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".
HTTP STYLE COMMUNICATION
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::Requests both to ftp and gopher servers, as well as to the
local file system.
The main attributes of the request objects are:
o method is a short string that tells what kind of request this is.
The most common methods are GET, PUT, POST and HEAD.
o uri is a string denoting the protocol, server and the name of the
"document" we want to access. The uri might also encode various
other parameters.
o headers contains additional information about the request and can
also used to describe the content. The headers are a set of
keyword/value pairs.
o content 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 code is a numerical value that indicates the overall outcome of the
request.
o message is a short, human readable string that corresponds to the
code.
o headers contains additional information about the response and
describe the content.
o content 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:
is_success()
The request was successfully received, understood or accepted.
is_error()
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 timeout specifies how much time we give remote servers to respond
before the library disconnects and creates an internal timeout
response.
o agent specifies the name that your application uses when it presents
itself on the network.
o from 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 parse_head specifies whether we should initialize response headers
from the "" section of HTML documents.
o proxy and no_proxy specify if and when to go through a proxy server.
o credentials provides 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.
An Example
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;
my $ua = LWP::UserAgent->new;
$ua->agent("MyApp/0.1 ");
# Create a request
my $req = HTTP::Request->new(POST => 'http://search.cpan.org/search');
$req->content_type('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->is_success) {
print $res->content;
}
else {
print $res->status_line, "\n";
}
The $ua is created once when the application starts up. New request
objects should normally created for each request sent.
NETWORK SUPPORT
This section discusses the various protocol schemes and the HTTP style
methods that headers may be used for each.
For all requests, a "User-Agent" header is added and initialized from
the "$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.
HTTP Requests
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 an "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