20-755: The Internet
Lecture 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
Lecture 9, 20-755: The Internet, Summer 1999
1
Today’s lecture
•
•
•
Dynamic content background (35 min)
Break (10 min)
Serving dynamic content with GET and POST
(40 min)
Lecture 9, 20-755: The Internet, Summer 1999
2
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.
Lecture 9, 20-755: The Internet, Summer 1999
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How programs run other
programs
•
Initially, foo is running
in process A with
process id (pid) of 325.
Lecture 9, 20-755: The Internet, Summer 1999
Process A
foo
pid = 325
4
How programs run other
programs
•
•
•
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.
Lecture 9, 20-755: The Internet, Summer 1999
Process A
foo
pid = 325
Process B
foo
pid = 326
5
How programs run other
programs
•
•
The instance of foo in
process B recognizes
that it is a clone.
Process B foo replaces
its code with the code
for bar.
Process A
foo
Process B
bar
Lecture 9, 20-755: The Internet, Summer 1999
pid = 325
pid = 326
6
How programs run other
programs
• 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.
Lecture 9, 20-755: The Internet, Summer 1999
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How programs run other
programs
# This is how program foo running in process A
# runs program bar in a new process B
# the parent executes this statement
$child_pid = fork();
# both parent and child run the if statement
if ($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
Lecture 9, 20-755: The Internet, Summer 1999
8
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
Lecture 9, 20-755: The Internet, Summer 1999
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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"; # dave
print "arg1 = $ARGV[1]\n"; # ohallaron
Lecture 9, 20-755: The Internet, Summer 1999
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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.
Lecture 9, 20-755: The Internet, Summer 1999
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Unix shell environment
variables
% printenv
PWD=/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
Lecture 9, 20-755: The Internet, Summer 1999
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Accessing environment
variables from PERL
•
Environment variables stored in a special
hash called “%ENV”
# sort and list the environment variables
foreach $key(sort keys %ENV) {
print “$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 hash
delete $ENV{“IPADDR”};
Lecture 9, 20-755: The Internet, Summer 1999
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Serving dynamic content
•
•
Client sends request to
server.
If request URI contains
the string “/cgi-bin”, then
the server assumes that
the request is for dynamic
content.
Lecture 9, 20-755: The Internet, Summer 1999
GET /cgi-bin/env.pl HTTP/1.1
client
server
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Serving dynamic content
•
The server creates a child
process and runs the
program identified by the
URI in that process
client
server
fork/exec
env.pl
Lecture 9, 20-755: The Internet, Summer 1999
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Serving dynamic content
•
•
The child runs and
generates the dynamic
content.
The server captures the
content of the child and
forwards it without
modification to the client
Lecture 9, 20-755: The Internet, Summer 1999
client
content
server
content
env.pl
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Serving dynamic content
•
•
The child terminates.
Server waits for the next
client request.
Lecture 9, 20-755: The Internet, Summer 1999
client
server
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Issues in serving dynamic
content
•
•
•
•
•
How does the client pass
program arguments to the
client
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.
Lecture 9, 20-755: The Internet, Summer 1999
request
content server
content
create
env.pl
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Break time!
Fish
Lecture 9, 20-755: The Internet, Summer 1999
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Today’s lecture
•
•
•
Dynamic content background (35 min)
Break (10 min)
Serving dynamic content with GET and POST
(40 min)
Lecture 9, 20-755: The Internet, Summer 1999
20
Issues in serving dynamic
content
•
•
•
•
•
How does the client pass
program arguments to the
client
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.
Lecture 9, 20-755: The Internet, Summer 1999
request
content server
content
create
env.pl
21
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.
Lecture 9, 20-755: The Internet, Summer 1999
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add.com:
THE Internet addition service!
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•
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!
Lecture 9, 20-755: The Internet, Summer 1999
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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">
Lecture 9, 20-755: The Internet, Summer 1999
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Serving dynamic content with GET
•
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!
Lecture 9, 20-755: The Internet, Summer 1999
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Serving dynamic content with GET
•
•
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);
Lecture 9, 20-755: The Internet, Summer 1999
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Serving dynamic content with GET
•
•
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.
Lecture 9, 20-755: The Internet, Summer 1999
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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)
Lecture 9, 20-755: The Internet, Summer 1999
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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 “_”)
Lecture 9, 20-755: The Internet, Summer 1999
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Serving dynamic content with GET
•
•
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)
Lecture 9, 20-755: The Internet, Summer 1999
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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";
Lecture 9, 20-755: The Internet, Summer 1999
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Serving dynamic content with
POST
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•
•
More complicated and less general than GET
Less frequently used because of the
complexity.
Only advantage is that it provides arbitrarylength argument lists
– older browsers and servers had unnecessary limits on
URI lengths in GET requests
– doesn’t seem to be a problem anymore
Lecture 9, 20-755: The Internet, Summer 1999
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Serving dynamic content with
POST
•
•
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”
Lecture 9, 20-755: The Internet, Summer 1999
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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>
Lecture 9, 20-755: The Internet, Summer 1999
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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: 34
CRLF
name=Dave+O%27Hallaron&num1=1&num2=2
Lecture 9, 20-755: The Internet, Summer 1999
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Serving dynamic content with
POST
•
•
Questions: How does the server pass the
arguments to the child?
Answer: Arguments are passed as one line
via stdin.
Lecture 9, 20-755: The Internet, Summer 1999
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Serving dynamic content with
POST
•
•
•
•
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.
Lecture 9, 20-755: The Internet, Summer 1999
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