- A properly constructed eye should contain every possible bit sequence from simple alternate 1’s and 0’s to isolated 1’s after long runs of 0’s, and all other patterns that may show up weaknesses in the design. Eye diagrams usually include voltage and time samples of the data acquired at some sample rate below the data rate.
- The sequence diagram shows an exemplar or “sample execution” of some portion of the system under specific conditions. Such an exemplar is commonly called a scenario, and a single sequence diagram generally shows a single scenario. 21 The messages may be synchronous (shown with a solid arrowhead), asynchronous (shown with an open arrowhead), or flow (shown with a dashed arrow line).
The sequence diagram represents the flow of messages in the system and is also termed as an event diagram. It helps in envisioning several dynamic scenarios. It portrays the communication between any two lifelines as a time-ordered sequence of events, such that these lifelines took part at the run time. In UML, the lifeline is represented by a vertical bar, whereas the message flow is represented by a vertical dotted line that extends across the bottom of the page. It incorporates the iterations as well as branching.
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Purpose of a Sequence Diagram
- To model high-level interaction among active objects within a system.
- To model interaction among objects inside a collaboration realizing a use case.
- It either models generic interactions or some certain instances of interaction.
Notations of a Sequence Diagram
An individual participant in the sequence diagram is represented by a lifeline. It is positioned at the top of the diagram.
Sequence Diagram Update Data
A role played by an entity that interacts with the subject is called as an actor. It is out of the scope of the system. It represents the role, which involves human users and external hardware or subjects. An actor may or may not represent a physical entity, but it purely depicts the role of an entity. Several distinct roles can be played by an actor or vice versa.
It is represented by a thin rectangle on the lifeline. It describes that time period in which an operation is performed by an element, such that the top and the bottom of the rectangle is associated with the initiation and the completion time, each respectively.
The messages depict the interaction between the objects and are represented by arrows. They are in the sequential order on the lifeline. The core of the sequence diagram is formed by messages and lifelines.
Following are types of messages enlisted below:
- Call Message: It defines a particular communication between the lifelines of an interaction, which represents that the target lifeline has invoked an operation.
- Return Message: It defines a particular communication between the lifelines of interaction that represent the flow of information from the receiver of the corresponding caller message.
- Self Message: It describes a communication, particularly between the lifelines of an interaction that represents a message of the same lifeline, has been invoked.
- Recursive Message: A self message sent for recursive purpose is called a recursive message. In other words, it can be said that the recursive message is a special case of the self message as it represents the recursive calls.
- Create Message: It describes a communication, particularly between the lifelines of an interaction describing that the target (lifeline) has been instantiated.
- Destroy Message: It describes a communication, particularly between the lifelines of an interaction that depicts a request to destroy the lifecycle of the target.
- Duration Message: It describes a communication particularly between the lifelines of an interaction, which portrays the time passage of the message while modeling a system.
A note is the capability of attaching several remarks to the element. It basically carries useful information for the modelers.
- Sequence fragments have been introduced by UML 2.0, which makes it quite easy for the creation and maintenance of an accurate sequence diagram.
- It is represented by a box called a combined fragment, encloses a part of interaction inside a sequence diagram.
- The type of fragment is shown by a fragment operator.
Types of fragments
Following are the types of fragments enlisted below;
|alt||Alternative multiple fragments: The only fragment for which the condition is true, will execute.|
|opt||Optional: If the supplied condition is true, only then the fragments will execute. It is similar to alt with only one trace.|
|par||Parallel: Parallel executes fragments.|
|loop||Loop: Fragments are run multiple times, and the basis of interaction is shown by the guard.|
|region||Critical region: Only one thread can execute a fragment at once.|
|neg||Negative: A worthless communication is shown by the fragment.|
|ref||Reference: An interaction portrayed in another diagram. In this, a frame is drawn so as to cover the lifelines involved in the communication. The parameter and return value can be explained.|
|sd||Sequence Diagram: It is used to surround the whole sequence diagram.|
Example of a Sequence Diagram
An example of a high-level sequence diagram for online bookshop is given below.
Any online customer can search for a book catalog, view a description of a particular book, add a book to its shopping cart, and do checkout.
Benefits of a Sequence Diagram
- It explores the real-time application.
- It depicts the message flow between the different objects.
- It has easy maintenance.
- It is easy to generate.
- Implement both forward and reverse engineering.
- It can easily update as per the new change in the system.
Sequence Diagram Editor
The drawback of a Sequence Diagram
- In the case of too many lifelines, the sequence diagram can get more complex.
- The incorrect result may be produced, if the order of the flow of messages changes.
- Since each sequence needs distinct notations for its representation, it may make the diagram more complex.
- The type of sequence is decided by the type of message.
There are four basic symbols that are used to represent a data-flow diagram.
A process receives input data and produces output with a different content or form. Processes can be as simple as collecting input data and saving in the database, or it can be complex as producing a report containing monthly sales of all retail stores in the northwest region.
Every process has a name that identifies the function it performs.
The name consists of a verb, followed by a singular noun.
- Apply Payment
- Calculate Commission
- Verify Order
- A rounded rectangle represents a process
- Processes are given IDs for easy referencing
A data-flow is a path for data to move from one part of the information system to another. A data-flow may represent a single data element such the Customer ID or it can represent a set of data element (or a data structure).
- Customer_info (LastName, FirstName, SS#, Tel #, etc.)
- Order_info (OrderId, Item#, OrderDate, CustomerID, etc.).
Data flow Example:
- Straight lines with incoming arrows are input data flow
- Straight lines with outgoing arrows are output data flows
Because every process changes data from one form into another, at least one data-flow must enter and one data-flow must exit each process symbol.
Rule of Data Flow
One of the rule for developing DFD is that all flow must begin with and end at a processing step. This is quite logical, because data can't transform on its own with being process. By using the thumb rule, it is quite easily to identify the illegal data flows and correct them in a DFD.
Sequence Diagram Update Data Set
|An entity cannot provide data to another entity without some processing occurred.|
|Data cannot move directly from an entity to a data story without being processed.|
|Data cannot move directly from a data store without being processed.|
|Data cannot move directly from one data store to another without being processed.|
Other frequently-made mistakes in DFD
A second class of DFD mistakes arise when the outputs from one processing step do not match its inputs and they can be classified as:
- Black holes - A processing step may have input flows but no output flows.
- Miracles - A processing step may have output flows but no input flows.
- Grey holes - A processing step may have outputs that are greater than the sum of its inputs
A data store or data repository is used in a data-flow diagram to represent a situation when the system must retain data because one or more processes need to use the stored data in a later time.
- Data can be written into the data store, which is depicted by an outgoing arrow
- Data can be read from a data store, which is depicted by an incoming arrow.
- Examples are: inventory, Accounts receivables, Orders, and Daily Payments.
Data Store Example
- A data store must be connected to a process with a data-flow.
- Each data store must have at least one input data-flow and at least one output data-flow (even if the output data-flow is a control or confirmation message).
An external entity is a person, department, outside organization, or other information system that provides data to the system or receives outputs from the system. External entities are components outside of the boundaries of the information systems. They represent how the information system interacts with the outside world.
- A rectangle represents an external entity
- They either supply data or receive data
- They do not process data
- A customer submitting an order and then receive a bill from the system
- A vendor issue an invoice
External Entity Example
- External entities also are called terminators because they are data origins or final destinations.
- An external entity must be connected to a process through a data-flow.