CSE 190: Internet E-Commerce Lecture 5

Exam: 3-Tier Architecture What are the three tiers? –Presentation, application, and data tier Presentation: responsible for generating HTML for the user interface Application: Executing the logic of the site (ordering a book, bidding on an auction, logging in a user) Data tier: Storing all of the persistent data for a user Presentation: purpose of –Firewall Forbids all TCP/IP traffic from entering or exiting the site’s network, unless it is an allowed service. E.g. allow only incoming HTTP requests, no FTP, telnet, SMTP. –Cache Stores commonly access pages (e.g. home page) in faster memory to improve performance –Load balancing server Distributes requests in round-robin fashion (or based upon machine load) to an array of servers –Web server –How to recover from failure? Application Tier –How to recover from failure? The load balancer recognizes that the machine is down and routes requests to a valid application server. Because all state is stored in the data tier, no data is lost when an app server crashes. Data Tier –Why can’t this be combined with application tier?

Exam: HTML Purpose; difference with other doc formats Structure of an HTML document General tag syntax – … Familiar with tags: html, head, body, H#, A, UL, OL, form, input, table, tr, td, br, img, center, frame, script – Components of URL URL encoding ( url?arg1=ab+cd&arg2=ef%20 ) Difference between relative and absolute URL –Relative URLs specify a document relative to the document that contains the URL. E.g. /home/index.html. –Absolute URLs specify a document without any need for context. They specify the complete protocol, domain, and filename. E.g.

Exam: Stylesheets Purpose What is meant by cascading? –More than one stylesheet may be applied at the same time to render a document. The more specific a style takes precedence over less specific styles. For example, inline styles precede internal styles which precede external styles which precede browser stylesheets. General CSS syntax –selector { property: value; } –Example: h1, h2, h3 { text-align: center; } Applying style to: –All tags of type X H1 example: H1 { color: red; margin-left: 20px; } –All elements of class X All elements of class X:.x { font-size: 18pt; } Specify an external stylesheet, internal stylesheet, and inline style –External: –Internal: ….. …. …. –Inline:

Exam: Javascript Purpose How to include in a page Writing text to HTML page Declaring variables –var greeting = “Welcome !” Declaring functions –function foo() { alert( “Foo Fighter” ); } Returning a value –function isEven(x) { return( x % 2 == 0 ); } Displaying an alert When the script executes

Exam: HTTP Status codes, classes of Purpose of: GET, POST, HEAD –HEAD does the same as GET, but without sending the response body. –GET is used for idempotent transactions, i.e. operations which leave the server state unchanged. POST is used for those which alter server state, such as logging in or buying an item. Idempotent: In arithmetic, multiplication by one is idempotent. For a server, GET applied to server-state yields the same server-state. But POST applied to server-state yields new-server- state. Syntax of: GET, POST –POST /cgi-bin/login.pl HTTP/1.0 Content-Length: 10 user=harry Request fields: Host, Content-Length –GET /home.asp HTTP/1.0 Host: –The host field allows multiple domains to be served by the same web server. It is a required field for all HTTP/1.1 clients to send. –Content-Length: Used by POST to indicate how many bytes are in the body of a POST request Response fields: Content-Type –Content-Type indicates the MIME type of a document, so the browser knows how to handle the response body. Examples include: text/html, text/plain, image/gif, image/jpeg, video/asf.

Exam: Web Servers Purpose of Apache multi-client model Apache configuration: Purpose of –Port Indicates what TCP/IP port the server is listening to. The default for HTTP is 80. –DocumentRoot Where to find the root document for a web site and its subdirectories and files. Usually: apache_install_directory/htdocs/ –ServerName Name of the machine where the web server is running –User HTTP authentication SSL Virtual Hosts CGI

Example Exam Questions (2 pts) Draw & explain the relationship between a load balancer and a cluster of web servers. (3 pts) Show the URL that represents a GET request for domain ucsd.edu and document /student/ski/home.asp with two arguments, task with value open and the second shibboleth with “ali baba” (no quotes). – (3 pts) Explain two advantages of stylesheets over plain HTML. Write an HTML file that applies an internal style to all DIV tags such that the attribute font-size is 18. – DIV { font-size: 18pt; } Look below This will be in 18pt

Example Exam (cot’d) (3 pts) Write an HTML page that contains a form taking username and password and submits it to ucsd.edu/form.cgi using HTTPS. Onlookers may not view the password. –… Enter your username: Enter your password: (3 pts) Write HTML code that uses Javascript to write “Hello World!” to the browser window. –… document.write( “Hello World!” ); (2 pts) Write a Javascript function that takes two numbers and returns their sum. (2 pts) Using Javascript to access the DOM, access the second form on the page and assign it to a variable. Username: var myform = document.forms[ 1 ]; // Show the HTML making up the form alert( "My form looks like this: " + myform.innerHTML );

Example Exam (cot’d) (3 pts) Show an HTTP/1.0 client send a request for a document hello.gif from an HTTP 1.1 server. Show both request and response headers. Indicate when the connection closes. –GET /hello.gif HTTP/1.0 HTTP/ OK Content-Type: image/gif Content-Length: 1018 GIF87aFDSU)(&#$)(&%(*$&)&#$%)…. Connection closed by server (2 pts) What is the purpose of DocumentRoot in Apache’s configuration file (httpd.conf)? (2 pts) What happens to your web server if a hostile user crashes a child process? How does the parent recover? Are pages still served? –Pages are still served. The other children process may still respond to user requests. The parent notices that the child has terminated by polling periodically. When the number of children falls below a threshold, it creates a new child process to handle requests. Since the parent never handles requests itself, delegating that task to its children, the parent rarely crashes. This leads to a more robust web server.