Web Application Architecture and Communication

1. Web Application Architectureand Communication

2. Displaying a Web page in a Browser • <a href= http://www.cems.uwe.ac.uk/~cjwallac/apps/poll2/tally.php?pollid=2 > Poll results</a>

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

4. 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

5. (Level 1 Diagram) a users information need information Browser such as 1 IE6, FoxFire presentation layer HTTP requests HTML files Apache or MS IIS 2 server application + layer PHP SQL requests tables 3 MySQL RDBMS persistence layer

6. Presentation layer arch • Decoding URLs : protocol/host/file • Host name converted to IP address(164.11.8.19) • www.dnsstuff.com • Issue request to remote server using appropriate protocol (usually HTTP) • accept the returned HTML (or JPEG, ..) file • Issue requests for any embedded links (<img src=red.gif/> • 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 (DHTML) • accept user input via a variety of controls on a form

7. Persistence layer arch • interaction with the database using standard languages e.g. SQL queries using database-specific protocol over TCP/IP • 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.

8. Application Layer arch • Server (Apache, IIS) • Identifying appropriate action to take – fetch a file, pass request to an interpreter • Sending output back to caller in MIME package • Support for: • thousands of concurrent users • multi-threading [ allow multiple processes to run concurrently] • caching [holding results in a temporary store to reduce re-calculation] • Server Script (e.g. in PHP) • Interacting with the server (accessing input and generating output) • 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

9. Making sense • We will be using a Patterns approach to structure our learning and analysis of systems and technology • 3-tier architecture is a ‘Layered architecture’ – we will study this in detail next week. • It depends on communication between programs in the layers, so we will look first at communications between processes.

10. 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

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

12. Call – reply pattern • Purpose • A local or remote process must be invoked, sending parameter values to the process and receiving a response back. • Multiple solutions • How parameter values are supplied • How the result is packaged for reply

13. 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 • Parameters identified by POSITION • 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”

14. 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 as a MIME package containing an HTML file • Content-type: text/html

15. 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 packaged as HTML in MIME

16. Server runs a PHP script • The web server is the caller • The getemp.php script is the callee • The parameter is empid=5 • Parameters are name/value couplets • 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

17. Simple Interaction Variations • How are parameters distinguished • By position • By name • How is the reply packaged? • How is the interface defined? • what parameters are required • what the effect of those parameters are • what replies to expect in what circumstances • Does the process always produce the same output for the same input or does is depend in past calls? • 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?

18. Sequence diagram of SMS bus times request

19. 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’