#SYSTEM SEQUENCE DIAGRAM ONLINE SHOPPING HOW TO#
#SYSTEM SEQUENCE DIAGRAM ONLINE SHOPPING CODE#

The terms identified in the use case text of Figure 6.57 are, in order of appearance:

The analyst is responsible for understanding what the real information needs of the system are, and for filtering the irrelevancies. Thus, not every noun, adjective, or verb should be considered as a model element. It is not advisable to represent in the conceptual model information that is irrelevant to the system. The analyst should mind the project goals and scope when looking for the elements of the conceptual model. Usually, these textual elements are composed by nouns such as “person,” “order,” “payment,” etc., or by expressions that denote nouns (known in linguistics as noun phrases), such as “payment authorization.” Moreover, sometimes even verbs may represent concepts, when verbs express an act that corresponds to a noun, as, for example, the verb “paying,” which corresponds to the noun “payment.” From those artifacts one can discover all textual elements that eventually refer to information to be managed. However, one of the most useful techniques is to look at the text of the expanded use cases or system sequence diagrams. The process of discovering the elements of the conceptual model may vary.

#SYSTEM SEQUENCE DIAGRAM ONLINE SHOPPING HOW TO#

Raul Sidnei Wazlawick, in Object-Oriented Analysis and Design for Information Systems, 2014 6.8.1 How to find concepts and attributes Finally, the quantity may be accepted as valid because the interface can compare it to the quantity available in stock that was returned with other information about the book.

For example, in Figure 8.2 observe that the cart ID received by the operation add2Cart is valid because it was returned by the command createCart the ISBN of the book is also valid because it was collected from a list returned by the searchBook query. Usually the flow of messages expressed in the system sequence diagram is a way to verify if preconditions are viable instead of exceptions. That way, the parameter would be validated before performing the command. This guarantee can be made by explicitly calling a query that verifies if the condition is true before calling the command, or by interface mechanisms that prevent the command from being called with invalid data.įor example, instead of allowing the user to type any string in an ISBN field, the user could be led to select from a list of valid ISBNs. Raul Sidnei Wazlawick, in Object-Oriented Analysis and Design for Information Systems, 2014 8.2.3 Precondition assuranceĪs the preconditions are not tested by the command that declares them, an external mechanism should exist to assure that they will be guaranteed before the command is called.

#SYSTEM SEQUENCE DIAGRAM ONLINE SHOPPING CODE#

It can be made because trying to obtain the code of a newly created object by other means would be counterproductive. It is one of the acceptable exceptions to that principle, because the command just creates a new object and returns its identification code. One remark must be made about the command createShoppingCart: as it creates a new domain object and returns its identification code, one could ask if this is not against the command-query separation principle. This is why the interface sends an ID as a parameter instead of the cart object itself. This means that the interface should not have direct access to the domain objects.

Why is the cart ID used and not the cart object itself? Are we not doing object-oriented design? The point is that the domain objects are encapsulated by the controller. When the order is ready to be completed, the interface sends to the controller the IDs of the customer and of the shopping cart (remember that the shopping cart is not yet associated to any customer).Īs the controller does not have state (transient memory), each operation sent to it that refers to the shopping cart must refer to its number ( cartId). It is the interface that holds the customer identification at this point. One point to notice is that as the controller has no transient memory, it does not register the information of who is shopping. At that point, if the user is not already identified an exception would be raised, as explained later. The order will be created only when the user chooses to stop shopping (finishing the order). Therefore, a shopping cart is created instead of an order at the initialization of that sequence diagram. For example, the fact that a user must be identified only when the order is finished prevents the system from creating an order before that point (if the order is really a concept that must be associated to a customer). System sequence diagram for the main flow of use case Order books using a stateless strategy.įrom this point it can be seen that the team is making a transition from analysis to design, because design decisions must be made in order to accommodate the requirements.

System sequence diagram online shopping