COMP 310/510 Fall 2013	Lec29: Use Cases and ChatApp API Design

Let's finish our discussion on generics, especially with regards to the DataPacket design we will be using.

Student-driven ChatApp Design

We will be working together as a class to design the ChatApp main interfaces and protocols, so class and lab attendence is mandatory!  

Each student is responsible for

•  for knowing the results of the class design discussion.   
•  for pointing out errors, ommissions and problems, both realized and potential.  
•  for suggesting solutions to those problems.

 The staff will make the latest version of the class-designed code available through a separate svn:externals connection.  

A Chat App Discussion discussion forum and chat room have been set up in Owlspace to facilitate out-of-class design discussions.  And as always, one can send a message to the class at comp310f12 at owlspace-ccm.rice.edu.

For reference today and in lab, be sure to have the ChatApp specifications handy.

Use Cases

A use case is an example of how a user might interact with an application.    To develop an application, one of the very first things that a developer must work with the customer to collect and analyze as many use cases as possible.    It is from these use cases that a picture is built up that tells the developer how exactly the user (or any external entity) will interact with the system being built and thus how the sytem needs to be built.

A Use Case Diagram (a standard type of UML diagram) is used to visually represent a use case.   The main components of Use Case diagrams are the "actors", the users or other external entities, and the various components of the system being built.  Arrows are used to indicate interactions between components. 

In class we will develop some use cases for our chat program.   These will be used to design our API below.

ChatApp Use Cases:

• Initial program start
• Initial connection
  • Switching chat room to the new chat room  (what if you are connected somewhere else already?)
  • Adding a new user to to the chat room
  • Notfiying all chat room participants of new user.
• Sending a message
  • Transmission status notification
  • Possible status notifications
• Disconnecting from a chat room
  • What if the user is the creator of the chat room?
• Dealing with unknown data types

Use Case References:

• Comp310 Java Resources page on Use Cases
• Wikipedia articles on Use Cases and Use Case Diagrams.
• http://www.objectmentor.com/resources/articles/usecases.pdf
• http://www.parlezuml.com/tutorials/usecases.htm (read to PDF docs that are linked here)

General Process for Designing the Sub-Systems (System Block Diagram) and API's in a System

1. Create a Use Case Diagram that shows all the relevant use cases.
2. Arrange the use cases into groups of related operations
3. Adjust the use cases to better fit the groupings from #2 above and/or adjust the groupings to clarify their semantics.
4. The groupings become the sub-systems and the diagram becomes the System Block Diagram
5.  The use cases that cross from one sub-system to another (note the directionality here) create the API's to the target sub-systems. 

The system block diagam shows the major, maximally decoupled, encapsulated sub-systems that make up the total system.   The system block diagram shows all the multiple "models" and "views" that the  controller in the MVC will have to construct and connect.

ChatApp API Design

We will also spend time today developing the ChatApp Application Programming Interface ("API").   An API is the set of interfaces and usage protocals that separate the various decoupled portions of a program.

Remember that at this stage, we have two distinct interaction modalities taht require well-defined interaction protocols:

• Initial connection protocol
  • Name of the interface
  • RMI bound name
  • Methods 
• Interface to chat room
  • Name of interface
  • Methods
  • Stub or fully serialized object?
  • Contains local connection or not?
  • The difference between the RMI room object vs the overall room management in a chat client?
  • Adding one's self to a room.
• Interface to individual users
  • Name of interface
  • Methods
• Message transmission protocol
  • "Well-known" data types?
  • Status types and behaviors
• View connectivity issues for transportable entities, e.g. data packet processing commands

© 2013 by Stephen Wong