Information Systems Development 3 Lecture 2

1. Information Systems Development 3Lecture 2 3-tier application architecture A technical view

2. Themes • Architecture : The large scale structure of a system, especially a computer system • Communications : The interaction between components • Description: Patterns and story-telling • Choice • Design decisions: design decisions which face a developer • Infrastructure choice: selecting the most appropriate software from the available range.

3. Displaying a Web page in a Browser • <a href= http://www.cems.uwe.ac.uk/~cjwallac/UFIE8V-2004/l2_3tier.html>Lecture 2</a> • URL is decoded : protocol/host/file • Page requested from host • Page is returned • HTML codes determine how text is rendered • If page contains links e.g. <img src=red.gif>, browser must repeat the same process on each

4. Dynamic Web page needed when: • consistent look and feel on each page of a large site • data is derived from a database • depends on real time • pages depend on user choice • business transactions e.g. e-commerce…

5. 3-tier architecture • APresentation layer using Browser technology • An Application layer using a web application server platform + application programs • A Persistence layer using a relational database or other data store technology

7. Presentation layer arch • Issue request to remote server • accept the returned HTML (or JPEG..) file • render (i.e. create a 2-d image ) the HTML • allow plug-ins to handle new file types • execute client-side scripts in JavaScript • support interaction between client-side scripts and the web page • accept user input via a variety of controls on a form

8. Persistence layer arch • interaction with the database using standard languages e.g SQL queries and table returns • define and modify the data structures (e.g. tables) themselves ( the Database Schema) • insert, update and delete data • maintain data persistently, with backup and recovery • handle transactions to support concurrent access to the database via locking et • optimise access by compilation of queries, indexing, replication of tables etc.

9. Application Layer arch • accepting requests from the Presentation layer • interpreting the requests according to business rules and past transactions from this client • requesting the appropriate data from the Persistence layer • computing derived data • creating the HTML (or GIF, MIDI..) for the page • providing run-time support for: • thousands of concurrent users • compilation to machine code • multi-threading [ allow multiple processes to run concurrently] • caching [holding results in a temporary store to reduce re-calculation]

10. Web Application Platforms • Server side includes - files with a .shtml extension • Cgi using Perl or other scripting language • PHP like SSI, a scripting language which is embedded in an HTML page. • Microsoft’s ASP (Active Server Pages ) • J2EE • JSP for page scripting • Java + class library • Multiple runtime vendors Tomcat, BEA • .NET is the latest technology from MS. • range languages (VB.NET, C#, , C++, Cobol... ) • compiled to a common intermediate code (MSIL) • Extensive class library • ASP.NET scripting page scripting

11. 3-tier Issues • Advantages: • Re-use of appropriate software • Software can be located on different machines for convenience and performance • Resolves one-many relationships – many clients – one application, many applications – one database • Challenges: • well-defined, standardised interfaces between layers required. • Software must be built to conform to interface standards • The designer has to decide where to locate specific functions

12. Communication • The ‘glue’ in this architecture is communication between software in the layers • A single request from a user results in a complex flurry of communications and executions • The flurry is composed of hundreds of simple interactions • Sequence diagrams can be useful to provide a simplified description

13. Simple Interaction • Call/reply pattern callee caller parameters function reply

14. A function call in PHP • <?php print date(“H:I”); ?> in a php script • the script is the caller • the date procedure is the callee • the string “H:I” is the parameter – • the function is to get the current date and time – • the reply – a string containing the time in the required format e.g. “13:45”

15. A browser requests a page • The user clicks on a link on a page. The link has an href=http://host/dir/page.html • The browser is the caller • The web server program(Apache or IIS) on the machine host is the callee. • The string “dir/page.html” is the parameter • The function is to read that file in its file store • The reply is the contents of the file

16. A browser requests a script to execute • The user clicks on a link on a page. href=http://host/dir/getemp.php?empno=5 • The browser is the caller • The web server on the machine host is the callee. • The string “dir/getemp.php?empno=5” is the parameter • The function when the suffix is php is to find the script, call it with the parameter and get the output. • The reply is the reply from getemp.php

17. Server runs a PHP script • The web server is the caller • The getemp.php script is the callee • The parameter is empid = 5 • The function is to find the details of employee 5 and format a report • to perform this function, the script calls mysql_connect procedures which in turn calls the mySQL server …. • The reply is HTML text

18. Simple Interaction Variations • How does the caller find the callee? • What does the caller do when is waiting for a reply? • What happens if the callee doesn’t reply? • How long should the caller wait until it thinks the callee isn’t going to reply? • Is a reply always required?

19. Sequence diagram of SMS bus times request

20. A Pattern • A pattern recurs in different guises • have the same overall structure and purpose, • with slight differences • Patterns help us understand and describe complex systems • Patterns ‘normalise’ descriptions – one thing in one place • Same unit plays different ‘roles’ • webserver is callee with respect to (wrt) browser, caller wrt getemp.php • Patterns are ‘fractal’

23. Other layered architectures • ANSI/SPARC Database model • The external or user level, characterised by Views • The Conceptual level, comprising the full corporate data model at a logical level • The Physical level, where data is stored in files

24. Channels between Layers • Layered communications model (Tanenbaum) • Application Layer • Transport Layer • Network Layer • Data Link Layer • Physical Layer • Presentation <> Middleware • HTTP (The protocol which supports GET and POST messages) carrying a limited number of types of content (MIME types) • Middleware <> Data stores • ODBC with SQL request and table returns (whole table or row at a time)

25. Review Questions • Place the following terms in the 3-tier architecture: • JavaScript, SSI, J2EE, MySQL, Oracle, CSS, PHP, ASP.NET, Flash, Java Applets, JSP, Dreamweaver, Visual .NET

26. Straw Poll application • Which parts belong to which tier of the 3-tier model script? • What is the ER model of the database? • What must be done to make selected improvements to this application? • Describe the interaction when a user votes; when the tally page is requested.

27. Platform Comparision • Enterprise solutions • J2EE versus .NET • Simple words such as “better” or “faster” are best used by simpletons. Life, especially when it involves critical enterprise applications, is more complicated. • PHP versus .NET • Language wars

28. Comparison criteria • Other considerations, such as the perception of the vendor as an “enterprise company”, conformance to standards, strength of the ecosystem around the product, the degree to which the product is a de facto standard amongst its class, stability of the product or technology, ease of use, IDE integration and other tool support, total cost of ownership (TCO), portability and lock-in (or lack thereof), scalability and many other such factors often outweigh Performance among our customers.