/usr/share/doc/swi-prolog-doc/UserGuide/httpd.html

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<H2><A NAME="sec:11.9"><SPAN class="sec-nr">11.9</SPAN> <SPAN class="sec-title">Playing 
WEB (HTTP) server</SPAN></A></H2>

<A NAME="sec:httpd"></A>

<P>Web presentation has attractive features. It is well accepted, 
standardised (if you stick to the basics) and network-transparent. Many 
people think you need a web-server like
<A class="url" href="http://www.apache.org">Apache</A> with some sort of 
server-scripting (<A NAME="idx:CGI:530">CGI</A>) to realise a server. 
This is not true. Any application capable of elementary TCP/IP 
communication can easily act as a web-server.

<P>Using <font size=-1>XPCE</font> for this task may be attractive for a 
number of reasons.

<P>
<UL>
<LI><I>Prototyping</I><BR>
As the <font size=-1>XPCE/P</font>rolog running on your desktop is the 
server you can use the full debugging capabilities of Prolog to debug 
your server application.

<P>
<LI><I>Including graphics</I><BR>
<font size=-1>XPCE</font> can generate GIF and JPEG images for your 
web-pages on the fly. You can include <font size=-1>XPCE</font> <A class="" href="summary.html#class:graphical">graphical</A> 
objects directly in the output and have the server library handle the 
required transformations.

<P>
<LI><I>Remote presentation</I><BR>
<font size=-1>XPCE</font> can be used as <A NAME="idx:groupware:531">groupware</A> 
server, presenting state of the applications and allowing remote users 
to interact using their web-browser<SUP class="fn">13<SPAN class="fn-text">Using 
the Unix/X11 version <font size=-1>XPCE</font> can manage windows on 
multiple desktops. For MS-Windows users this is not supported.</SPAN></SUP>

<P>
<LI><I>Report generation</I><BR>
Applications may to use HTML as framework for report generation. Though 
rather weak in its expressiveness, the advantage is the wide support on 
presentation and distribution applications.
</UL>

<P>We start with a small demo, illustrating frames and text.

<P><A NAME="fig:httpd"></A>
<CENTER>
<IMG SRC="httpd.gif">
</CENTER>
<TABLE ALIGN=center WIDTH="75%"><TR><TD>
<B>Figure 28 : </B>Mozilla showing <font size=-1>XPCE</font> generated 
figure</TABLE>

<PRE class="code">
:- module(my_httpd,
          [ go/1
          ]).
:- use_module(library(pce)).
:- use_module(library('http/httpd')).
:- use_module(library('http/html_write')).
:- use_module(library('draw/importpl')).

%       Create server at Port
<A NAME="idx:go1:532"></A>
go(Port) :-
        new(_, my_httpd(Port)).
<A NAME="idx:myhttpd:533"></A>
:- pce_begin_class(my_httpd, httpd, "Demo Web server").</PRE>

<P><B><CODE>-&gt;</CODE>request</B> is sent after the super-class has 
received a complete request header. We get the `path' and have a Prolog 
predicate generating the replies.

<PRE class="code"><A NAME="idx:myhttpdsendrequest:534"></A>
request(HTTPD, Request:sheet) :-&gt;
        "A request came in."::
        get(Request, path, Path),
        reply(Path, HTTPD).

:- discontiguous
        reply/2.</PRE>

<P><B><CODE>-&gt;</CODE>reply_html</B> takes &lt;<VAR>Module</VAR>&gt;:&lt;<VAR>DCGRuleSet</VAR>&gt; 
to formulate a reply. This uses the html_write library, converting a 
complex Prolog term into a formatted HTML document. The complex term can 
invoke additional DCG rulesets, providing nicely structured 
content-generation.

<PRE class="code">
reply('/', HTTPD) :- !,
        send(HTTPD, reply_html, my_httpd:frames).

frames --&gt;
        html(html([ head(title('Demo')),
                    frameset([cols('25%,75%')],
                             [ frame([ src('/index'),
                                       name(index)
                                     ]),
                               frame([ src('/blank'),
                                       name(body)
                                     ])
                             ])
                  ])).

<A NAME="idx:reply2:535"></A>
reply('/blank', HTTPD) :-
        send(HTTPD, reply_html, my_httpd:blank).

blank --&gt;
        page(title('Blank'),
             []).
<A NAME="idx:reply2:536"></A>
reply('/index', HTTPD) :-
        send(HTTPD, reply_html, my_httpd:index).

index --&gt;
        page(title('Index'),
             [ a([ href('/text'), target(body) ],
                 [ 'Show text' ]),
               br([]),
               a([ href('/picture'), target(body) ],
                 [ 'Show picture' ])
             ]).
<A NAME="idx:reply2:537"></A>
reply('/text', HTTPD) :-
        send(HTTPD, reply_html, my_httpd:text).

text --&gt;
        page(title('Text'),
             [ p(['Just showing a little text'])
             ]).</PRE>

<P>Reply a graphical object. The server translates the graphical to a 
GIF or JPEG bitmap and provides the proper HTTP reply header. You can 
also embed graphicals into the HTML structures used above.

<P>The drawing itself is exported from the demo program PceDraw and 
turned into an XPCE graphical using the support library draw/importpl.

<PRE class="code"><A NAME="idx:reply2:538"></A>
reply('/picture', HTTPD) :-
        make_picture(Gr),
        send(HTTPD, reply, Gr, 'image/gif').
<A NAME="idx:makepicture1:539"></A>
make_picture(Dev) :-
        new(Dev, device),
        drawing(xpcenetscape, Drawing),
        realise_drawing(Dev, Drawing).

%       Drawing imported from PceDraw

drawing(xpcenetscape,
        [ compound(new(A, figure),
                   drawing([ display(box(137, 74)+radius(17),
                                     point(0, 0)),
                             display(text('XPCE', center, normal),
                                     point(52, 30))
                           ]),
                   point(163, 183)),
          compound(new(B, figure),
                   drawing([ display(box(137, 74)+radius(17),
                                     point(0, 0)),
                             display(text('Netscape', center, normal),
                                     point(42, 30))
                           ]),
                   point(350, 183)),
          connect(connection(A,
                             B,
                             handle(w, h/2, link, east),
                             handle(0, h/2, link, west)) +
                    arrows(both))
        ]).

:- pce_end_class(my_httpd).</PRE>

<H3><A NAME="sec:11.9.1"><SPAN class="sec-nr">11.9.1</SPAN> <SPAN class="sec-title">Class <B>httpd</B></SPAN></A></H3>

<P>The library <CODE>library(http/httpd)</CODE> defines the class <B>httpd</B>. 
This subclass of <A class="" href="summary.html#class:socket">socket</A> 
deals with most of the HTTP protocol details, breaking down HTTP 
requests and encapsulating responses with the proper headers. The class 
itself is an <EM>abstract</EM> class, a subclass needs to be created and 
some of the <EM>virtual methods</EM> needs to be refined to arrive at a 
useful application.

<DL>
<DT><STRONG>httpd -&gt;initialise:</STRONG> <VAR>Port:[int]</VAR></DT>
<DD class="defbody">
Create a server and bind it to <VAR>Port</VAR>. If <VAR>Port</VAR> is 
omitted a free port is chosen. With a specified port, 8080 is a commonly 
used alternative to the standard 80 used by web-servers. If you have a 
web-server running on the same machine you may can generate a page on 
your website redirecting a page to this server. The URI of this server 
is <TT>http://&lt;<VAR>host</VAR>&gt;/&lt;<VAR>Port</VAR>&gt;</TT>.</DD>
<DT><STRONG>httpd -&gt;accepted:</STRONG> <VAR></VAR></DT>
<DD class="defbody">
This is sent after a connection has been accepted. The system 
implementation logs the new connection if debugging is enabled. You can 
refine or redefine this method, asking for the <A NAME="idx:socketgetpeername:540"></A>`<B>socket<CODE>&lt;-</CODE>peer_name</B>' 
and sending <B><CODE>-&gt;</CODE>free</B> to the socket if you want to 
restrict access.</DD>
<DT><STRONG>httpd -&gt;request:</STRONG> <VAR>Data:sheet</VAR></DT>
<DD class="defbody">
This is sent from <B><CODE>-&gt;</CODE>input</B> after a complete 
request-header is received.
<B><CODE>-&gt;</CODE>input</B> decodes the header-fields, places them in <VAR>Data</VAR> 
and then calls <B><CODE>-&gt;</CODE>request</B>. The attribute-names in 
the sheet are downcase versions of the case-insensitive request fields 
of the HTTP header. In addition, the following fields are defined:

<P>
<CENTER>
<TABLE BORDER=2 FRAME=box RULES=groups>
<TR VALIGN=top><TD COLSPAN=2 ALIGN=center><B>Fields that are always 
present</B></TR>
<TBODY>
<TR VALIGN=top><TD>request</TD><TD><CODE>GET</CODE>, <CODE>POST</CODE>, 
etc. I.e. the first word of the request-header. In most cases this will 
be
<CODE>GET</CODE>. </TD></TR>
<TR VALIGN=top><TD>path</TD><TD>The `path' part of the request. This is 
normally used to decide on the response. If the path contains a ? 
(question mark) this and the remaining data are removed and decoded to 
the `form' attribute. </TD></TR>
<TR VALIGN=top><TD>form</TD><TD>If the request is a <CODE>GET</CODE> 
request with form-data, the form attribute contains another sheet 
holding the decoded form-data. Otherwise <B><CODE>&lt;-</CODE>form</B> 
holds <A NAME="idx:nil:541"></A><B>@nil</B>. </TD></TR>
<TR VALIGN=top><TD>http_version</TD><TD>Version of the HTTP protocol 
used by the client. Normally
<CODE>1.0</CODE> or <CODE>1.1</CODE>. </TD></TR>
<TBODY>
<TR VALIGN=top><TD COLSPAN=2 ALIGN=center><B>Other fields</B></TR>
<TBODY>
<TR VALIGN=top><TD>user</TD><TD>If authorisation data is present, this 
contains the user-name. If this field is present, the password field is 
present too. </TD></TR>
<TR VALIGN=top><TD>password</TD><TD>Contains the decoded password 
supplied by the user. </TD></TR>
</TABLE>

</CENTER>

<P>After decoding the request, the user should compose a response and 
use
<B><CODE>-&gt;</CODE>reply</B> or <B><CODE>-&gt;</CODE>reply_html</B> to 
return the response to the client.</DD>
<DT><STRONG>httpd -&gt;reply:</STRONG> <VAR></VAR></DT>
<DD class="defbody">
Send a reply. This method or <B><CODE>-&gt;</CODE>reply_html</B> is 
normally activated at the end of the user's <B><CODE>-&gt;</CODE>request</B> 
implementation. Data is one of:

<P>
<UL>
<LI><I>A string or source_sink</I><BR>
If the reply is a <A class="" href="summary.html#class:string">string</A>, <A class="" href="summary.html#class:text_buffer">text_buffer</A>, <A class="" href="summary.html#class:resource">resource</A> 
or <A class="" href="summary.html#class:file">file</A>, the data in this 
object will be returned. Unless otherwise specified <B><CODE>-&gt;</CODE>reply</B> 
assumes the data has <A NAME="idx:mimetype:542">mime-type</A> <CODE>text/plain</CODE>.

<P>
<LI><I>A pixmap</I><BR>
If the reply is a <A class="" href="summary.html#class:pixmap">pixmap</A> 
(or can be converted automatically, for example any <A class="" href="summary.html#class:graphical">graphical</A>), 
this image is encoded as GIF or JPEG and sent with the corresponding <A NAME="idx:imagegif:543">image/gif</A> 
or <A NAME="idx:imagejpeg:544">image/jpeg</A> mime-type. For more 
information on image save-types, see <A NAME="idx:imagesendsavein:545"></A>`<B>image<CODE>-&gt;</CODE>save_in</B>'.
</UL>

<P><VAR>Type</VAR> is the mimi-type returned and tells the browser what 
to do with the data. This should correspond with the content of <VAR>Data</VAR>. 
For example, you can return a PNG picture from a file using

<PRE class="code">
        send(HTTPD, reply, file('pict.png'), 'image/png'),
</PRE>

<P><VAR>Status</VAR> is used to tell the client in a formal way how the 
request was processed. The default is <CODE>200 OK</CODE>. See the 
methods below for returning other values.

<P><VAR>Header</VAR> is a <A class="" href="summary.html#class:sheet">sheet</A> 
holding additional name-value pairs. If present, they are simply added 
to the end of the reply-header. For example if you want to prevent the 
browser caching the result you can use

<PRE class="code">
        send(HTTPD, reply, ...,
             sheet(attribute('Cache-Control', 'no-cache'))),
</PRE>

</DD>
<DT><STRONG>httpd -&gt;reply_html:</STRONG> <VAR></VAR></DT>
<DD class="defbody">
Uses the <CODE>library(http/html_write)</CODE> library to translate <VAR>Term</VAR> 
into HTML text using DCG rules and then invokes <B><CODE>-&gt;</CODE>reply</B> 
using the <VAR>Type</VAR>
<CODE>text/html</CODE>. <VAR>Status</VAR> and <VAR>Header</VAR> are 
passed unmodified to <B><CODE>-&gt;</CODE>reply</B>.
</DD>
</DL>

<P>In addition to the principal methods above, a number of methods are 
defined for dealing with abnormal replies such as denying permission, 
etc.

<DL>
<DT><STRONG>httpd -&gt;forbidden:</STRONG> <VAR>What:[name]</VAR></DT>
<DD class="defbody">
Replies with a <CODE>403 Forbidden</CODE> message. <VAR>What</VAR> may 
be provided to indicate what is forbidden. Default is the path from the 
current
<B><CODE>&lt;-</CODE>request</B>.</DD>
<DT><STRONG>httpd -&gt;authorization_required:</STRONG> <VAR></VAR></DT>
<DD class="defbody">
Challenges the user to provide a name and password. The only method 
provided is <CODE>Basic</CODE>. <VAR>Realm</VAR> tells the user for 
which service permission is requested. On all subsequence contacts from 
this client to this server the <B><CODE>-&gt;</CODE>request</B> data 
contains the <CODE>user</CODE> and
<CODE>password</CODE> fields. The demo implementation of <B><CODE>-&gt;</CODE>request</B> 
in
<B>httpd</B> contains the following example code:

<PRE class="code">
request(S, Header:sheet) :-&gt;
        "Process a request.  The argument is the header"::
        (   get(Header, path, '/no')
        -&gt;  send(S, forbidden, '/no')
        ;   get(Header, path, '/maybe')
        -&gt;  (   get(Header, value, user, jan),
                get(Header, value, password, test)
            -&gt;  send(S, reply, 'You hacked me')
            ;   send(S, authorization_required)
            )
        ;   send(S, reply, 'Nice try')
        ).</PRE>

</DD>
<DT><STRONG>httpd -&gt;not_found:</STRONG> <VAR>What:[char_array]</VAR></DT>
<DD class="defbody">
Reply with a <CODE>404 Not Found</CODE> message, using the request-path 
as default for <VAR>What</VAR>.</DD>
<DT><STRONG>httpd -&gt;moved:</STRONG> <VAR>Where:char_array</VAR></DT>
<DD class="defbody">
Reply with a <CODE>301 Moved Permanently</CODE>. Normally the client 
will retry the request using the URL returned in <VAR>Where</VAR>.</DD>
<DT><STRONG>httpd -&gt;server_error:</STRONG> <VAR>What:[char_array]</VAR></DT>
<DD class="defbody">
Reply with a <CODE>500 Internal Server</CODE> using `<VAR>What</VAR> as 
additional information to the user. This is the default reply if <B><CODE>-&gt;</CODE>request</B> 
fails or raised an exception.
</DD>
</DL>

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