrams and collaboration diagrams. Sequence diagrams shows object interaction in timely manner(so no need of numbering the messages). But collaboration diagram doesn't show object interaction in timely manner. (need to numbering the messages).
NOTE:
Now Collaboration diagrams are also called as communication diagrams.
A Sequence diagram is dynamic, and, more importantly, is time ordered. A Collaboration diagram is very similar to a Sequence diagram in the purpose it achieves; in other words, it shows the dynamic interaction of the objects in a system. A distinguishing feature of a Collaboration diagram is that it shows the objects and their association with other objects in the system apart from how they interact with each other. The association between objects is not represented in a Sequence diagram.
Link : http://www.developer.com/design/article.php/3102981/Collaboration-Diagram-in-UML.htm
Use sequence diagrams to show time and to emphasize the sequence or
the ordering of the interactions.
Use communication diagrams to emphasize the links among the
participants. Communication diagrams can be a helpful supplement
to the object diagrams.
What's the difference between sequence diagrams and scenario diagrams in UML?
I think you mean System Sequence Diagram (SSD) and Sequence Diagram.
The purpose of SSD is to illustrate a particular use-case scenario in a visual format where the system appears as a black box.
Sequence Diagram, on the other hand, shows how system objects interact over time for achieving a particular scenario of a use-case.
"Scenarios" per your link aren't diagrams, they're narrative text. They're also called "Use Case Narratives" among other terms. They are approximately equivalent to other behaviour specifications (sequence diagrams, activity diagrams).
There are generally three differences:
Format. Scenarios are textual, sequence/activity diagrams are, well, diagrams
Content. Textual narrative is often used to describe the sequence of interactions between the user and the "system" in Use Case descriptions. Hence there are usually only two participants (User & System). In contrast, sequence diagrams are primarily used to show how behaviour is distributed among objects. Consequently there will usually be more than two participants.
Formality. Narratives are often used earlier in the cycle to get an idea of what's needed. Consequently they're often less formal than Sequence diagrams - which typically show inter-object messages ("method calls").
For examples of tools that do this, take a look at websequencediagrams.com. Or search for "free UML tools".
There is no diagram type called scenario. Maybe you are thinking of collaboration?
Sequence and collaboration diagrams are actually the same: the Sequence depicts the chronology while the Collaboration depicts the interactions between the collaborators. Together (a tool that was around a decade ago) used to let you literally just change the setting and see the diagram in the other mode.
I use OmniGraffle (Mac). It's great for class diagrams. Has support for Sequence diagrams but not Collaboration.
Is an activity diagram is considered algorithmic?
Strange question indeed. Taking a look at dictionary definitions:
Algorithmic: of or relating to or having the characteristics of an algorithm
Algorithm(1): a precise rule (or set of rules) specifying how to solve some problem
Algorithm(2): an effective method for solving a problem expressed as a finite sequence of steps.
So: can Activity Diagrams be used to describe "a method for solving problems expressed as a finite sequence of steps"? Yes - that's their purpose. They support all the usual control structures required to describe algorithms: sequence (A followed by B), alternation (either A or B), iteration. They also provide explicit support for parallel activity.
Areas where they may be considered weak:
The execution semantics are not precisely and unambiguously defined. Whilst the UML spec does provide some semantics, it leaves some points open.
There's no pre-defined set of primitive types defined for Activity Diagrams. Thus, the lowest level primitive activities (adding integers, concatenating strings, etc.) aren't pre-defined. In that sense Activity Diagrams on their own aren't computationally complete.
However: those are nitpicking theoretics. In practice, Activity Diagrams are a popular means to represent algorithms: from the design of software procedures to specifying business processes.
Of course, it all comes down to your definition of algorithmic. If you go with the general definitions above then ADs are algorithmic. If you go with something more specific they might not be.
hth.
OMG Meta-Models (i.e. UML, SysML, etc.), starting from the Meta-Meta-Modeling language by means of which they are described (MOF), consists of two different classes of models respectively aimed at definining:
The static structure of a system, or architecture (i.e. Class Diagram, Component Diagram, Block Diagram, etc.);
The dynamic of the system, or behaviours (i.e. sequence diagram, activity diagrams, state machine diagrams, object interaction diagrams, etc.)
Activity diagrams belong to the second class so they are aimed at describing behaviours.
an algorithmic is only of type of behaviour that can be described by means of an activity diagram
Activity diagrams complemented with the new UML executable standards can be used to represent any algorithm. Activity diagrams alone can be used to represent the overall control flow but not really the details of data management
I've had this discussion with my professor at college about UML diagrams .
He believes that sequence diagrams should be drawn before getting to class diagrams, but I think the opposite . I think after finishing the usecase diagram , the next diagram should be class diagram and after that we should get to sequence diagram.
Rational rose requires us to use the classes in sequence diagram, which are already in class diagram.
Can anyone help me with this?
I think you're both wrong. They should be drawn at the same time. As you're drawing your sequence diagram, you will undoubtedly come up with properties that you will need to keep track of the state, or that you hadn't thought of if you do the class diagram in a vacuum.
Of course, this is highly subjective and personal, but years of real-world experience (as opposed to academic theory) have taught me to work on both at the same time. MAYBE starting with the class diagram, but the class diagram invariably changes when you start going through process flows.
Well it depends a lot on how you plan to do things. I think it's a subjective matter. If you rather explain the actions performed for your usecases and after this has been done write the classes based on what you need to perform the sequences your professor is right.
But if you prefer to determine what the structure of your classes are and then adapt the action sequence to this then you would do the class diagram first and later the sequence.
In my experience I do them concurrently. I put the fundamental attributes to the class diagram but not the actions, and while I do the sequence diagram I add the methods and attributes that I need to the class diagram.
There is no one standard answer. There are several opinions, approaches and methods. In Unified Process I believe you first identify use case and then make realizations for them, e.g. sequence diagrams. As in use cases, there are actors and the systems and/or its parts interacting ina sequence. Actually this interactions should help you decompose your design and get to classes. Once you have classes on the analysis level, you can go further to design classess and design interactions. However these are quite a lot to draw in a diagram, most of the times code is the best documentation on this level, even generated diagrams are too large and more difficult to understand then code itself.
To prepare sequence diagram you need classes not the class diagram
You can prepare empty classes on the fly while in process of preparing sequence diagram....
Identification class objects in be part of preparing sequence or you can try to identify your objects before hand....
sequence is logical process while class diagram is end output
I don't think there is an order for creating diagrams since both are two different views of a system: class diagram structural (static) and sequence is behavioral (dynamic). I would start with sequence diagrams since you will discover more classes to create as you go through sequences. Do whatever makes sense for you at the time. If your doing more object oriented programming, I would consider doing classes before sequences.
Structural and behavioural models for all but the very simplest systems are naturally created simultaneously and iteratively, refining both over time.
You may have some method of "object discovery" such as CRC cards, which will yield a set of initial classes, with collaborations (the classes they interact with) and responsibilities, which informs both the methods they will need, and internal behaviour/state/activity.
You may then want to explore the use cases and scenarios using sequence or communication diagrams, this will expose the details of the required object communication and therefore inform the generation of public methods and relationships to refine the class diagram while at the same exploring the system behaviour which may yield further objects and classes to be created.
You may also want to explore the internal behaviour of classes, especially if they have stateful and/or active behaviour; activity ans state-machine diagrams are useful for this.
Either way I doubt that the use of Rational Rose is really the determinant of diagram creation order. Rational may require the classes on a sequence diagram to exist, but I imagine that they need not actually appear on a class diagram; they can probably equally be created on the sequence diagram and then later placed on a class diagram, or even created in the project explorer or whatever the equivalent is in that tool. Even if the only method of creating a class is to place it on a class diagram, that does not require you to refine and complete the class or its relationships before exploring interactions on a sequence diagram.
You need to buy some clothes, how you will proceed. You start choosing clothes first or you will decide first where to go? At the same time, will you go to shoe store if you want to buy shirt.
So both are iterative, but definitely first step is sequence on very high (component) level then drill down to class level sequence
You should first decide your application flow,means you should first draw a sequence diagram. It will show the flow of your application after this you should go for class diagram.