LWP(3) User Contributed Perl Documentation LWP(3)
NAME
LWP - The World-Wide Web library for Perl
SYNOPSIS
use LWP;
print "This is libwww-perl-$LWP::VERSION\n";
DESCRIPTION
The libwww-perl collection is a set of Perl modules which provides a
simple and consistent application programming interface (API) to the
World-Wide Web. The main focus of the library is to provide classes
and functions that allow you to write WWW clients. The library also
contain modules that are of more general use and even classes that help
you implement simple HTTP servers.
Most modules in this library provide an object oriented API. The user
agent, requests sent and responses received from the WWW server are all
represented by objects. This makes a simple and powerful interface to
these services. The interface is easy to extend and customize for your
own needs.
The main features of the library are:
+o Contains various reusable components (modules) that can be used
separately or together.
+o Provides an object oriented model of HTTP-style communication.
Within this framework we currently support access to http, https,
gopher, ftp, news, file, and mailto resources.
+o Provides a full object oriented interface or a very simple
procedural interface.
+o Supports the basic and digest authorization schemes.
+o Supports transparent redirect handling.
+o Supports access through proxy servers.
+o Provides parser for robots.txt files and a framework for
constructing robots.
+o Supports parsing of HTML forms.
+o Implements HTTP content negotiation algorithm that can be used both
in protocol modules and in server scripts (like CGI scripts).
+o Supports HTTP cookies.
+o Some simple command line clients, for instance "lwp-request" and
"lwp-download".
HHTTTTPP SSTTYYLLEE CCOOMMMMUUNNIICCAATTIIOONN
The libwww-perl library is based on HTTP style communication. This
section tries to describe what that means.
Let us start with this quote from the HTTP specification document
:
+o The HTTP protocol is based on a request/response paradigm. A client
establishes a connection with a server and sends a request to the
server in the form of a request method, URI, and protocol version,
followed by a MIME-like message containing request modifiers, client
information, and possible body content. The server responds with a
status line, including the message's protocol version and a success
or error code, followed by a MIME-like message containing server
information, entity meta-information, and possible body content.
What this means to libwww-perl is that communication always take place
through these steps: First a request object is created and configured.
This object is then passed to a server and we get a response object in
return that we can examine. A request is always independent of any
previous requests, i.e. the service is stateless. The same simple
model is used for any kind of service we want to access.
For example, if we want to fetch a document from a remote file server,
then we send it a request that contains a name for that document and
the response will contain the document itself. If we access a search
engine, then the content of the request will contain the query
parameters and the response will contain the query result. If we want
to send a mail message to somebody then we send a request object which
contains our message to the mail server and the response object will
contain an acknowledgment that tells us that the message has been
accepted and will be forwarded to the recipient(s).
It is as simple as that!
The Request Object
The libwww-perl request object has the class name "HTTP::Request". The
fact that the class name uses "HTTP::" as a prefix only implies that we
use the HTTP model of communication. It does not limit the kind of
services we can try to pass this request to. For instance, we will
send "HTTP::Request"s both to ftp and gopher servers, as well as to the
local file system.
The main attributes of the request objects are:
+o The mmeetthhoodd is a short string that tells what kind of request this
is. The most common methods are GGEETT, PPUUTT, PPOOSSTT and HHEEAADD.
+o The uurrii is a string denoting the protocol, server and the name of
the "document" we want to access. The uurrii might also encode various
other parameters.
+o The hheeaaddeerrss contain additional information about the request and can
also used to describe the content. The headers are a set of
keyword/value pairs.
+o The ccoonntteenntt is an arbitrary amount of data.
The Response Object
The libwww-perl response object has the class name "HTTP::Response".
The main attributes of objects of this class are:
+o The ccooddee is a numerical value that indicates the overall outcome of
the request.
+o The mmeessssaaggee is a short, human readable string that corresponds to
the code.
+o The hheeaaddeerrss contain additional information about the response and
describe the content.
+o The ccoonntteenntt is an arbitrary amount of data.
Since we don't want to handle all possible code values directly in our
programs, a libwww-perl response object has methods that can be used to
query what kind of response this is. The most commonly used response
classification methods are:
issuccess()
The request was was successfully received, understood or accepted.
iserror()
The request failed. The server or the resource might not be
available, access to the resource might be denied or other things
might have failed for some reason.
The User Agent
Let us assume that we have created a request object. What do we
actually do with it in order to receive a response?
The answer is that you pass it to a user agent object and this object
takes care of all the things that need to be done (like low-level
communication and error handling) and returns a response object. The
user agent represents your application on the network and provides you
with an interface that can accept requests and return responses.
The user agent is an interface layer between your application code and
the network. Through this interface you are able to access the various
servers on the network.
The class name for the user agent is "LWP::UserAgent". Every libwww-
perl application that wants to communicate should create at least one
object of this class. The main method provided by this object is
request(). This method takes an "HTTP::Request" object as argument and
(eventually) returns a "HTTP::Response" object.
The user agent has many other attributes that let you configure how it
will interact with the network and with your application.
+o The ttiimmeeoouutt specifies how much time we give remote servers to
respond before the library disconnects and creates an internal
timeout response.
+o The aaggeenntt specifies the name that your application should use when
it presents itself on the network.
+o The ffrroomm attribute can be set to the e-mail address of the person
responsible for running the application. If this is set, then the
address will be sent to the servers with every request.
+o The ppaarrsseehheeaadd specifies whether we should initialize response
headers from the section of HTML documents.
+o The pprrooxxyy and nnoopprrooxxyy attributes specify if and when to go through
a proxy server.
+o The ccrreeddeennttiiaallss provide a way to set up user names and passwords
needed to access certain services.
Many applications want even more control over how they interact with
the network and they get this by sub-classing "LWP::UserAgent". The
library includes a sub-class, "LWP::RobotUA", for robot applications.
AAnn EExxaammppllee
This example shows how the user agent, a request and a response are
represented in actual perl code:
# Create a user agent object
use LWP::UserAgent;
$ua = LWP::UserAgent->new;
$ua->agent("MyApp/0.1 ");
# Create a request
my $req = HTTP::Request->new(POST => 'http://search.cpan.org/search');
$req->contenttype('application/x-www-form-urlencoded');
$req->content('query=libwww-perl&mode=dist');
# Pass request to the user agent and get a response back
my $res = $ua->request($req);
# Check the outcome of the response
if ($res->issuccess) {
print $res->content;
}
else {
print $res->statusline, "\n";
}
The $ua is created once when the application starts up. New request
objects should normally created for each request sent.
NNEETTWWOORRKK SSUUPPPPOORRTT
This section discusses the various protocol schemes and the HTTP style
methods that headers may be used for each.
For all requests, a "User-Agent" header is added and initialized from
the $ua->agent attribute before the request is handed to the network
layer. In the same way, a "From" header is initialized from the
$ua->from attribute.
For all responses, the library adds a header called "Client-Date".
This header holds the time when the response was received by your
application. The format and semantics of the header are the same as
the server created "Date" header. You may also encounter other
"Client-XXX" headers. They are all generated by the library internally
and are not received from the servers.
HHTTTTPP RReeqquueessttss
HTTP requests are just handed off to an HTTP server and it decides what
happens. Few servers implement methods beside the usual "GET", "HEAD",
"POST" and "PUT", but CGI-scripts may implement any method they like.
If the server is not available then the library will generate an
internal error response.
The library automatically adds a "Host" and a "Content-Length" header
to the HTTP request before it is sent over the network.
For a GET request you might want to add a "If-Modified-Since" or "If-
None-Match" header to make the request conditional.
For a POST request you should add the "Content-Type" header. When you
try to emulate HTML