1. 1 CS 501 Spring 2007 CS 501: Software Engineering Lecture 14 System Architecture and Design 2

2. 2 CS 501 Spring 2007 Administration

3. 3 CS 501 Spring 2007 Architectural Styles An architectural style is system architecture that recurs in many different applications. See: Mary Shaw and David Garlan, Software architecture: perspectives on an emerging discipline. Prentice Hall, 1996

4. 4 CS 501 Spring 2007 Architectural Style: Pipe Example: A three-pass compiler Parser Lexical analysis Code generation Output from one subsystem is the input to the next.

5. 5 CS 501 Spring 2007 Architectural Style: Master File Update Master file update Data input and validation Mailing and reports Example: billing system for electric utility Advantages: Efficient way to process batches of transactions. Disadvantages: Information in master file is not updated immediately. Customer services

6. 6 CS 501 Spring 2007 Architectural Style: Repository Repository Input components Transactions Example: A digital library Advantages: Flexible architecture for data-intensive systems. Disadvantages: Difficult to modify repository since all other components are coupled to it.

7. 7 CS 501 Spring 2007 Architectural Style: Repository with Storage Access Layer Data Store Input components Transactions Advantages: Data Store subsystem can be changed without modifying any component except the Storage Access. Storage Access This is sometimes called a "glue" layer Repository

8. 8 CS 501 Spring 2007 Data Intensive Systems: Merger of Two Banks Each bank has a database with its customer accounts. The databases are used by staff at many branches and for back-office processing. These systems are examples of Repository Architectural Style. The requirement is to integrate the two banks so that they appear to the customers to be a single organization and to provide integrated service from all branches. This is an example of working with legacy systems.

9. 9 CS 501 Spring 2007 Merger of Two Banks: Options ??? A B

10. 10 CS 501 Spring 2007 Merger of Two Banks: Interface between the Databases Accounts database Batch input Teller transactions Accounts database Batch input Teller transactions Bank A Bank B Both systems follow the repository style

11. 11 CS 501 Spring 2007 Merger of Two Banks: Architectural Options I.Convert everything to System A convert databases retrain staff enhance System A (software and hardware) discard System B II.Build an interface between the databases in System A and System B III.Extend client software so that it can interact with either System A or System B database

12. 12 CS 501 Spring 2007 Merger of Two Banks: Interface between the Databases Accounts database Batch input Teller transactions Accounts database Batch input Teller transactions Bank ABank B Data exchange API Problem Accounts databases are rarely exactly equivalent. Data trans- form

13. 13 CS 501 Spring 2007 Data Intensive Systems: Distributed Data Distributed Data Data is held on several computer systems. A transaction may need to assemble data from several sources.

14. 14 CS 501 Spring 2007 Architectures for Distributed Computing An application that is running on one computer wishes to use data or services provided by another: Network connection private, public, or virtual private network location of firewalls Protocols point-to-point, multicast, broadcast message passing, RPC, distributed objects stateful or stateless Performance quality of service

15. 15 CS 501 Spring 2007 Architectural Style: Client/Server Web example: Serving static pages Firefox client Apache server The control flows in the client and the server are independent. communication between client and server follows a protocol. In a peer-to-peer architecture, the same component acts as both a client and a server.

16. 16 CS 501 Spring 2007 Architectural Style: Three Tier Architecture Web example: Serving dynamic pages Each of the tiers can be replaced by other components that implement the same interfaces Presentation tier Application tier Database tier

17. 17 CS 501 Spring 2007 Distributed Computing: Multicast User interface service User Databases 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). Three tier architecture: Broadcast searching

18. 18 CS 501 Spring 2007 Distributed Computing: Stateless Protocol v. Stateful Stateless protocol Example: http Open connection Send message Return reply Close connection State in http must be sent with every message (e.g., as parameter string) Cookies are a primitive way of retaining some state

19. 19 CS 501 Spring 2007 Distributed Computing: Stateless Protocol v. Stateful Stateful (session) protocol Example: Z39.50 Open connection Begin session Interactive session End session Close connection Client and server remember the results of previous transactions (e.g., authentication, partial results) until session is closed.

20. 20 CS 501 Spring 2007 Networks: Quality of Network Services Criteria in choosing a network architecture Performance Maximum throughput Latency Variations in throughput Real-time media (e.g., audio) Business Suppliers, cost Trouble shooting and maintenance

21. 21 CS 501 Spring 2007 Networks: Choices Public Internet: Ubiquitous -- worldwide Low cost Private network: Security / reliability Predictable performance Choice of protocols (not constrained to TCP/IP)

22. 22 CS 501 Spring 2007 Networks: Routers and Other Network Computing Interoperation with third party devices => remote devices may have faulty software Restart after total failure Defensive programming -- must survive => erroneous or malicious messages => extreme loads => time outs, dropped packets, etc. Evolution of network systems => Support for several versions of protocols

23. 23 CS 501 Spring 2007 Networks: Firewall Public network Private network Firewall A firewall is a computer at the junction of two network segments that: Inspects every packet that attempts to cross the boundary Rejects any packet that does not satisfy certain criteria, e.g., an incoming request to open a TCP connection an unknown packet type Firewalls provide security at a loss of flexibility and a cost of system administration.

24. 24 CS 501 Spring 2007 Distributed Data: Replication Replication Several copies of the data are held in different locations. Mirror: Complete data set is replicated Cache: Dynamic set of data is replicated (e.g., most recently used) With replicated data, the biggest problems are concurrency and consistency.

25. 25 CS 501 Spring 2007 Distributed Computing: Distributed Caches.edu server cornell.edu server cs.cornell.edu server First attempt to resolve www.cs.cornell.edu 1 2 3 The Domain Name System

26. 26 CS 501 Spring 2007 Distributed Computing: Distributed Caches.edu server cornell.edu server cs.cornell.edu server Better method 3 1 almaden.ibm.com cornell.edu ece.cmu.edu ibm.com acm.org.edu 2 Local cache local DNS server The Domain Name System

27. 27 CS 501 Spring 2007 Distributed Computing: Distributed Caches For details of the actual protocol read: Paul Mockapetris, "Domain Names - Implementation and Specification". IETF Network Working Group, Request for Comments: 1035, November 1987. http://www.ietf.org/rfc/rfc1035.txt?number=1035 The Domain Name System

28. 28 CS 501 Spring 2007 Distributed Computing: Intermittent Connectivity This is an example of an epidemic protocol. Such protocols are especially useful in networks with intermittent connectivity, e.g., mobile computing. The biggest problem is ensuring that the data is distributed effectively. Example: Usenet

29. 29 CS 501 Spring 2007 Time-Critical Systems A real time (time-critical) system is a software system whose correct functioning depends upon the results produced and the time at which they are produced. A soft real time system is degraded if the results are not produced within required time constraints A hard real time system fails if the results are not produced within required time constraints

30. 30 CS 501 Spring 2007 Time-Critical System: Buffering a CD Controller for Automobile Input block Output block 1 2 3 4 5 6 7 Circular buffer

31. 31 CS 501 Spring 2007 Time Critical System: Architectural Style - Daemon Daemon Example: Web server The daemon listens at port 80 When a message arrives it: spawns a processes to handle the message returns to listening at port 80 Spawned process

32. 32 CS 501 Spring 2007 Time Critical System: Architectural Style - Model/Controller/View Model Controller View Example: An unmanned aircraft Controller: Sends control signals to the aircraft and receives instrument readings. Model: Translates data received from and sent to the aircraft into a model of flight performance. It uses domain knowledge about the aircraft and flight. View: Displays information about the aircraft to the user.

33. 33 CS 501 Spring 2007 Time-Critical System: Autonomous Land Vehicle Sensors GPS Sonar Laser Signal processing ModelControl signals Steer Throttle Controls

34. 34 CS 501 Spring 2007 Time Critical System: Autonomous Land Vehicle Model Sensors View Extend model/controller/view Controller

35. 35 CS 501 Spring 2007 Software Considerations In some types of system architecture, non-functional requirements of the system may dictate the software design and development process.

36. 36 CS 501 Spring 2007 Software Considerations: Time-Critical System Developers of advanced time-critical software spend almost all their effort developing the software environment: Monitoring and testing -- debuggers Crash restart -- component and system-wide Downloading and updating Hardware troubleshooting and reconfiguration etc., etc., etc.

37. 37 CS 501 Spring 2007 Software Considerations: Performance Resource considerations may dictate software design and implementation: Low level language (e.g., C) where programmer has close link to machine Inter-process communication may be too slow (e.g., C fork). May implement special buffering, etc., to control timings

38. 38 CS 501 Spring 2007 Software Considerations: Multi-Threading Several similar threads operating concurrently: Re-entrant code -- separation of pure code from data for each thread May be real-time (e.g., telephone switch) or non-time critical The difficult of testing real-time, multi-threaded systems may determine the entire software architecture. Division into components, each with its own acceptance test.

39. 39 CS 501 Spring 2007 Software Considerations: Embedded Real-time Systems Software and hardware are combined to provide an integrated unit, usually dedicated to a specific task: Digital telephone Automobile engine control GPS Scientific instruments Seat bag controller The software may be embedded in the device in a manner that cannot be altered after manufacture.

40. 40 CS 501 Spring 2007 Software Considerations: Embedded Real-time Systems Design of embedded systems requires close understanding of hardware characteristics Special purpose hardware requires special tools and expertise. Some functions may be implemented in either hardware of software (e.g., floating point unit) Design requires separation of functions Distinction between hardware and software may be blurred. Hardware v. Software

41. 41 CS 501 Spring 2007 Software Considerations: Continuous Operation Many systems must operate continuously Software update while operating Hardware monitoring and repair Alternative power supplies, networks, etc. Remote operation These functions must be designed into the fundamental architecture.

42. 42 CS 501 Spring 2007 Coupling and Cohesion Coupling is a measure of the dependencies between two subsystems. If two systems are strongly coupled, it is hard to modify one without modifying the other. Cohesion is a measure of dependencies within a subsystem. If a subsystem contains many closely related functions its cohesion is high. An ideal breakdown of a complex system into subsystems has low coupling between subsystems with high cohesion within subsystems. *