CSCE 515: Computer Network Programming ------ Web Servers & Dynamic Web Documents Wenyuan Xu http://www.cse.sc.edu/~wyxu/csce515f07.html Department of Computer Science and Engineering University of South Carolina Reference: http://www.cs.rpi.edu/~hollingd/netprog/notes/dyn_doc/dyn_doc.ppt

Web Server „

Talks HTTP

„

Looks at METHOD, URI to determine what the client wants.

„

For GET, URI often is just the path of a file (relative to some directory on the web server).

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CSCE515 – Computer Network Programming

GET /foo/blah /

usr

bin

foo

www

fun

blah 2007

CSCE515 – Computer Network Programming

etc

gif

In the good old days... „

Years ago… the WWW was made up of (mostly) static documents. … Each

URL corresponded to a single file stored on some hard disk.

„

Today - many of the documents on the WWW are built at request time. … URL

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doesn’t correspond to a single file.

CSCE515 – Computer Network Programming

Dynamic Documents „

Dynamic Documents can provide: … automation

of web site maintenance … customized advertising … database access ! a e id l o Co … shopping carts … date and time service

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CSCE515 – Computer Network Programming

Web Programming Writing programs that create dynamic documents has become very important. „ There are a number of general approaches: „

… Create „

custom server for each service desired.

Each is available on different port.

… Have

web server run external programs. … Develop a real smart web server „

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SSI, scripting, server APIs. CSCE515 – Computer Network Programming

Custom Server Write a TCP server that watches a “well known” port for requests. „ Develop a mapping from http requests to service requests. „ Send back HTML (or whatever) that is created/selected by the server process. „ Have to handle http errors, headers, etc. „

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CSCE515 – Computer Network Programming

An Example Custom Server We want to provide a time and date service. „ Anyone in the world can find out the date and time (according to our computer)!!! „

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CSCE515 – Computer Network Programming

Custom Server request „

We don’t care what is in the http request, our reply doesn’t depend on it.

„

We assume the request comes from a browser that wants the content formatted as an HTML document.

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CSCE515 – Computer Network Programming

WWW based time and date server Copyright @2003 DaveH Enterprises

… Listen

on a well known TCP port. … Accept a connection. … Find out the current time and date … Convert time and date to a string … Send back some http headers (Content-Type) … Send the string wrapped in HTML formatting. … Close the connection. loop forever 2007

CSCE515 – Computer Network Programming

Accessing our custom server. We can publish the URL to our server, or embed links to the server in other HTML documents. „ We need to make sure the server is always running (on the published host and port). „ Once we are famous we can include advertisements and make money! „

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CSCE515 – Computer Network Programming

Another Example „

Keep track of how many times our server is hit each day.

„

Report on the number of hits our server got on any day in the past!

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CSCE515 – Computer Network Programming

The Request and Reply „

The reply now does depend on the request.

„

We have to remember that the request comes from a HTTP client, so we need to accept HTTP requests.

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CSCE515 – Computer Network Programming

Time & Date Hit Server Each request comes as a string (URI) specifying a resource. ! t n a li p „ Our requests will look like this: m o C 2K Y /mm/dd/yyyy „ An example URL for our service: „

http://www.timedate.com:4567/02/10/2000

„

We will get a request like: GET /02/10/2000 HTTP/1.1

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CSCE515 – Computer Network Programming

Fancy means $$$ „

We want to provide a table that lists the number of hits received each hour of the day in question timedate.com hit report for 01/17/1999 hour 12-1AM 1-2AM 2-3AM

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number of hits 4,320 18,986 246

CSCE515 – Computer Network Programming

HTML Basics „

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HTML Tables: …

,

…

,

…

,

…

,

start/end a table start/end a table row start/end a table cell start/end table header cell

CSCE515 – Computer Network Programming

timedate.com Hit Table hour 12-1AM 1-2AM 2-3AM 2007

number of hits 4,320 18,986 246

CSCE515 – Computer Network Programming

New code … Record … Read

the “hit” in database.

request - parse request to

month,day,year … Lookup … Send

back some http headers (Content-Type)

… Create … Close

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hits for month,day,year in database.

HTML table and send back to client.

the connection.

CSCE515 – Computer Network Programming

Drawbacks to Custom Server Approach „

We might have lots of ideas custom services. … Each

requires dedicated address (port) … Each needs to include: basic TCP server code „ parsing HTTP requests „ error handling „ headers „ access control (might want users to pay each time they check the time and date!) „

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CSCE515 – Computer Network Programming

Another Approach Take a general purpose Web server (that can handle static documents) and have it process requested documents as it sends them to the client. „ The documents could contain commands that the server understands (the server includes some kind of interpreter). „

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CSCE515 – Computer Network Programming

Example Smart Server Have the server read each HTML file as it sends it to the client. „ The server could look for this: „

<SERVERCODE> some command

The server doesn’t send this part to the client, instead it interprets the command and sends the result to the client. „ Everything else is sent normally. „

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CSCE515 – Computer Network Programming

Example Commands <SERVERCODE> Time <SERVERCODE> Date <SERVERCODE> Hitlist <SERVERCODE> Include file <SERVERCODE> randomfile directory

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CSCE515 – Computer Network Programming

Example Document <TITLE>timedate.com Home Page

Welcome to timedate.com

<SERVERCODE> include fancygraphic The current time is <SERVERCODE> time .

Today is <SERVERCODE> date . Visit our sponser: <SERVERCODE> random sponsor

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CSCE515 – Computer Network Programming

Real Life - Server Side Includes „

Many real web servers support this idea (but not the syntax I’ve shown).

„

Server Side Includes (SSI) provides a set of commands that a server will interpret.

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CSCE515 – Computer Network Programming

SSI Configuration „

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Typically the server is configured to look for commands only in specially marked documents (so normal documents aren’t slowed down).

CSCE515 – Computer Network Programming

SSI Directives SSI commands are called directives „ Directives are embedded in HTML comments. A comment looks like this: „

„

A directive looks like this:

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CSCE515 – Computer Network Programming

Some SSI Directives echo: inserts the value of an environment variable into the page. SSI servers keep a number of useful things in environment variables: DATE_LOCAL

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CSCE515 – Computer Network Programming

SSI echo example The GMT time is: The local time is: Today is

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CSCE515 – Computer Network Programming

SSI Directives include: inserts the contents of a text file.

flastmod: inserts the time and date that a file was last modified. This file last modified This file last modified 2007

CSCE515 – Computer Network Programming

SSI Directives (cont.) exec: runs an external program and inserts the output of the program. Current users:

Danger! Danger! Danger!

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CSCE515 – Computer Network Programming

SSI Example
It is now:
Today is:


This file last modified

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CSCE515 – Computer Network Programming

More Power „ „

Some servers support elaborate scripting languages. Scripts are embedded in HTML documents, the server interprets the script: … Microsoft „

Active Server Pages (ASP)

VBScript, .asp

… JaveServer „

java, .jsp

… Hypertext „

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Pages (JSP)

Preprocessor (PHP)

.php

CSCE515 – Computer Network Programming

Server Mapping and APIs Some servers include a programming interface that allows us to extend the capabilities of the server by writing modules. „ Specific URLs are mapped to specific modules instead of to files. „ We could write our timedate.com server as a module and merge it with the web server. „

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CSCE515 – Computer Network Programming

External Programs „

Another approach is to provide a standard interface between external programs and web servers. … We

can run the same program from any web server. … The web server handles all the http, we focus on the special service only. … It doesn’t matter what language we use to write the external program.

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CSCE515 – Computer Network Programming

Common Gateway Interface CGI is a standard interface to external programs supported by most (if not all) web servers. „ The interface that is defined by CGI includes: „

… Identification

of the service (external

program). … Mechanism for passing the request to the external program.

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CSCE515 – Computer Network Programming

CGI Programming „

CGI programs are often written in scripting languages (perl, tcl, etc.),

„

JAVA Servlets … java

technology’s answer to CGI … Install software that implements Java Servlet „ „ „

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Apache Tomcat JavaServer Web Development Kit Sun’s Java Web Server

CSCE515 – Computer Network Programming

An Servlets example import java.io.* import javax.servlet.*; import javax.servlet.http.*; public class HelloWWW extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println("\n" + "\n" + "<TITLE>Hello WWW\n" + "\n" + "

Hello WWW

\n" + ""); } }

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CSCE515 – Computer Network Programming

Result

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CSCE515 – Computer Network Programming

SSI vs. CGI „

SSI: … Static

HTML + embedded command … add small amounts of dynamic content to pages, without doing a lot of extra work „

CGI … Generate

a complete HTML document in real time

… Powerful

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CSCE515 – Computer Network Programming

Concurrent Server Design Alternatives

Concurrent Server Design Alternatives One child process per client Spawn one thread per client Preforking multiple processes Prethreaded Server

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CSCE515 – Computer Network Programming

One child per client •

Traditional Unix server: … TCP:

after call to accept(), call fork(). … UDP: after recvfrom(), call fork(). … Each process needs only a few sockets. … Small requests can be serviced in a small amount of time. •

Parent process needs to clean up after children!!!! (call wait() ).

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CSCE515 – Computer Network Programming

One thread per client •

•

•

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Almost like using fork - call pthread_create instead. Using threads makes it easier (less overhead) to have sibling processes share information. Sharing information must be done carefully (use pthread_mutex)

CSCE515 – Computer Network Programming

Example main(int argc, char **argv) { listenfd=socket(…) Bind(listenfd…) Listen(listenfd,LISTENQ); Signal(SIGGHLD, sig_chld); Signal(SIGINT,sig_int); For( ; ;) { connfd = Accept(listenfd, …); if ( (pid = Fork())==0) { Close(listendf); doit(connfd); Close(connfd); exit(0); } } Close(connfd); }

Process version 2007

main(int argc, char **argv) { pthread_t tid; listenfd=socket(…) Bind(listenfd…) Listen(listenfd,LISTENQ); For( ; ;) { connfd = Accept(listenfd, …); Pthread_creat(&tid, NULL, &doit, (void *) connfd); } } static void doit (void *arg) { … Close( (int) arg); return NULL; }

CSCE515 – Computer Network Programming

Thread version

Prefork()’d Server •

Creating a new process for each client is expensive.

•

We can create a bunch of processes, each of which can take care of a client.

•

Each child process is an iterative server.

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CSCE515 – Computer Network Programming

Prefork()’d TCP Server • •

• •

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Initial process creates socket and binds to well known address. Process now calls fork() a bunch of times. All children call accept(). The next incoming connection will be handed to one child.

CSCE515 – Computer Network Programming

listen() Sum of both queues cannot exceed backlog

Server accept

TCP

Completed connection queue

3-way handshake complete

Incomplete connection queue

arriving SYN

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CSCE515 – Computer Network Programming

Preforking •

•

•

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As the book shows, having too many preforked children can be bad. Using dynamic process allocation instead of a hard-coded number of children can avoid problems. The parent process just manages the children, doesn’t worry about clients.

CSCE515 – Computer Network Programming

Sockets library vs. system call •

A preforked TCP server won’t usually work the way we want if sockets is not part of the kernel: … calling

accept() is a library call, not an atomic operation.

•

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We can get around this by making sure only one child calls accept() at a time using some locking scheme.

CSCE515 – Computer Network Programming

Prethreaded Server •

Same benefits as preforking.

•

Can also have the main thread do all the calls to accept() and hand off each client to an existing thread.

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CSCE515 – Computer Network Programming

What’s the best server design for my application? •

Many factors: … expected

number of simultaneous clients. … Transaction size (time to compute or lookup the answer) … Variability in transaction size. … Available system resources (perhaps what resources can be required in order to run the service). 2007

CSCE515 – Computer Network Programming

Server Design •

•

•

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It is important to understand the issues and options. Knowledge of queuing theory can be a big help. You might need to test a few alternatives to determine the best design.

CSCE515 – Computer Network Programming

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Related Documents

hour	number of hits
12-1AM	4,320
1-2AM	18,986