Architectural patterns

1. Architectural patterns Architectural patterns

2. Patterns • Architectural patterns • Fundamental structural organization for software systems. • High-level subdivision of the system. • Highest level of pattern • Design patterns • Organization of classes • Intermediate level of pattern • Idioms • Organization within a single class or a few classes • Lowest level of pattern • Some architectural patterns • Layers • Pipes and filters • Broker • Model-View-Controller (MVC) • Master-slave Architectural patterns

3. Categories of architectural patterns • From mud to structure • Patterns: Layers, “Pipes and filters” • Distributed systems • Patterns: Broker, Master-Slave • Interactive systems • Patterns: Model-View-Controller (MVC) Architectural patterns

4. Architectural Pattern - Layers • Decompose overall system task into cooperating subtasks Examples • TCP/IP, and other protocol stacks • Java application, JVM, OS, physical machine • Information systems: Presentation, Controller, Model, Data • Upper layer asks lower layer for service • Interfaces between layers must be stable • Standardization: Public or proprietary • Java: “interface” is a keyword of the language • Error handling • Exceptions • Special values for signaling errors, like null or -1 Architectural patterns

5. Layers consequences • Benefits • Reuse of layers • Support for standardization • A network protocol specifies a layer, but does not implement it! • Dependencies are kept local • Exchangeability • An implementation of a layer can be exchanged with another implementation • With the same interface • Liabilities • Lower efficiency • Difficult to establish correct granularity of layers • Layers inside layers might help Architectural patterns

6. Architectural Pattern - Pipes and filters • System process a stream of data. • Each processing step is encapsulated in a filter component. • Examples • A shell in an operating system, like Linux/UNIX or MS-DOS • find “a” data.txt | sort | more • Find all lines with “a” in data.txt | sort the lines | show the lines, one screen at a time • Compilation of a Java program • Lexical analysis | syntax analysis | semantic analysis | code generation • Generally • Data source | filter | … | filter | data sink • The vertical bar | is called a “pipe”. • Implementation • Cooperating processes • No shared memory: Uses standard input (System.in) and standard output (System.out) • Cooperating threads • Shared memory: Bounded buffers • Example: The bounded buffer exercise Architectural patterns

7. Pipes and filters, consequences • Benefits • Flexibility by filter exchange • Plug-in a new filter, and you have another application • Flexibility by recombination • Reuse of filter components • Useful for prototyping • Efficient in parallel processing • Liabilities • Sharing information is hard • Data transformation overhead • Data must be adapted to the next filter • Error handling Architectural patterns

8. Distributed systems • A layered system can be distributed • Each layer running on a separate computer • Called “multi-tiered” system • A “pipes and filters” system can be distributed • Each filter running on a separate computer • Pipes are network connections Architectural patterns

9. Architectural Pattern - Broker • Coordinates communication between distributed components • Decouples clients and servers • Broker features • Register / deregister servers • Locate servers • Forward messages • Examples • CORBA • Common Object Request Broker Architecture • Some chat / messenger systems Architectural patterns

10. Broker, consequences • Benefits • Location transparency • Clients do not need to know where servers are. • Servers can be moved to other computers. • Changeability and extensibility • Servers can be changed • Keep the same interface • Reusability • Components can be reused in other application • Liabilities • Efficiency • Fault tolerance • Broker is not working => nothing is working Architectural patterns

11. Architectural Pattern – Master-Slave • Divide and conquer • Master use ‘same’ subservice’ from slaves • Master functions • Split work • Call slaves • Combine results • Examples • Parallel processing • Fault tolerance • Computational accuracy Architectural patterns

12. Master-Slave, consequences • Benefits • Faster computation • Split the problem over threads and machines. • Easy scalability. • Robustness • Slaves can be duplicated • Correctness • Slave can be implemented differently to minimize sementic errors • Liabilities • Communication overhead • Not all problems can be divided Architectural patterns

13. Interactive systems • Interaction with the user • Through graphical user interfaces • System responds to events (user inputs) • Functional core of the system must be kept independent of the user interface • You must be able to add a new user interface to the system. • A single system can have many user interfaces • PC interface • Web interface • PDA interface • Mobil phone interface Architectural patterns

14. Architectural Pattern Model-View-Controller (MVC) • Divides an application into 3 parts • Model • Core functionality and data • View • Displays information to the user • Same model component can have many views • Controller • Handles user input • User interface = View + controller • Changes in the model are automatically propagated to the view • Use observer-observable design pattern Architectural patterns

15. Model-View-Controller in Java • The Java GUI frameworks Swing (and AWT) uses a modified version of MVC • Each Swing component has • Model • Holding the state of the component • View • The visual part • Controllers • Are attached by programmers • Example: JButton • Model: ButtonModel getModel() • View: ButtonUI getUI() • Controllers: addActionListener(…), addItemListener(…), attChangeListener(…) • Example, JavaBeans: AccountConstrained (Model) + AccountFrame (View + controller) Architectural patterns

16. Model-View-Controller, consequences • Benefits • Multiple views for the same model • Synchronized views • “Pluggable” views and controllers • Exchangeability of “look and feel” • Port to a new windowing platform does not change the model. • Liabilities • Increased complexity • Excessive number of updates • Intimate connection between view and controller • Close coupling of (view, controller) and model • Both view and controller make calls to model Architectural patterns

17. Distributed systems • Advantages • Resource sharing • Printers, files (HTTP), etc. • Openness • Using standard protocols • Scalability • Fault tolerance • Replication • Disadvantages • Complexity • Security • Manageability • Unpredictability Architectural patterns

18. Client-server vs. distributed objects • Client-Server • Clients and servers are treated differently • Distributed objects • Interacting objects • Location is irrelevant (taken care of by middleware) • Build on top of client/server • Like distributed library • Java RMI • CORBA Architectural patterns

19. Service-oriented architecture • SOA • Web services • HTTP for transportation • SOAP structured data exchange • UDDI for discovery • WSDL for description • XML used everywhere • Service registry • Service provider • Service requestor Architectural patterns

20. References • Buschmann et al.Pattern-Oriented Software Architecture, Volume1, Wiley 1996 • Martin FowlerPatterns of Enterprise Application Architecture, Addison Wesley 2003 • Ian SommervilleSoftware Engineering, 7th edition, Addison Wesley 2004 11. Architectural Design 12. Distributed System Architectures • Lethbridge & Langaniere Object-Oriented Software Engineering, 2nd edition, McGraw-Hill 2005 9. Architecting and designing software Architectural patterns