2002.09_Advanced Web Applications with Tclhttpd.pdf

PROGRAMMING

Tclhttpd

Delivery Service

A number of techniques are now

Programmed in Tcl,the Tclhttpd web server is an ideal platform for advanced

available for web applications in

Tcl. In addition to CGI scripts, Tcl

modules for Apache or fully featured

application servers, you might like to take

a look at Tclhttpd. The webserver is a 100

per cent Tcl code development with a

long history, and this is reflected in the

current version. The Tcl webserver’s

functions provide an ideal platform for

advanced web applications. This article

demonstrates various approaches for the

creation of HTML pages and also in the

processing of requests.

Tclhttpd’s origins go back to 175 lines

of Tcl that Brent Welch wrote in the mid

90s. The codebase has grown since to

encompass something in the region of

12,000 lines, and this does not include

the extensive Tcllib library. This stable

codebase supports speedy deployment in

various areas. Tclhttpd can:

• Serve static web sites

• Run Server Side Includes

• Link individual URLs, even whole

directories, or various MIME types with

Tcl scripts

• Embed Tcl code in HTML

• Read and write cookies

• Manage sessions

• Authenticate users

• Evaluate forms

• Upload files to servers

• Support email

The development activities were never

intended to rival the king of the hill,

Apache. If you are having to contend with

several hundred requests per second, Tcl

modules such as mod_tcl [5] or

mod_websh [6] for Apache are definitely

a better bet. But if you are looking to

develop web applications for small to

medium volume web sites, Tclhttpd will

provide you with a solid base from which

you can work.

web applications thatcan also profitfrom the ease and speed of development

thatTcl offers.Tcl library functions and numerous extensions are available

including a library thattakes the hard work outof generating HTML code.

BY CARSTEN ZERBST

of traffic. Other reference applications

include a global network for meteorologi-

cal data from airports or the Medusa

Project [4] that accesses a large-scale

database. But if you talk to the users, you

will normally find that Tcl has mainly

been used for internal projects.

Sourceforge has the Tclhttpd source

files [1]. There are two versions: the all-

inclusive variant tclhttpd-3.2-dist

including Tcl, Thread and Tcllib [3] and

the current version tclhttpd-3.3.1 . The

older package’s advantage is ease of

installation. The following steps are all

that you need to do to get a web server up

and running:

Suitable for

projects of all sizes

But Tclhttpd does not need to hide its

light under a bushel – after all it does

host http://www.tcl.tk and this website

has to cope with a considerable volume

# tar -xzf tclhttpd3.2-dist.tarU

.gz

# cd tclhttpd3.2-dist/tclhttpd3U

# make

# make install

# cd bin

September 2002 www.linux-magazine.com

Web applications with the Tcl web server

Tclhttpd

PROGRAMMING

# wish httpd.tcl

Running with 256 file U

descriptor limit

httpd started on port 8015

# Do not create threads

Config threads 0

Config main [file join U

[Config home] httpdthread.tcl]

The Direct_Url /listing2 .html listing2

command assigns the URL http:// local-

host:8015/listing2.html to the listing2

procedure. The Tcl procedure creates and

returns the required page. The variables

available in the script, for example env ,

are interesting, as they are being used to

store information on the current client

connection. The html::tableFromArray

command formats the content of the

global variable producing the result

shown in Figure 1.

Just launch your browser and point it at

http://localhost:8015 to view the sample

files that show you some of the package’s

capabilities. You can configure the server

via the tclhttpd .rc file; the following is a

listing that contains an example of some

of the options available.

# Logfile: /usr/local/httpd/log

Config LogFile /usr/local/U

httpd/log

Config LogFlushMinutes 0

When you start out on a development

project, it makes sense to use the content

in the sample directory, to leverage the

control panel and statistics features. The

control panel reads variables from

browsers or reloads the libraries, and

both these functions are very useful for

when it comes to debugging.

Tclhttpd can create web page content

dynamically at runtime and it supports

various approaches to do so. The easiest

way to go is to configure a Direct_Url :

The server will then pass requests for the

URL to the configured Tcl procedure. In

contrast to a CGI script the server will not

spawn a new process but it will run the

procedure directly in its own server

process. This allows you to use variables

from the server for counters or to open

database connections.

# Sample configuration

# httpd running as user 500

# in group 100

Config uid 500

Config gid 100

Direct_Url /listing2.html U

listing2

proc listing2 {args} {

puts stderr $args

# httpd listening on port 8015,U

normal hostname

Config host [info hostname]

Config port 8015

set html "<html>"

append html "<body>"

append html [html::tableFromU

Array ::env "border=1" *]

append html "</body>U

</html>"

return $html

# Custom scripts in

# .../custom directory

Config library [file join U

[Config home] .. custom]

}

# HTML files in

# /usr/local/httpd/htdocs

Config docRoot /usr/local/U

httpd/htdocs

The script must be stored in the contrib

directory in order for the server to find it.

Tclhttpd reads all the scripts in this direc-

tory automatically at startup.

During the development phase the lib

directory is also useful. Scripts stored in

this directory still need to be loaded

explicitly in the main script, but this

allows you to reload them later from the

control panel with the Reload Source

function, which provides a facility for

on the fly code modification. If an error

is discovered in a script the Tclhttpd

displays the debugging information

directly as an HTML page for easy

viewing.

DirectUrl Dynamics

The next listing shows a simple example.

Figure 1:The procedure detailed in Listing†2 outputs the contentof the global Tcl variable ::env .The vari-

able contains entries thatyou should recognize from CGI scripts,such as HTTP_USER_AGENT .

ElegantTemplates

Templates are a more elegant solution

than using Direct Url and comprise an

HTML document with embedded Tcl

code. The Tcl elements are encapsulated

in brackets – the return value of the func-

tion that immediately precedes the

closing bracket is passed to the HTML

page. You can use the value of the vari-

able in the whole template and not only

in the Tcl elements.

If you want to work with templates,

you will need to place a copy of the .tml

file and the libtml directory taken from

www.linux-magazine.com September 2002

PROGRAMMING

Tclhttpd

the distribution in the htdocs directory.

The sample template in the next listing

first defines a variable, that contains the

value of the env in HTML code. Lower

down in the template, the content is then

integrated into the page using $later . The

last section of the template contains the

date of the last modification. We can see

that a template can therefore encompass

a varying mixture of scripts, variables

and HTML.

Tclhttpd automatically updates the

saved version if the template is newer

than the saved results, or if the browser

calls the template directly. As you can see

in the following listing, the [Doc_ U

Dynamic] can be used to suppress the

caching functionality.

Templates provide an easy migration

path that you can use to gradually

upgrade static web sites with dynamic

functions. In addition to simple counters,

navigation toolbars would be obvious

candidates, since a single procedure

could create them for the whole site.

htdocs/libtml/sunscript.tcl contains a

sample of source code from the former

Sunscript page.

set message "no project U

selected"

}

]

<hr>

$message

<form action=$page(url) U

method=POST>

Text: <input type=text

[html::formValue text]> <br>

Project: [html::radioSet U

project { } {

"Project 1" project1

"Project 2" project2

}] <p>

<input value="Send" U

type=submit>

</form> <p>

<html>

<head> <title> SimpleU

Template</title> </head>

<body>

A simple template.

[ set later "watch this"

html::tableFromArray U

::env "border=1" *

Interactive Templates

Web pages with user input are the next

hurdle for a web application to overcome.

The interactions of this kind basically

comprise of two elements: an HTML page

provides the user interface in the browser

and a script running on the server that

evaluates the input. It makes sense for a

template to create the form and evaluate

it, allowing the template in turn to return

a modified version of the formula in case

of input errors. In case of valid input, the

template would then transfer the browser

to a different page.

Input was:

[html::tableFromList U

[ncgi::nvlist] "border=1"]

</body>

</html>

]

<p>

$later

<hr>

Last change

[clock format [file mtime U

$::env(PATH_TRANSLATED)]]

</body>

</html>

The sample script in the listing above

shows how a template can edit a form.

Although the template is short, it pro-

vides heaps of functionality. First the

[Doc _Dynamic] command prevents the

form from caching the template, which

would make no sense at all.

The next Tcl block handles the data

input using a few functions from the Ncgi

package in Tcllib. For example,

ncgi::empty checks whether an entry for

the project field in the form exists. In this

case the request is passed via Doc_Redi-

Templates use the .tml file suffix and are

stored just like normal HTML documents

in htdocs. When a browser requests the

listing3.html, the server first actions the

listing3.tml template and then returns the

result. Tclhttpd addtionally writes the

result to listing3.html on the hard disk

and uses it for any further requests. This

kind of caching is particularly practical

for templates that either perform some

complex calculations or contain slow

database queries.

<html>

<head> <title>Entries<U

/title></head>

<body>

[Doc_Dynamic]

[ if {![ncgi::empty U

project] } {

Doc_Redirect [ncgi::valueU

project].html?[ncgi::query]

} else {

TABLE 1: INSTRUCTIONS FOR HTML AND CGI

Instruction

Meaning

HTML

html::h1 Title

Produces a heading,also "html::h2" and "html::h3"

html::tableFromArray ArrayName

Produces a HTML table from a Tcl array

html::checkbox Name Value

Produces a Checkbox

html::trxtInputName Parameter

Produces a textinput

Ncgi

ncgi::cookie Cookie Returns a listof values for Cookie

ncgi::setCookie - name Name -Value Value Sets a Cookie

ncgi::empty Name

Indicates whether an inputvalue is present

Figure 2:Mozilla displaying the contentof a cookie

thatoriginated in the template of the “cookie”list-

ing above.The cookie was setby the server at

192.168.42.150 ,is called test and contains the value 42 .

ncgi::value Key

Returns the CGI value identified by Key

ncgi::nvlist

Returns all the query as a name,value list

ncgi::query

Returns the raw query data

September 2002 www.linux-magazine.com

Tclhttpd

PROGRAMMING

Figure 3:The googbar looks like the Google home page,butin factitis a Tcl

program thatredirects search operations directly to Google.

Figure 5: dndspy (bottom) shows the MIME types and actions allowed with drag

objects.Grabbing a message (flying page) from Evolution (top).

Figure 4:The BWidgets demo application shows the capabilities of this

toolkit.The widgets are written in Tcl and only use Tk.

Data crumbs

Having to retype

data is clumsy, and

not all browsers

can or want to

store user input.

Although it is easy

to store data on the

server, it is by no means trivial to assign it

to a specific user. HTTP is stateless and

requests are normally independent of any

previous requests.

Cookies provide a solution to this

known problem: The server asks the

browser to store some data (a cookie) on

the client. If the user revisits the site, and

has not deleted the cookie meanwhile,

the server can read the cookie it asked the

browser to store.

The template in the next listing first

checks the browser for the answer cookie.

If the cookie exists, it outputs the value, if

not, the template attempts to write a

cookie. Apart from the server that sets the

cookie values the browser itself can also

read the cookie, as you can see in Figure

2. Cookies can often be troublesome,

especially from a legal point of view (data

protection and so on); they also pose a

security risk. You will want to try to avoid

using them as a building block of your

application logic.

if {[string length U

[Doc_Cookie answer]] > 0} {

set html "Cookie answer is U

[Doc_Cookie answer]"

} else {

Doc_SetCookie -name answer U

-value 42 -path $page(url)

set html "Set Cookie U

answer=42"

}]

</body>

</html>

rect to the HTML page for this project. If

this does not occur, a message is then

stored in the variable message and it is

written to the page by $message, which is

the message construct.

The middle section inside the template

contains a simple form, whose input can

be returned to the template by the use of

HTTP-POST . The form contains a text

input box and radio buttons, which have

been assigned to project1 and project2 ,

and are labeled Project 1 and Project 2 . At

the end of the script the user input is for-

matted. This is then output so that it can

be used for debugging purposes.

When a user first opens the listing4.html

page, no project has been selected. In this

case clicking on the Send button will

return the user to the same page. Any

entries in the text boxes are retained due

to html::formValue text . Users must first

select a project before the input validity

check can redirect them to another page.

The Html and Ncgi packages comprise

solutions for numerous tasks that involve

formulae and their evaluation. Table 1

contains some practical functions. The

complete documentation for the packages

is available in Tcllib[3].

Although open networks are a good

thing, you might need to implement

access restrictions for part of your web

site. In addition to the .htaccess files you

will be familiar with from Apache,

Tclhttpd supports authentication by a Tcl

procedure. A file called .tclaccess , which

is stored in the individual directories,

takes care of this. You need to set the

variables realm and callback here.

set realm "tickle"

set callback aok?

proc aok? {sock realm user U

password} {

if {[string match $password U

tickle]} {

return 1

} else {

return 0

<html> <head> <title>U

Cookie</title> </head>

<body> <h1>Cookie</h1>

[ Doc_Dynamic

}

www.linux-magazine.com September 2002

PROGRAMMING

Tclhttpd

Figure 6:Toucan a GUI developer interface for Palm programs.The IDE and any software created with it

are based on Tcl.

additionally show you how to upload files

and use image maps. You can obtain

additional information, which is available

from [2]. Amongst the other interesting

snippets, you will note an excerpt from a

book by Brent Welch, Tclhttpd’s author.

Sourceforge also offers a mailing list that

provides competent answers to complex

issues, but still finds time to deal with

beginners’ questions.

In some cases you may not need to go

to all the trouble of developing a new

application yourself. The Infocetera[12]

site provides a complete Groupware

application, including a calendar, address

book, room planner, task planner, and

other modules. The whole application is

based on Tclhttpd.

Our next informative TCL article will

soon be appearing on your pages of Linux

Magazine from out of the murky depths

of server applications. We will be looking

into both Tk and the BWidget set. The

image in Figure 4 should serve to whet

your appetite for this widget. It provides

capable new widgets by using only the Tk

standard widgets and pure Tcl code.

The callback variable contains the

name of the Tcl procedure responsible for

authentication. The browser displays the

content of realm as a text during the login

dialog. This data is also passed as an

argument to the callback procedure. Our

example accepts users that supply the

password tickle . A real application would

access a user password entry in a file,

database or LDAP directory. After logging

in the user name is stored in the

::env(REMOTE_USER) variable.

Sessions are a more advanced variant.

Here, Tclhttpd uses its own interpreter for

the individual sessions. This provides for

data separation between the sessions,

and so retaining the data for each of the

individual sessions.

INFO

[1] Tclhttpd home page: http://sourceforge. U

net/projects/tclhttpd/

[2] Informationen on Tclhttpd: http://www. U

tcl.tk/software/tclhttpd/

[3] Home pages for Tcllib and BWidgets:

http://sourceforge.net/projects/tcllib/

[4] Medusa project: http://ciheam.maich. U

gr/medusa/

[5] Tcl Apache module: http://tcl.apache. U

org/mod_tcl/mod_tcl.html

[6] Websh: http://websh.com

[7] Googbar: http://www.geddy.hpg.ig.com. U

br/software/googbar/

[8] Toucan: http://home.attbi.com/ U

~maccody/

[9] Ora Tcl: http://oratcl.sourceforge.net

[10] Tcl XML: http://tclxml.sourceforge.net

[11] Tk DND: http://www.iit.demokritos.gr/ U

~petasis/

[12] Infocetera: http://www.infocetera.com

More Info

The capabilities described in this article

cover only a small portion of this web

server’s total functionality. The sample

files bundled with the Tclhttpd package

Tcl News

Nextrelease anticipated

Tcl 8.4 will be leaving beta in the Autumn.Although I have notnoticed any errors for a long time

now,the Tcl coreteam prefers to waitand deliver an absolutely perfectversion.A whole bunch of

new applications in and for Tcl are available rightnow.The BWidgets and additional GUI elements

such as Tree and Combobox – which will be featured in the nextissue of TCL – are now available in

version 1.4 [3].The demo in Figure 4 mightwhetyour appetite.

Ora Tcl and Tcl XML

There is also news on these two packages which were covered in our lastTCL article:The latestver-

sion of the Ora Tcl[9] database extensions supports the full range of Oracle 9i features and the Tcl

XML package on Sourceforge[10] now comprises xmlgen,providing a new approach to generating

XML and HTML.xmlgen provides a language map between Tcl and XML:Instead of using elements

and attributes,you can now work on a higher level with application objects.

Lots of Little Tools

Apartfrom all these server oriented treats there is news on several smaller tools.Toucan,a devel-

oper interface for Palm programs[8] requires only a minimal hardware platform.Both the

developer interface and any applications developed with itare based on Tcl (Figure 6).The Googbar

(Figure 3),which can launch Google searches[7] has an extremely small memory footprint.Tkdnd,a

drag & drop extension for Tk[11] needs even less space on screen.Itruns on the XDND protocol used

by Gnome and KDE applications and also on Windows. dndspy (Figure 5) is also included.This pro-

gram shows data traffic in the DND protocol

Carsten Zerbst

works for Atlantec

on the PDM ship

building system.He

is also interested in

Tcl/Tk usage and

applications.

September 2002 www.linux-magazine.com

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