fcl-web

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What is fpWeb

fpWeb can be used to build cgi applications. We need more details here: what other functionality does it have, how does it compare to other frameworks/tools, e.g. Brook

Using fpWeb together with Lazarus

Installing the weblaz fpWeb Lazarus Package

The first step to do is installing the package which comes in the path lazarus/components/fpweb/weblaz.lpk. As usual with design-time packages, you'll have to rebuild Lazarus.

Creating a CGI application

After the weblaz package is installed, a very simple CGI web application which displays an HTML page can be created by going to the Lazarus menu "File->New...". From the list of possible applications select "CGI Application" as in the image below, which will create a main CGI project file and a fpweb web module.

New cgi.PNG

The TFPWebModule allows you to manipulate properties and events using the Object Inspector.

To add code to show the page, a request handler should be added. To do this, double click the OnRequest property in the object inspector, as in the image below:

Webmodule.PNG

In the event handler one should write the HTML code which will be displayed by the browser. To avoid mixing Pascal and HTML, this page can be loaded from the directory the CGI executable is in by using AResponse.Contents.LoadFromFile().

The type of the response should be set in AResponse.ContentType. For HTML pages this should have the value 'text/html;charset=utf-8'.

Finally, Handled should be set to True to indicate that the request was successfully handled (and return a 200 OK status code to the browser). After adding this code, the web module should look like this:

unit mainpage;
 
{$mode objfpc}{$H+}
 
interface
 
uses
  Classes, SysUtils, FileUtil, HTTPDefs, websession, fpHTTP, fpWeb; 
 
type
 
  { TFPWebModule1 }
 
  TFPWebModule1 = class(TFPWebModule)
    procedure DataModuleRequest(Sender: TObject; ARequest: TRequest;
      AResponse: TResponse; var Handled: Boolean);
  private
    { private declarations }
  public
    { public declarations }
  end; 
 
var
  FPWebModule1: TFPWebModule1; 
 
implementation
 
{$R *.lfm}
 
{ TFPWebModule1 }
 
procedure TFPWebModule1.DataModuleRequest(Sender: TObject; ARequest: TRequest;
  AResponse: TResponse; var Handled: Boolean);
begin
  AResponse.ContentType := 'text/html;charset=utf-8';
  AResponse.Contents.LoadFromFile(ExtractFilePath(ParamStr(0)) + 'mainpage.html');
  Handled := True;
end;
 
begin
  RegisterHTTPModule('TFPWebModule1', TFPWebModule1);
end.

Deploying the CGI application

This section assumes the Apache web server is used. Of course, other web servers that support CGI (nginx, cherokee) can be used, too.

Apache can be downloaded here: http://httpd.apache.org/download.cgi or installed using your distribution's package manager.

The default installation of Apache will treat all files located in its cgi-bin directory as CGI programs, so the user won't be able to access plain HTML files placed there. This directory can be set in the file httpd.conf in the following section:

   #
   # ScriptAlias: This controls which directories contain server scripts.
   # ScriptAliases are essentially the same as Aliases, except that
   # documents in the target directory are treated as applications and
   # run by the server when requested rather than as documents sent to the
   # client.  The same rules about trailing "/" apply to ScriptAlias
   # directives as to Alias.
   #
   ScriptAlias /cgi-bin/ "C:/Program Files/Apache Software Foundation/Apache2.2/cgi-bin/"

If you place an executable called "mywebpage.cgi" in this directory, then the page can be accessed as http://localhost/cgi-bin/mywebpage.cgi (or remotely with the corresponding IP address or domain name).

fcl-web with Lazarus on Windows produces .exe files. For Apache to serve these files you have to add this:

    AddHandler cgi-script .exe

And to serve your executables from another directory, you add this:

   ScriptAlias /bin/ "C:/lazarus_projects/test-fclweb/bin/"
   <Directory "C:/lazarus_projects/test-fclweb/bin/">
       AllowOverride None
       Options None
       Order allow,deny
       Allow from all
   </Directory>

Formatting the HTML and Reading Query fields

The previous example just showed a plain HTML page. One might wish to e.g.

  • dynamically change the HTML page output, and
  • as read the variables the browser passed to the webpage in Query fields (in e.g. HTTP GET and HTTP POST actions).

A simple solution for the first problem is simply using the standard Pascal routine Format and adding %s or %d in the HTML file in the appropriate places which will receive a custom value.

Reading GET data

To read the GET variables one can use ARequest.QueryFields, which is a TStrings descendent. Each variable will be in a separate line in the TStrings in the format variablename=value, similarly to how they are shown in the browser page address. To search for a specific variable one can use ARequest.QueryFields.Values[], passing the variable name in the brackets to receive its value back, instead of parsing the string manually.

The resulting code:

procedure TFPWebModule1.DataModuleRequest(Sender: TObject; ARequest: TRequest;
  AResponse: TResponse; var Handled: Boolean);
var
  HexText, AsciiText: string;
begin
  HexText := ARequest.QueryFields.Values['hex'];
  AsciiText := HexToAnsii(HexText);
 
  AResponse.ContentType := 'text/html;charset=utf-8';
  AResponse.Contents.LoadFromFile(ExtractFilePath(ParamStr(0)) + 'mainpage.html');
  AResponse.Contents.Text := Format(AResponse.Contents.Text,
    [HexText, AsciiText]);
  Handled := True;
end;

Reading POST data

Data submitted by POST requests can be obtained from TRequest.Content. It will come without the request headers - in other words it contains the body of the request.

Submitted form data can also be accessed using TRequest.ContentFields which is the content parsed as fields separated by & and decoded. For example, the following set of form values:

login: dfg 345&&
login_senha: ====
email: dfg

Will be encoded in 'APPLICATION/X-WWW-FORM-URLENCODED' like this (see wikipedia POST (HTTP)):

login=dfg+345%26%26&login_senha=%3D%3D%3D%3D&email=dfg

And will be available in TRequest.ContentFields via the line index or using the Values property, which is more convenient:

TRequest.ContentFields[0]: login=dfg 345&&
TRequest.ContentFields[1]: login_senha=====
TRequest.ContentFields[2]: email=dfg
TRequest.ContentFields.Values['email']: dfg

If using other mime-types than 'MULTIPART/FORM-DATA' and 'APPLICATION/X-WWW-FORM-URLENCODED' (the types supported by HTML forms):

  • content is only available in TRequest.Content, not TRequest.ContentFields
  • use of these mime-types raises an exception for FPC 2.4.2 (fixed in FPC 2.5+).

Note that HTTP POST can also send Query fields (in the URL), e.g. http://server/bla?question=how, and those are accessed by TRequest.QueryFields as explained in the previous section.

procedure TFPWebModule1.DataModuleRequest(Sender: TObject; ARequest: TRequest;
  AResponse: TResponse; var Handled: Boolean);
var
  lData: String;
begin
  lData := ARequest.Content;

Reading POST binary data

To receive data on the server that has been POSTed e.g. as multipart/form-data, use something like this:

procedure TMainWebModule.TFPWebActions2Request(Sender: TObject;
  ARequest: TRequest; AResponse: TResponse; var Handled: Boolean);
var
  i: Integer;
begin  
  // Process all received files
  for i := 0 to ARequest.Files.Count - 1 do
  begin
    // Doing something else than writeln is highly recommended ;)
    writeln ('Received Filename: '+ARequest.Files[i].LocalFileName);
  end;
 
  Handled := true;
end;

The client can send data using e.g. FileFormPost.

Using multiple modules

If there is only one module in the web application, all requests will be directed to this module.

As your web application grows, multiple modules can be used. A new module can be added by choosing 'File - New' and then one of 'Web module' or 'HTML Web Module'.

FCL-web uses the URL to determine how a HTTP request should be handled. It must therefore know which web-modules exist in the application. To achieve this, each module must be registered.

Each module is registered with fcl-web in the initialization section of the unit it is defined in:

RegisterHTTPModule('location', TMyModule);

The module will then be invoked if an URL of the form

http://www.mysite.org/mycgi.cgi/location

or

http://www.mysite.org/mycgi.cgi?module=location

is used.

If multiple modules are present, the name of the module must appear in the URL, or an error will be raised.

This behaviour can also be forced for applications that have only a single module by setting the Application's property AllowDefaultModule to false:

Application.AllowDefaultModule := False;

In that case, the fcl-web application will always require the name of the module in the URL.

The name of the request variable that determines the module name (by default, this is 'module') can be set in the Application.ModuleVariable property. The following code

Application.ModuleVariable := 'm';

ensures that the following URL is directed to TMyModule:

http://www.mysite.org/mycgi.cgi?m=location

If all this is not enough to determine the module to which the request should be passed, the Application.OnGetModule event can be used. It is of type TGetModuleEvent:

type
  TGetModuleEvent = Procedure (Sender : TObject; ARequest : TRequest;
                               Var ModuleClass : TCustomHTTPModuleClass) of object;

Creating an event handler for this event allows fine control over the module that is created to handle the request: the request (passed in ARequest) can be examined, and the 'ModuleClass' variable must be set to the class of the module that should handle the request.

If 'ModuleClass' is 'Nil' on return, an error will be sent to the browser.

Using Actions

A module can be used to group certain kinds of actions that logically belong together. Imagine a module TUserModule that is used to handle user management in the webpages. There can be multiple actions associated with a user:

  • Creating
  • Deleting
  • Editing
  • Displaying

These different actions can be handled by the same module. One can determine the action from the URL manually, as in:

http://mysite/mycgi.cgi/user?action=delete&id=12

This can be automated.

In order to make it easier to distinguish between various actions, the module has a property actions: this is a collection, in which each item is associated with a different response to the request. The actions have various properties:

Name 
The name of the action. The URL will be examined to determine the name of the action.
Content 
A string. If set, this is sent to the browser.
Contents 
A stringlist. If set, this is sent to the browser.
ContentProducer 
If set, the contentproducer will handle the request.
Default
if set to 'True', then this action is the default action. That means that if FCL-Web cannot determine the action name, this action will be executed.
Template 
If set, this template will be processed, and the results sent to the browser.

There are also some events:

BeforeRequest 
executed before the request is processed. Can be used to set the 'Content' or other properties.
AfterRequest 
executed after the request is processed.
OnRequest 
an event handler to handle the request. If set, the handler is used to handle the request.

Again, as in the case of multiple modules, the URL is used to determine which action to execute. The part right after the module part ("user" in this example) is used:

http://mysite/mycgi.cgi/user/delete&id=12

would execute the action named 'delete'. This is equivalent to the URL

http://mysite/mycgi.cgi/user?action=delete&id=12

The 'ActionVar' property of the module can be used to set the name of the request variable to use. Setting

UserModule.ActionVar := 'a';

reduces the above URL to

http://mysite/mycgi.cgi/user?a=delete&id=12

If there is only one module in the application, the URL can be shortened to

http://mysite/mycgi.cgi/delete&id=12

Using HTML Templates

For information about using templates, template tags and template tag parameters to generate response pages, please refer to the fptemplate.txt file under your FPC directory in /packages/fcl-base/texts/.

Example projects that demonstrate using templates can be found under your FPC directory in /packages/fcl-web/examples/fptemplate/ (see the README.txt in there for more). (In earlier versions, these examples were in the Lazarus directory in /components/fpweb/demo/fptemplate/)

Tips & troubleshooting

Sending result codes

To send a different HTTP response than 200 OK, use AResponse.Code and AResponse.CodeText, e.g.

AResponse.Code:=404;
AResponse.CodeText:='Document not found';

Sending binary data

An approach that seems to work to send e.g. a tiff file from the web server to the client - adapted from $(fpcdirectory)\packages\fcl-web\examples\jsonrpc\demo1\wmdemo.pp - something like:

var
 
AResponse.ContentStream := TMemoryStream.Create;
try
  AResponse.ContentStream.LoadFromFile('/tmp/sample.tiff');
  AResponse.ContentType := 'image/tiff'; //or whatever MIME type you want to send
// to do: there is an fpweb example that gets the mime type from the file extension...
  AResponse.ContentLength:=AResponse.ContentStream.Size; //apparently doesn't happen automatically?
  AResponse.SendContent;
finally
  AResponse.ContentStream.Free;
end;
Handled := true;


Error: Could not determine HTTP module for request

You may have multiple modules and multiple actions. If you specify an URL with only 1 item, like:

http://localhost/cgi-bin/somemodule

then fpweb assumes you're specifying an action. If you don't have a default module set, you will get a 500 internal server error (Could not determine HTTP module for request)

You can modify this behaviour to let fpweb map to a module name instead by setting the application's PreferModuleName property to true.

Error: response code 500 Internal Server error when trying to handle DELETE method

In FPC 2.6.2 and lower, fcl-web does not accept the DELETE method and generates an error.

Notes

  • The cgiapp unit is deprecated, please use fpcgi as much as possible.
  • If you deploy your CGI application and get errors like "Error: No action name and no default action", you should make sure there's an action assigned to the URL, or catch non-supported actions with an action marked Default. In both cases, an OnRequest event handler for the action that sets Handled:=true should be used.

See also

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