11.9 Playing WEB (HTTP) server

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 Apache with some sort of server-scripting (CGI) to realise a server. This is not true. Any application capable of elementary TCP/IP communication can easily act as a web-server.

Using XPCE for this task may be attractive for a number of reasons.

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

Figure 28 : Mozilla showing XPCE generated figure
:- 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

go(Port) :-
        new(_, my_httpd(Port)).

:- pce_begin_class(my_httpd, httpd, "Demo Web server").

->request is sent after the super-class has received a complete request header. We get the `path' and have a Prolog predicate generating the replies.

request(HTTPD, Request:sheet) :->
        "A request came in."::
        get(Request, path, Path),
        reply(Path, HTTPD).

:- discontiguous

->reply_html takes <Module>:<DCGRuleSet> 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.

reply('/', HTTPD) :- !,
        send(HTTPD, reply_html, my_httpd:frames).

frames -->
        html(html([ head(title('Demo')),
                             [ frame([ src('/index'),
                               frame([ src('/blank'),

reply('/blank', HTTPD) :-
        send(HTTPD, reply_html, my_httpd:blank).

blank -->

reply('/index', HTTPD) :-
        send(HTTPD, reply_html, my_httpd:index).

index -->
             [ a([ href('/text'), target(body) ],
                 [ 'Show text' ]),
               a([ href('/picture'), target(body) ],
                 [ 'Show picture' ])

reply('/text', HTTPD) :-
        send(HTTPD, reply_html, my_httpd:text).

text -->
             [ p(['Just showing a little text'])

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.

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

reply('/picture', HTTPD) :-
        send(HTTPD, reply, Gr, 'image/gif').

make_picture(Dev) :-
        new(Dev, device),
        drawing(xpcenetscape, Drawing),
        realise_drawing(Dev, Drawing).

%       Drawing imported from PceDraw

        [ 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)),
                             handle(w, h/2, link, east),
                             handle(0, h/2, link, west)) +

:- pce_end_class(my_httpd).

11.9.1 Class httpd

The library library(http/httpd) defines the class httpd. This subclass of socket deals with most of the HTTP protocol details, breaking down HTTP requests and encapsulating responses with the proper headers. The class itself is an abstract class, a subclass needs to be created and some of the virtual methods needs to be refined to arrive at a useful application.

httpd ->initialise: Port:[int]
Create a server and bind it to Port. If Port 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 http://<host>/<Port>.
httpd ->accepted:
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 `socket<-peer_name' and sending ->free to the socket if you want to restrict access.
httpd ->request: Data:sheet
This is sent from ->input after a complete request-header is received. ->input decodes the header-fields, places them in Data and then calls ->request. 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:

Fields that are always present
requestGET, POST, etc. I.e. the first word of the request-header. In most cases this will be GET.
pathThe `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.
formIf the request is a GET request with form-data, the form attribute contains another sheet holding the decoded form-data. Otherwise <-form holds @nil.
http_versionVersion of the HTTP protocol used by the client. Normally 1.0 or 1.1.
Other fields
userIf authorisation data is present, this contains the user-name. If this field is present, the password field is present too.
passwordContains the decoded password supplied by the user.

After decoding the request, the user should compose a response and use ->reply or ->reply_html to return the response to the client.

httpd ->reply:
Send a reply. This method or ->reply_html is normally activated at the end of the user's ->request implementation. Data is one of:

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

        send(HTTPD, reply, file('pict.png'), 'image/png'),

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

Header is a sheet 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

        send(HTTPD, reply, ...,
             sheet(attribute('Cache-Control', 'no-cache'))),
httpd ->reply_html:
Uses the library(http/html_write) library to translate Term into HTML text using DCG rules and then invokes ->reply using the Type text/html. Status and Header are passed unmodified to ->reply.

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

httpd ->forbidden: What:[name]
Replies with a 403 Forbidden message. What may be provided to indicate what is forbidden. Default is the path from the current <-request.
httpd ->authorization_required:
Challenges the user to provide a name and password. The only method provided is Basic. Realm tells the user for which service permission is requested. On all subsequence contacts from this client to this server the ->request data contains the user and password fields. The demo implementation of ->request in httpd contains the following example code:
request(S, Header:sheet) :->
        "Process a request.  The argument is the header"::
        (   get(Header, path, '/no')
        ->  send(S, forbidden, '/no')
        ;   get(Header, path, '/maybe')
        ->  (   get(Header, value, user, jan),
                get(Header, value, password, test)
            ->  send(S, reply, 'You hacked me')
            ;   send(S, authorization_required)
        ;   send(S, reply, 'Nice try')
httpd ->not_found: What:[char_array]
Reply with a 404 Not Found message, using the request-path as default for What.
httpd ->moved: Where:char_array
Reply with a 301 Moved Permanently. Normally the client will retry the request using the URL returned in Where.
httpd ->server_error: What:[char_array]
Reply with a 500 Internal Server using `What as additional information to the user. This is the default reply if ->request fails or raised an exception.