Lecture 9, 20-755: The Internet, Summer 1999 1 20-755: The Internet Lecture 9: Web Services II David...

Lecture 9, 20-755: The Internet, Summer 1999 1

20-755: The InternetLecture 9: Web Services II

David O’Hallaron

School of Computer Science and

Department of Electrical and Computer Engineering

Carnegie Mellon University

Institute for eCommerce, Summer 1999


Today’s lecture

• Dynamic content background (35 min)

• Break (10 min)

• Serving dynamic content with GET and POST (40 min)


How programs run other programs

• Recall that a process is an instance of a running program.

• Suppose a process A, which is running program foo, wants to run the program bar.

• Two-step procedure:– First, process A creates a new process B that is a clone

of A

» A and B are independent processes running concurrently on the machine.

» A is the parent, B is the child.

» Each has a unique process id (pid)

– Second, process B recognizes that it is a clone, overwrites foo with bar, and transfers control to the first instruction in bar.



• Initially, foo is running in process A with process id (pid) of 325.

foo

Process A

pid = 325



• Next, program foo running in process A clones a copy of itself.

• So now we have two identical independent processes (A and B) running the same code.

• A can wait immediately for B to complete, or do other work in the meantime.

foo

Process A

foo

Process B

pid = 325

pid = 326



• The instance of foo in process B recognizes that it is a clone.

• Process B foo replaces its code with the code for bar.

foo

Process A

bar

Process B

pid = 325

pid = 326



•pid = fork()– creates a clone of the current process.

– returns a 0 to the child process.

– returns the positive integer process ID of the child to the parent.

•exec(objfile)– replaces the current running program with the code in the

executable file objfile.

– exec never returns to the caller unless there is an error.

» e.g., if it can’t locate objfile.



# This is how program foo running in process A# runs program bar in a new process B # the parent executes this statement$child_pid = fork();

# both parent and child run the if statementif ($child_pid == 0) { # Only the child executes this code print “I’m the child\n” exec(bar); # the child only gets to this point if the # exec fails die “can’t exec bar: $!”; }# the parent continues here


Perl abstractions for fork and exec

• backquote operator– $output = `foo`;

» runs the executable program foo and returns the contents of STDOUT to variable $output.

• system command– system(“foo”, $arg1, arg2);

» runs executable program date.

» output goes to wherever STDOUT is currently going (e.g., the screen)

– system($prog > mydate.txt”)

» redirects output to file mydate.txt


How programs pass info to the programs they create

• Command line arguments– the exec operator can pass a list of ASCII arguments to

the program that it run

» exec(“foo.pl”, “dave”, “ohallaron”);

#!/usr/local/bin/perl5 -w # Array @ARGV holds the arguments.# Acessing @ARGV returns the number of array elements# $0 is the name of the perl script (foo.pl)# $ARGV[0] is the first array element (argument)# $ARGV[1] is the second array element (argument)if (@ARGV != 2) { print "usage: $0 first last\n"; exit; } print "arg0 = $ARGV[0]\n"; # daveprint "arg1 = $ARGV[1]\n"; # ohallaron


How programs pass info to the programs they create

• Environment variables– Each process maintains a set of “environment variables”

» list of ASCII (name,value) pairs.

» represent long term conditions or preferences.

– A forked process gets an exact duplicate of the parent’s environment variables.


Unix shell environment variables

% printenvPWD=/usr/droh/afs/TERM=emacs EMACS=t MANPATH=/usr/man:/usr/local/man:/usr/local/apache/man:/usr/X11R6/man PRINTER=iron login_done=1 HOSTNAME=kittyhawk.cmcl.cs.cmu.edu HOSTTYPE=i386_linux3 HOST=kittyhawk.cmcl.cs.cmu.edu SHLVL=2 KRBTKFILE=/tkt/3478-030d-379b6ada PATH=.:/usr/droh/bin:/usr/sbin:/sbin:/usr/local/apache/bin: /usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:/usr/etc:/etc: /usr/X11R6/bin USER=droh SHELL=/usr/local/bin/tcsh HOME=/usr/droh


Accessing environment variables from PERL

• Environment variables stored in a special hash called “%ENV”

# sort and list the environment variablesforeach $key(sort keys %ENV) { print “$key=$ENV{$key}\n”;}

# add a new (key,value) pair to the environment hash%ENV{“IPADDR”} = “128.1.194.242”;

# delete a (key,value) pair from the environment hashdelete $ENV{“IPADDR”};


Serving dynamic content

client server

• Client sends request to server.

• If request URI contains the string “/cgi-bin”, then the server assumes that the request is for dynamic content.

GET /cgi-bin/env.pl HTTP/1.1



client server

• The server creates a child process and runs the program identified by the URI in that process

env.pl

fork/exec



client server• The child runs and

generates the dynamic content.

• The server captures the content of the child and forwards it without modification to the client

env.pl

content

content



client server

• The child terminates.

• Server waits for the next client request.


Issues in serving dynamic content

• How does the client pass program arguments to the server?

• How does the server pass these arguments to the child?

• How does the server pass other info relevant to the request to the child?

• How does the server capture the content produced by the child?

• These issues are addressed by the Common Gateway Interface (CGI) specification.

client server

content

content

request

create

env.pl


Break time!

Fish


Today’s lecture

• Dynamic content background (35 min)

• Break (10 min)

• Serving dynamic content with GET and POST (40 min)


Issues in serving dynamic content

• How does the client pass program arguments to the server?

• How does the server pass these arguments to the child?

• How does the server pass other info relevant to the request to the child?

• How does the server capture the content produced by the child?

• These issues are addressed by the Common Gateway Interface (CGI) specification.

client server

content

content

request

create

env.pl


CGI

• Because the children are written according to the CGI spec, they are often called CGI programs.

– Because many CGI programs are written in Perl, they are often called CGI scripts.

• However, CGI really defines a simple standard between the client (browser), the server, and the child process.


add.com: THE Internet addition service!

• Ever needed to add two numbers together and you just can’t find your calculator?

• Try Dr. Dave’s addition service at add.com!– Takes as input your name, and two numbers you want to

add together.

– Returns their sum in a tasteful personalized message.

– After the IPO we’ll expand to multiplication!


Serving dynamic content with GET

• Question: How does the client pass arguments to the server?

• Answer: The arguments are appended to the URI

• Can be encoded directly in a URL typed to a browser or a URL in an HTML link

– http://add.com/cgi-bin/add.pl?Dave+O’Hallaron&1&2

– add.pl is the program on the server that will do the addition.

– argument list starts with “?”

– arguments separated by “&”

– spaces represented by “+”

• Can also be generated by an HTML form<form method=get action="http://add.com/cgi-bin/post.pl">



• URL: – http://add.com/cgi-bin/add.pl?Dave+O’Hallaron&1&2

• Result:

Mr. Dave O'Hallaron, Welcome to add.com!

The answer is: 1 + 2 = 3

Please come again soon! Tell your friends!



• Question: How does the server pass these arguments to the child?

• Answer: In environment variable QUERY_STRING– a single string containing everything after the “?”

– for add.com: QUERY_STRING = “Dave+O’Hallaron&1&2”

## Child code that parses the add.com arguments#$args = $ENV{QUERY_STRING}; $args =~ s/\+/ /; #replaces + with “ “($name, $a1, $a2) = split(/&/, $args);



• Question: How does the server pass other info relevant to the request to the child?

• Answer: in a collection of environment variables defined by the CGI spec.


Some CGI environment variables

• General– SERVER_SOFTWARE

– SERVER_NAME

– GATEWAY_INTERFACE (CGI version)

• Request specific– SERVER_PORT

– REQUEST_METHOD (GET, POST, etc)

– QUERY_STRING (contains args)

– REMOTE_HOST (domain name of client)

– REMOTE_ADDR (IP address of client)

– CONTENT_TYPE (for POST, type of data in message body, e.g., text/html)

– CONTENT_LENGTH (length in bytes)


Some CGI environment variables

• In addition, the value of each header of type type received from the client is placed in environment variable HTTP_type

– Examples:

» HTTP_ACCEPT

» HTTP_HOST

» HTTP_USER_AGENT (any “-” is changed to “_”)



• Questions: How does the server capture the content produced by the child?

• Answer: The child writes its content to stdout.

## server code that runs child and captures stdout#

# run the child and put its dynamic content in $child_output$child_output = `add.pl`;

# send the child’s dynamic content back to the client$connfd->print($output)


Putting it all together:The CGI script for

GET requests to add.com

#!/usr/local/bin/perl5 $args = $ENV{QUERY_STRING}; $args =~ s/\+/ /; ($name, $a1, $a2) = split(/&/, $args); print "Content-type: text/html\n\n"; print "<html><head></head><body>\n"; print "<h3>Mr. $name, Welcome to add.com!</h3>\n"; print "<b>The answer is: $a1 + $a2 = ", $a1+$a2, "</b><br>\n"; print "<p><i>Please come again soon! Tell your friends!</i>\n"; print "</body></html>\n";


Serving dynamic content with POST

• More complicated and less general than GET

• Less frequently used because of the complexity.

• Only advantage is that it provides arbitrary-length argument lists

– older browsers and servers had unnecessary limits on URI lengths in GET requests

– doesn’t seem to be a problem anymore



• Question: How does the client pass arguments to the server?

• Answer: In the message body of the HTTP request generated by a form.

– space converted to “+”

– puctuation converted to “%asciihexvalue”

» e.g., apostrophe becomes “%27”


add.com HTML form(form.html)

<html> <body> <form method=post action="http://add.com/cgi-bin/post.pl"> <p>Name <input name="name" type=text SIZE="48"> <p>num1 <input name="num1" type=text SIZE="6"> <p>num2 <input name="num2" type=text SIZE="6"> <p><input type=submit> <input type=reset> </form> </body> </html>


HTTP request generated by add.com form

POST /cgi-bin/post.pl HTTP/1.1 Accept: */* Referer: http://add.com/form.html Accept-Language: en-us Content-Type: application/x-www-form-urlencoded Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 (compatible; MSIE 4.01; Windows 98) Host: add.com Content-Length: 34CRLFname=Dave+O%27Hallaron&num1=1&num2=2



• Questions: How does the server pass the arguments to the child?

• Answer: Arguments are passed as one line via stdin.



• Question: How does the server pass other info relevant to the request to the child?

• Answer: As with GET, in a collection of environment variables defined by the CGI spec.

• Question: How does the server capture the content produced by the child?

• Answer: As with GET, via stdout.

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Lecture 9, 20-755: The Internet, Summer 1999 1 20-755: The Internet Lecture 9: Web Services II David...

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