Cornell University Compu1ng and Informa1on Science CS 5150 So(ware Engineering 13. Three Popular Architectural Styles William Y. Arms
Three Popular Architectural Styles Example 1: Three Tier Architecture This architecture is an extension of the client/server model. It is the standard architecture for small and medium sized web sites.
A Very Basic Web Site This very basic web system has a client/server architecture. WebBrowser WebServer HTTP dependency realiza.on interface
Web Server with Data Store The basic client/server web site returns only fixed HTML pages. ARach the server to a data store, so that it can respond to requests from the client and return suitable content. Data Server Web browser Advantage Server-side code can respond to user requests by accessing data, configuring pages, validaQng informaQon, etc.
Component Diagram These components might be located on a single node Database WebBrowser WebServer Server ODBC HTTP Since components are replaceable binary elements: • Any web browser can access the web site. • The database server can be replaced by another database that supports the same interface.
Architectural Style: Three Tier Architecture Database Presentation Application tier tier tier External services Each of the tiers can be replaced by other components that implement the same interfaces.
Three Tier Architecture: Broadcast Searching User Databases User interface service This is an example of a multicast protocol. The primary difficulty is to avoid troubles at one site degrading the entire system (e.g., every transaction cannot wait for a system to time out).
Extending the Web with Executable Code that can be Downloaded html Java Data Script Server Web browser Executable code in a scripQng language such as JavaScript can be downloaded from the server. This can be done within the three Qer architecture.
Web Browser with JavaScript Web Browser HTMLRender JavaScript HTTP Each package HTTP contains several classes. The Presentation Tier has become more complex. Since it still supports the same interface it is still a replaceable binary component.
Three Popular Architectural Styles Example 2: Master File Update This architecture is an alternative to the repository model. It is very widely used in data processing systems.
Master File Update OrganizaQon ApplicaQon Master file TransacQons Cornell University registraQon students add/drop, record grade NYSEG uQlity billing householders pay bill, read meter Tompkins Trust banking customers deposit, withdraw Characteristics • Many complex types of transaction, including reversals (corrections). • Correctness of master file is vital to the success of the organization. • Batch updating of master file is acceptable (e.g., overnight). • Must be able to answer customer queries, without waiting for full update.
Master File Update: Transactions Example: Electricity U1lity Billing Requirements analysis idenQfies many transacQon types: • Create account / close account • Meter reading • Payment received • Other credits / debits • Check cleared / check bounced • Account query • CorrecQon of error • etc., etc., etc.,
Master File with Batch Processing: Dataflow Model New Bills, transactions reports, etc. Input and Pending Master file Output validation transactions update process read only Master Checkpoints, file audit trail • Input and validation process runs throughout the day. It processes transactions when they arrive. • Master file update program runs once per day (usually at night). • Output process is run after the master file update finishes.
Batch Processing: Input and Validation Paper errors transac.on read Input and Pending Data input write validation transactions read only Master Network file transac.on Because the input and validation process is able to read the master file, it can check that the transaction is compatible with the master file, e.g., file has a record for the customer.
Benefits of Batch Processing with Master File Update Advantages: • Backup and recovery have fixed checkpoints. • BeRer management control of operaQons. • Efficient use of staff and hardware. • Error detecQon and correcQon is simplified. Disadvantages: • InformaQon in master file is not updated immediately. • No good way to answer customer inquiries.
Online Inquiry Customer service New transac.on representaQve read only Input and Pending Master validation transactions file • Customer calls the organizaQon and speaks to a customer service representaQve. • The representaQve can read the master file and the pending file, but cannot change them. • OpQonal. Customer can read selected parts of the master file and pending file without going through customer service.
Online Inquiry: Use Case AnswerCustomer <<includes>> CustomerRep NewTransacQon • The representaQve can read the master file and pending file, but not make changes to them. • If the representaQve wishes to change informaQon in the master file, a new transacQon is created and sent to the input and validaQon process. • The representaQve can create an annotaQon, e.g., a note of a telephone conversaQon with a customer. It is treated as a new transacQon.
Three Popular Architectural Styles Example 3: Model/View/Controller (MVC) This architecture is used to control a complex user interface. It is the standard architecture for mobile apps and widely used in robotics.
Model/View/Controller (MVC) The definition of Model/View/Controller (MVC) is in a state of flux. The term is used to describe a range of architectures and designs. • Some are system architectures, where the model, view, and controller are separate components. • Some are program designs, with classes called model, view, and controller.
Model/View/Controller as a System Architecture Model State State query change External services View control View Controller
Example: a Drone The drone is a small unmanned airplane. • The drone is flown by a pilot who sits at a computer on the ground. • The drone is controlled by radio signals from the ground staQon (e.g, changes to throRle, ailerons, flaps, etc.). • Sensors on the airplane send radio signals to the ground staQon (e.g., air speed, GPS coordinates, control surface sefngs, etc.). This example is based on a CS 5150 project.
Drone: View The pilot sits at a computer on the ground with a user interface that represents the cockpit controls of a light airplane. This is the view. Pilot From a CS 5150 project
Drone: Model For the drone, the model: • maintains a record of the state of the drone, e.g., speed, fuel. • models future changes of state, e.g., rate of turn, changes of speed. • updates the state based on data from the controller, e.g., changes in control sefngs. • updates the view with informaQon to be displayed in the user interface. In general, a different drone will need a different model, but the view and controller are separate components and will not necessarily change.
Drone: Controller Scenario: The pilot wishes to change the flap sefngs to 20 degrees. This will allow slower speed for landing. • The view sends a message to the controller, setFlaps(20). • The controller sends a message to the airplane, setFlaps(20). • The airplane sets the flaps and sends a message to the controller, flapsAreSet(20). • The controller relays the message to the model, flapsAreSet(20). • The model updates the state informaQon and recalculates the stall speed. • The model noQfies the view, flapsAreSet(20), and also the predicted change to the stall speed. • The view displays the changed sefngs on the pilot’s console.
View View The view presents the state of the interface to the user. It subscribes to the model, which noQfies it of events that change the state. • renders data from the model for the user interface • provides editors for properQes, such as text fields, etc. • receives updates from the model • sends user input to the controller A given model may support a choice of alternaQve views. The CS 5150 project was to create a new view for the pilot. Since this was a separate component, it could replace the old view with no other changes to the system.
Model Model The model records the state of the applicaQon and noQfies subscribers. It does not depend on the controller or the view. • stores the state of the applicaQon in suitable data structures or databases • responds to instrucQons to change the state informaQon • noQfies subscribers of events that change the state • may be responsible for validaQon of informaQon
Controller External Controller services The controller is the part of the system that manages user input and navigaQon within the applicaQon. • defines the applicaQon behavior • maps user acQons to changes in the state of the model • interacts with external services via APIs • may be responsible for validaQon of informaQon Different frameworks handle controllers in different ways. In parQcular, there are several ways to divide responsibiliQes between the model and the controller, e.g., data validaQon, external APIs.
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