1. Advanced Distributed Software Architectures and Technology group ADSaT 1 Application Architectures Ian Gorton, Paul Greenfield

2. Advanced Distributed Software Architectures and Technology group ADSaT 2 Aim Cover some typical N-tier application architectures Look at their strengths and weaknesses Look at a sample complex application

3. Advanced Distributed Software Architectures and Technology group ADSaT 3 N-Tier Applications Layered Architectures –Client layer taking care of user interfaces –Some number of business logic layers implementing business processes and rules –Data layer for accessing data and implementing data integrity rules

4. Advanced Distributed Software Architectures and Technology group ADSaT 4 N-tier Architectures Client layer –Static Web pages –Web-based through CGI programs –Active client-side Web pages Scripting and applets –Active server-side Web pages –Traditional client applications –Background/batch processes

5. Advanced Distributed Software Architectures and Technology group ADSaT 5 N-tier Architectures Business logic –Multiple layers possible Web scripts, server components –Distribute and replicate as necessary –Re-use of logic a primary goal Data access –Components enforce data integrity –Isolate business logic from database –Can be partitioned and replicated

6. Advanced Distributed Software Architectures and Technology group ADSaT 6 N-tier Example Order component Customer component Payment component Order component Goods component Stock component Delivery component MTS*MTS* DCOM/COM*DCOM/COM* Web server Scripted Web pages User interface app Print invoicesPrint ship requests Background processes * Or other transactional ORB of your choice Business Logic Layer Data Access Layer Client Access Layer

7. Advanced Distributed Software Architectures and Technology group ADSaT 7 Pooling Keep a shared pool of expensive or scarce resources –Database connections (ODBC, …) –Objects (COM+, EJB, …) Allocate from pool only when needed Release as soon as not needed Makes stateless transactional object designs really feasible

8. Advanced Distributed Software Architectures and Technology group ADSaT 8 DB Connection Pooling Database connections are… –Expensive to allocate (performance) –Limited in number (scalability)

9. Advanced Distributed Software Architectures and Technology group ADSaT 9 Object Pooling Client keeps reference to object Allocate new object on every method call –Stateless, transactional objects Discard objects at commit time Objects recycled to reduce costs –May only help with expensive objects

10. Advanced Distributed Software Architectures and Technology group ADSaT 10 Object Pooling

11. Advanced Distributed Software Architectures and Technology group ADSaT 11 Architectural Patterns Common models for building systems –Stateless or stateful? –Synchronous or asynchronous? –Data layer access Complexity and correctness –Recovery after failure –Scalability and parallelism –Simple, working and obviously correct is a good goal

12. Advanced Distributed Software Architectures and Technology group ADSaT 12 Stateful Session Server object Clients Server object Factory New Client

13. Advanced Distributed Software Architectures and Technology group ADSaT 13 Characteristics One server object dedicated for each client Server object can hold client- specific information (state) New clients request a Factory object to create them a server object. Can be distributed. Transactions can span multiple calls

14. Advanced Distributed Software Architectures and Technology group ADSaT 14 Issues How long do server objects live? Life of client? Forever? reusable What happens if a server object/node dies unexpectedly? How do clients find a factory? How does load balancing work if load becomes uneven?

15. Advanced Distributed Software Architectures and Technology group ADSaT 15 Stateless Session Server object pool Clients

16. Advanced Distributed Software Architectures and Technology group ADSaT 16 Characteristics Any client can use any server Server object pool can be distributed Server objects maintain no state on behalf of client, and pool DB connections Each client-server interaction is a single transaction

17. Advanced Distributed Software Architectures and Technology group ADSaT 17 Design Issues How big is server object pool? Static Dynamic How do clients decide which server object to connect to? Finding servers? Length of time they use a server Closely related to load balancing...

18. Advanced Distributed Software Architectures and Technology group ADSaT 18 Stateful v Stateless Stateless scales better in practice –Stateful may be faster Stateless is easier to build Stateful can be very useful in some applications: –greatly benefit from data caching –load balancing is not too much of an issue

19. Advanced Distributed Software Architectures and Technology group ADSaT 19 Entity Pattern Customer table Customer Server object Customer Server object Clients Customer Server object Factory 1 2

20. Advanced Distributed Software Architectures and Technology group ADSaT 20 Characteristics Server objects represent specific ‘rows’ in the database Client use a factory to create a server object Server objects have a get/set style interface for each attribute of the database row

21. Advanced Distributed Software Architectures and Technology group ADSaT 21 Issues How is concurrency control handled? 2 clients access same row at same time Scalability - many remote accesses to get/set individual attributes Plus all issues of stateful servers

22. Advanced Distributed Software Architectures and Technology group ADSaT 22 State(ful/less) + Entity Customer table Customer Server object Customer Server object Server object Server object

23. Advanced Distributed Software Architectures and Technology group ADSaT 23 Characteristics Combines service-based patterns with entity pattern Clients access state(ful/less) server objects (see previous slides) Server objects create entity objects as needed Server objects isolated from DBMS - a good thing...

24. Advanced Distributed Software Architectures and Technology group ADSaT 24 Issues All of the previous ones… Need to rely on DBMS locking to handle concurrency Need to be careful with DBMS deadlocks Scales only if server object and entity object in same process/node

25. Advanced Distributed Software Architectures and Technology group ADSaT 25 Synchronous Messaging Server object Client Request Queue Reply Queue

26. Advanced Distributed Software Architectures and Technology group ADSaT 26 Characteristics Client writes a service request to a queue in a transaction, and indicates where to write the results Server object removes request, performs service, and writes results to a reply queue (all in a transaction) Client removes results from reply queue in a transaction

27. Advanced Distributed Software Architectures and Technology group ADSaT 27 Issues Emulating synchronous behaviour with asynchronous technology Breaking 1 transaction up in to 3 client has to remember state for recovery purposes How do we scale up to many clients? many request and reply queues many server objects many queues per server object

28. Advanced Distributed Software Architectures and Technology group ADSaT 28 Asynchronous Messaging Queue Server object Client Request Queue State(ful/less) Server object

29. Advanced Distributed Software Architectures and Technology group ADSaT 29 Characteristics Client makes synchronous call to server object Server object updates DBMS, places request on queue and returns results to client in a transaction Queue server object takes request from queue and updates DBMS in a transaction

30. Advanced Distributed Software Architectures and Technology group ADSaT 30 Issues Used for delayed processing when part of a synchronous transaction is slow write slow part to queue transactionally no need to delay client, process request some time later (but will get processed) Very commonly deployed architecture, one of the strengths of messaging

31. Advanced Distributed Software Architectures and Technology group ADSaT 31 Example

32. Advanced Distributed Software Architectures and Technology group ADSaT 32 Web-based Clients Paul Greenfield

33. Advanced Distributed Software Architectures and Technology group ADSaT 33 The Web A single standard user interface technology –Web browsers A common client platform –Using HTML to get to a Web server A common protocol Except for …. –Many variations and standards to choose from….

34. Advanced Distributed Software Architectures and Technology group ADSaT 34 Static Web Pages Static HTML –Simple forms –No complex logic –Simple HTML is very portable across browsers –Very limited as a tool for building user interfaces No way to include user logic Presentation/forms only

35. Advanced Distributed Software Architectures and Technology group ADSaT 35 CGI Scripts Running logic on the server –The traditional UNIX way –URL references a CGI program rather than a web page –Web server runs a program or script (C, Perl, …) when the page is requested –Program/script runs and returns HTML –Complex to write and scales poorly

36. Advanced Distributed Software Architectures and Technology group ADSaT 36 ISAPI and NSAPI Call a library rather than starting a program –Microsoft and Netscape APIs –Much faster than CGI (no startup time) –Call returns HTML code has to write HTML strings –Still hard to write and complex –Java servlets are a later equivalent

37. Advanced Distributed Software Architectures and Technology group ADSaT 37 Client-side Scripting Add logic to the HTML –Logic in Jscript, VBscript, … –Script sees Web page as an object Manipulate the page being displayed Change text, check values, hide fields,… –Portability? MS, Netscape; v3, v4, v5, … –Useful for building dynamic, responsive Web-based user interfaces

38. Advanced Distributed Software Architectures and Technology group ADSaT 38 Applets Downloadable code run inside the Web page –Java applets, ActiveX controls –Severe security restrictions? Java sandbox (no files, little networking, …) Code signing (ActiveX and Java) –Relegated to niche usage Graphics, expanding indexes, viewers

39. Advanced Distributed Software Architectures and Technology group ADSaT 39 Server-side Scripting Scripted Web pages with the Web server running the script –Easier programming than CGI Script running within a Web page –Script can call on business components Fits in with n-tier model Script does UI/presentation functions –Can produce portable HTML –ASP (Microsoft) and JSP (SUN)

40. Advanced Distributed Software Architectures and Technology group ADSaT 40 N-tier Clients Simple browsers on desktops –Good chance of portability –Simple HTML Server-side scripted Web pages –ASP or JSP –Calling on transactional components for business processes (COM, EJB)

41. Advanced Distributed Software Architectures and Technology group ADSaT 41 Next Week… CORBA