Wapp - A Web-Application Framework for TCL

1.0 Introduction

Wapp is a simple and lightweight framework that strives to simplify the construction of web application written in TCL. The same Wapp application can be launched in multiple ways:

• From the command-line (ex: tclsh app.tcl). In this mode, The wapp-app find an available TCL port on localhost, starts an in-process web server listening on that port, and then launches the users default web browser directed at localhost:$port

• As a CGI program

• As an SCGI program

• As a stand-alone web server

All four methods of launching the application provide the same interface to the user.

1.0 Hello World!

Wapp is designed to be easy to use. A hello-world program is as follows:

package require wapp
proc wapp-default {} {
   wapp "<h1>Hello, World!</h1>\n"
}
wapp-start $::argv

The application defines one or more procedures that accept HTTP requests. For an HTTP request where the initial portion of the URI is "abcde", the procedure named "wapp-page-abcde" will be invoked to construct the reply. If no such procedure exists, "wapp-default" is invoked instead.

The procedure generates a reply using one or more calls to the "wapp" command. Each "wapp" command appends new text to the reply.

The "wapp-start" command starts up the built-in web server.

To run this application, but the code above into a file named "main.tcl" and then run type "tclsh main.tcl" at the command-line. That should cause the "Hello, World!" page to appear in your web browser.

1.1 A Slightly Longer Example

Information about each HTTP request is encoded in the global ::wapp() array variable. The following sample program shows the information available in ::wapp().

package require wapp
proc wapp-default {} {
  global wapp
  wapp "<h1>Hello, World!</h1>\n"
  wapp-unsafe "<p>See the <a href='$wapp(BASE_URL)/env'>Wapp "
  wapp "Environment</a></p>"
}
proc wapp-page-env {} {
  wapp "<h1>Wapp Environment</h1>\n"
  wapp "<p><ul>\n"
  foreach var [lsort [array names ::wapp]] {
    wapp <li>
    wapp-escape-html "wapp($name) = $::wapp($name)\n"
  }
  wapp "</ul></p>"
}
wapp-start $::argv

In this application, the default "Hello, World!" page has been extended with a hyperlink to the /env page. The "wapp-unsafe" command works exactly the same as "wapp" (it appends its argument text to the web page under construction) except that the argument to "wapp-unsafe" is allowed to contain TCL variable and command expansions. The "wapp" command will generate a warning if its argument contains TCL variable or command expansions, as a defense against accidental XSS vulnerabilities.

The /env page is implemented by the "wapp-page-env" proc. This proc generates an HTML unordered list where each element of the list describes a single value in the global ::wapp() array. The "wapp-escape-html" command is like "wapp" and "wapp-unsafe" except that "wapp-escape-html" escapes HTML markup so that it displays correctly in the output.

1.2 The ::wapp() Global Variable

To better understand how the ::wapp() array works, try running the previous sample program, but extend the /env URL with extra path elements and query parameters. For example: [http://localhost:8080/env/longer/path?q1=5&title=hello+world%21]

Notice how the query parameters in the input URL are decoded and become elements of the ::wapp() array. The same thing occurs with POST parameters and cookies - they are all converted into entries in the ::wapp() array variable so that the parameters are easily accessible to page generation procedures.

The ::wapp() variable contains additional information about the request, roughly corresponding to CGI environment variables. To prevent environment information from overlapping and overwriting query parameters, all the environment information uses upper-case names and all query parameters are required to be lower case. If an input URL contains an upper-case query parameter (or POST parameter or cookie), that parameter is silently omitted from the ::wapp() variable

The ::wapp() variable contains the following environment values:

• HTTP_HOST
  The hostname (or IP address) and port that the client used to create the current HTTP request. This is the first part of the request URL.

• HTTP_USER_AGENT
  The name of the web-browser or other client program that generated the current HTTP request.

• HTTPS
  If the HTTP request arrived of SSL, then this variable has the value "on". For an unencrypted request, the variable does not exist.

• REMOTE_ADDR
  The IP address and port from which the HTTP request originated.

• SCRIPT_NAME
  In CGI mode, this is the name of the CGI script in the URL. In other words, it is the initial part of the URL path that identifies the CGI script. For other modes, this variable is an empty string.

• PATH_INFO
  The part of the URL path that follows the SCRIPT_NAME. For all modes other than CGI, this is exactly the URL pathname.

• REQUEST_URI
  The URL for the inbound request, without the initial "http://" or "https://" and without the HTTP_HOST.

• REQUEST_METHOD
  "GET" or "HEAD" or "POST"

• BASE_URL
  The text of the request URL through the SCRIPT_NAME. This value can be prepended to hyperlinks to ensure that the correct page is reached by those hyperlinks.

• PATH_HEAD
  The first element in the PATH_INFO. The value of PATH_HEAD is used to select one of the "wapp-page-XXXXX" commands to run in order to generate the output web page.

• PATH_TAIL
  All of PATH_INFO that follows PATH_HEAD.

1.3 Additional Wapp Commands

The following utility commands are available for use by applications built on Wapp:

• wapp-start ARGLIST
  Start up the application. ARGLIST is typically the value of $::argv, though it might be some subset of $::argv if the containing application has already processed some command-line parameters for itself.

• wapp TEXT
  Add TEXT to the web page output currently under construction. TEXT must not contain any TCL variable or command substitutions.

• wapp-unsafe TEXT
  Add TEXT to the web page under construction even though TEXT does contain TCL variable and command substitutions. The application developer must ensure that the variable and command substitutions does not allow XSS attacks.

• wapp-encode-html TEXT
  Add TEXT to the web page under construction after first escaping any HTML markup contained with TEXT.

• wapp-encode-url TEXT
  Add TEXT to the web page under construction after first escaping any characters so that the result is safe to include as the value of a query parameter on a URL.

• wapp-mimetype MIMETYPE
  Set the MIME-type for the generated web page. The default is "text/html".

• wapp-reply-code CODE Set the reply-code for the HTTP request. The default is "200 Ok".

• wapp-redirect TARGET-URL
  Cause an HTTP redirect to the specified URL.

• wapp-reset
  Reset the web page under construction back to an empty string.

• wapp-set-cookie [-path PATH] [-expires DAYS] NAME VALUE
  Cause the cookie NAME to be set to VALUE.

• wapp-send-hex HEX
  Cause the HTTP reply to be binary that is constructed from the hexadecimal text in the HEX argument. Whitespace in HEX is ignored. This command is useful for returning small images from a pure script input. The "wapp-file-to-hex" command can be used at development time to generate appropriate HEX for an image file.

• wapp-cache-control CONTROL
  The CONTROL argument should be one of "no-cache", "max-age=N", or "private,max-age=N", where N is an integer number of seconds.

• wapp-etag ETAG
  Set the expiration tag for the web page.

• wapp-send-file FILENAME
  Make the content of the file FILENAME be the HTTP reply.

• wapp-send-query DB SQL
  Run the SQLite query SQL on the DB database connection and make the HTTP reply be the value of the first column of the first row in the result.

• wapp-set-csp POLICY
  Set the Content Security Policy for the application. This command only works for command-line and server modes. This command is a no-op for CGI and SCGI since there is no standard way of communicating the desired content security policy back to the server in those instances.

• wapp-debug-port PORT
  For debugging use only: open a listening TCP socket on PORT and run an interactive TCL shell on connections to that port. This allows for interactive debugging of a running instance of the Wapp server. This command is a no-op for short-lived CGI programs, obviously. Also, this command should only be used during debugging, as otherwise it introduces a severe security vulnerability into the application.

• wapp-safety-check
  Examine all TCL procedures in the application and report errors about unsafe usage of "wapp".

1.4 Design Rules

All global procs and variables used by Wapp begin with the four character prefix "wapp". Procs and variable intended for internal use begin with the seven character prefix "wappInt".