Examples and Usage

Some examples of Rivet usage follow. Some prior Tcl knowledge is assumed. If you don't know much Tcl, don't worry, it's easy, and there are some good resources available on the web that will get you up to speed quickly. Go to the web sites section and have a look.

Example 1. Hello World

As with any tool, it's always nice to see something work, so let's create a small "Hello World" page.

Assuming you have Apache configured correctly, create a file called hello.rvt where Apache can find it, with the following content:

puts "Hello World"

If you then access it with your browser, you should see a blank page with the text "Hello World" (without the quotes) on it. The command puts is the good old Tcl command for terminal printing, which appends to the argument string the control characters needed to open a newline. When you don't have to run through complex elaboration and the output can be lumped in a single string (just like in the 'Hello World' example) you can draw on the popular shorthand syntax for string output that comes handy in many cases

<?= "Hello World" ?>

which is translated into

puts -nonewline "Hello World"

Example 2. Generate a Colorful Table

In another simple example, we dynamically generate a table selecting a different background color for each cell. The font color is determined through a simple CSS rule embedded in a HTML <style> element. Create the file colortable.rvt and put the following code in it

        td { 
            font-size: 12px;
            font-family: monospace;
            text-align: center;
            padding-left: 3px;
            padding-right: 3px
        td.bright { color: #eee; }
        td.dark   { color: #222; }
# we create a 9x9 table selecting a different background for each cell

for {set i 0} { $i < 9 } {incr i} {
    puts "<tr>"
    for {set j 0} {$j < 9} {incr j} {

        set r [expr int(255 * ($i + $j) / 16)] 
        set g [expr int(255 * (8 + $j - $i) / 16)]
        set b [expr int(255 * (abs (4 - $i) + abs(4 - $j)) / 8)]

# determining the background luminosity (YIQ space of NTSC) and choosing
# the foreground color accordingly in order maintain maximum contrast

        if { [expr ($r*0.29894)+($g*0.58704)+($b*0.11402)] > 148.0} {
            set cssclass "dark"
        } else {
            set cssclass "bright"

        set cell [format "%3d %3d %3d" $r $g $b]

        puts [format \
            "<td bgcolor=\"%02x%02x%02x\" class=\"%s\">%s</td>" \
                                            $r $g $b $cssclass $cell]
    puts "</tr>"

If you read the code, you can see that this is pure Tcl. We could take the same code, run it outside of Rivet, and it would generate the same HTML

The result should look something like this:

Example 3. Variable Access

Here, we demonstrate how to access variables set by GET or POST operations.

Given an HTML form like the following:

<form action="vars.rvt">
        <td><input name="title" /></td>
        <td><input name="salary" /></td>
        <td><input name="boss" /></td></tr>
          <select name="skills" multiple="multiple">
        <td><input type="submit" /></td>

We can use this Rivet script to get the variable values:

set errlist {}
if { [::rivet::var exists title] } {
    set title [::rivet::var get title]
} else {
    set errlist "You need to enter a title"

if { [::rivet::var exists salary] } {
    set salary [::rivet::var get salary]
    if { ! [string is digit $salary] } {
        lappend errlist "Salary must be a number"
} else {
    lappend errlist "You need to enter a salary"

if { [::rivet::var exists boss] } {
    set boss [::rivet::var get boss]
} else {
    set boss "Mr. Burns"

if { [::rivet::var exists skills] } {
    set skills [::rivet::var list skills]
} else {
    lappend errlist "You need to enter some skills"

if { [llength $errlist] != 0 } {
    foreach err $errlist {
        puts "<b> $err </b>"
} else {
    puts "Thanks for the information!"
          <td><?= $title ?></td>
          <td><?= $boss ?></td>
          <td><?= $salary ?></td>
          <td><?= $skills ?></td>

The first statement checks to make sure that the boss variable has been passed to the script, and then does something with that information. If it's not present, an error is added to the list of errors.

In the second block of code, the variable salary is fetched, with one more error check - because it's a number, it needs to be composed of digits.

The boss variable isn't required to have been sent - we set it to "Mr. Burns" if it isn't among the information we received.

The last bit of variable handing code is a bit trickier. Because skills is a listbox, and can potentially have multiple values, we opt to receive them as a list, so that at some point, we could iterate over them.

The script then checks to make sure that errlist is empty and outputting a thankyou message. If errlist is not empty, the list of errors it contains is printed.

Example 4. File Upload

The ::rivet::upload command endows Rivet with an interface to access files transferred over http as parts of a multipart form. The following HTML in one file, say, upload.html creates a form with a text input entry. By clicking the file chooser button the file browser shows up and the user selects the file to be uploaded (the file path will appear in the text input). In order to make sure you're uploading the whole file you must combine the action of the enctype and method attributes of the <form...> tag in the way shown in the example. Failure to do so would result in the client sending only the file's path, rather than the actual contents.

<form action="foo.rvt" enctype="multipart/form-data" method="post">
<input type="file" name="MyUpload"></input>
<input type="submit" value="Send File"></input>

In the script invoked by the form (upload.rvt) upload ?argument ...? commands can be used to manipulate the various files uploaded.

::rivet::upload save MyUpload /tmp/uploadfiles/file1
puts "Saved file [::rivet::upload filename MyUpload] \
	([::rivet::upload size MyUpload] bytes) to server"

Don't forget that the apache server must have write access to the directory where files are being created. The Rivet Apache directives have a substantial impact on the upload process, you have to carefully read the docs in order to set the appropriate directives values that would match your requirements.

It is also important to understand that some upload commands are effective only when used in a mutually exclusive way. Apache stores the data in temporary files which are read by the upload save ?upload name? ?filename? or by the upload data ?upload name? command. Subsequent calls to these 2 commands using the same ?upload name? argument will return no data on the second call. Likewise upload channel ?upload name? will return a Tcl file channel that you can use in regular Tcl scripts only if you haven't already read the data, for example with a call to the upload data ?upload name? command.

Example 5. File Download

In general setting up a data file for being sent over http is as easy as determining the file's URI and letting Apache's do all that is needed. If this approach fits your design all you have to do is to keep the downloadable files somewhere within Apache's DocumentRoot (or in any of the directories Apache has right to access).

When a client sends a request for a file, Apache takes care of determining the filetype, sends appropriate headers to the client and then the file content. The client is responsible for deciding how to handle the data accordingly to the "content-type" headers and its internal design. For example when browsers give up trying to display a certain "content-type" they display a download dialog box asking for directions from the user.

Rivet can help if you have more sofisticated needs. For instance you may be developing an application that uses webpages to collect input data. This information might be passed on to scripts or programs for processing. In this case a real file representing the data doesn't exist and the content is generated on demand by the server. In other circumstances you may need to dynamically inhibit the download of specific files and hide them away, Your scripts may expunge from the pages every link to these files (your pages are dynamic, aren't they?) and move them out of way, but it looks like a cumbersome solution.

Putting Tcl and Rivet in charge of the whole download mechanism helps in building cleaner and safer approaches to the download problem.

In this example a procedure checks for the existence of a parameter passed in by the browser. The parameter is the name (without extension) of a pdf file. Pdf files are stored in a directory whose path is in the pdf_repository variable.

This code is reported as an example of how to control the protocol using the headers command.

# Code example for the transmission of a pdf file. 

if {[::rivet::var exists pdfname]} {
    set pdfname [::rivet::var get pdfname]

# let's build the full path to the pdf file. The 'pdf_repository'
# directory must be readable by the apache children

    set pdf_full_path [file join $pdf_repository ${pdfname}.pdf]
    if {[file exists $pdf_full_path]} {

# Before the file is sent we inform the client about the file type and
# file name. The client can be proposed a filename different from the
# original one. In this case, this is the point where a new file name
# must be generated.

        ::rivet::headers type                       "application/pdf"
        ::rivet::headers add Content-Disposition    "attachment; filename=${pdfname}.pdf"
        ::rivet::headers add Content-Description    "PDF Document"

# The pdf is read and stored in a Tcl variable. The file handle is
# configured for a binary read: we are just shipping raw data to a
# client. The following 4 lines of code can be replaced by any code
# that is able to retrieve the data to be sent from any data source
# (e.g. database, external program, other Tcl code)

        set paper       [open $pdf_full_path r]
        fconfigure      $paper -translation binary
        set pdf         [read $paper]
        close $paper

# Now we got the data: let's tell the client how many bytes we are
# about to send (useful for the download progress bar of a dialog box)

        ::rivet::headers add Content-Length  [string length $pdf]

# Let's send the actual file content

        puts $pdf
    } else {
        source pdf_not_found_error.rvt
} else {
    source parameter_not_defined_error.rvt

Before the pdf is sent the procedure sets the Content-Type, Content-Disposition, Content-Description and Content-Length headers to inform the client about the file type, name and size. Notice that in order to set the Content-Type header Rivet uses a specialiezed form of the headers command. Headers must be sent before data gets sent down the output channel. Messing with this prescription causes an error to be raised (in fact the protocol itself is been violated)

More information about the meaning of the mime headers in the http context can be found at http://www.w3.org/Protocols/rfc2616/rfc2616.html

Example 6. XML Messages and Ajax

The headers command is crucial for generating XML messages that have to be understood by JavaScript code used in Ajax applications.

Ajax is a web programming technique that heavily relies on the abilty of a web browser to run in backround JavaScript functions. JavaScript functions can be run as callbacks of events generated by a user interaction but they can also react to other I/O events, for example network events. Modern browsers endow JavaScript with the ability to build http GET/POST requests to be sent to a remote webserver. Generally these requests refer to scripts (e.g. Tcl scripts run by Rivet) which inherit as variables the arguments encoded in the request. The output produced by these scripts is sent back to the browser where callbacks functions extract information and hand it down to functions that directly manipulate a page's DOM. Therefore through Ajax becomes possible to build web applications that are more responsive and flexible: instead of going through the cycle of request-generation-transfer-display of a whole page, Ajax scripts request from a webserver only the essential data to be displayed. Ajax emphasizes the requirement of separation between data and user interface, saves the server from sending over the same html code and graphics if only a fraction of a page has to be updated, allows the programmer to design flexible solutions for complex forms and makes possible to find new innovative approaches to simple problems (e.g. Google tips that show up as you type in a query). A downside of this approach is the large number of complexities, subtleties and incompatibilities that still exist in the way different versions of popular browsers handle the DOM elements of a page.

JavaScript can handle the communication between client and server through an instance of a specialized object. For quite a long time 2 approaches existed, the non-IE world (Firefox,Safari,Opera...) used the XMLHttpRequest class to create this object, whereas IE (before IE7) used the ActiveXObject class. With the release of IE7 Microsoft introduced native support for XMLHttpRequest class objects thus enabling programmers with a unique method for the development of dynamic pages.

By creating an instance of this class a POST or GET request can be sent to the server and the response is stored in a property ('returnedText') of the communication object. It's become widely customary to encode these responses in XML messages. You can invent your own message structure (either based on XML or anything else), but one has to be aware that if the http headers are properly set and the message returned to the client is a well formed XML fragment, also the property XMLResponse is assigned with a reference to an object that represents the DOM of the XML response. By means of the XML W3C DOM interface the programmer can easily manipulate the data embedded in the XML message.

In this example a Rivet script initializes an array with the essential data regarding a few of the major composers of the european music. This array plays the role of a database. The script sends back to the client two types of responses: a catalog of the composers or a single record of a composer.

# Ajax query servelet: a pseudo database is built into the dictionary 'composers' with the
# purpose of emulating the role of a real data source. 
# The script answers with  2 types of responses: a catalog of the record ids and a database 
# entry matching a given rec_id. The script obviously misses the error handling and the
# likes. Just an example to see rivet sending xml data to a browser. The full Tcl, JavaScript
# and HTML code are available from http://people.apache.org/~mxmanghi/rivet-ajax.tar.gz

# This example requires Tcl8.5 or Tcl8.4 with package 'dict' 
# (http://pascal.scheffers.net/software/tclDict-8.5.2.tar.gz)

# A pseudo database. rec_id matches a record in the db

set composers [dict create  \
                1 {first_name Claudio middle_name "" last_name Monteverdi   \
                    lifespan 1567-1643 era Renaissance/Baroque}             \
                2 {first_name Johann middle_name Sebastian last_name Bach   \
                    lifespan 1685-1750 era Baroque }                        \
                3 {first_name Ludwig middle_name "" last_name "van Beethoven" \
                    lifespan 1770-1827 era Classical/Romantic}              \
                4 {first_name Wolfgang middle_name Amadeus last_name Mozart \
                    lifespan 1756-1791 era Classical }                      \
                5 {first_name Robert middle_name "" last_name Schumann      \
                    lifespan 1810-1856 era Romantic} ]

# we use the 'load' argument in order to determine the type of query
# load=catalog:         we have to return a list of the names in the database
# load=composer&amp;res_id=<id>: the script is supposed to return the record
#               having <id> as record id

if {[::rivet::var exists load]} {

# the xml declaration is common to every message (error messages included)

    set xml "<?xml version=\"1.0\" encoding=\"ISO-8859-1\"?>\n"
    switch [::rivet::var get load] {
        catalog {
            append xml "<catalog>\n"
            foreach nm [dict keys $composers] {
                set first_name  [dict get $composers $nm first_name]
                set middle_name [dict get $composers $nm middle_name]
                set last_name   [dict get $composers $nm last_name]
                append xml "    <composer key=\"$nm\">$first_name "
                if {[string length [string trim $middle_name]] > 0} {
                    append xml "$middle_name "
                append xml "$last_name</composer>\n"
            append xml "</catalog>\n"
        composer {
            append xml "<composer>\n"
            if {[::rivet::var exists rec_id]} {
                set rec_id [::rivet::var get rec_id]
                if {[dict exists $composers $rec_id]} {
                    foreach {k v} [dict get $composers $rec_id] {
                        append xml "<$k>$v</$k>\n"
            append xml "</composer>\n"

# we have to tell the client this is an XML message. Failing to do so
# would result in an XMLResponse property set to null

    ::rivet::headers type "text/xml"
    ::rivet::headers add Content-Length [string length $xml]
    puts $xml

For sake of brevity the JavaScript and HTML will not listed here. They can be downloaded (along with the Tcl script) stored in the rivet-ajax.tar.gz archive. By simply opening this tar archive in a directory accessible by your apache server and pointing your browser to the rivetService.html page you should see a page with a drop-down list. Every time a different name is picked from the list a new query is sent and logged in the apache access.log file, even though the html is never reloaded.

Example 7. A Calendar Utility

Rivet comes with a Calendar package that provides classes for printing calendar tables in various forms.

The HtmlCalendar class prints a calendar table in a similar form the Unix program 'cal' does. Example: the following code

package require Calendar

proc ::cal_cell_attributes { day month year weekday } {
    if {$weekday == 3} {
        return [list class curr_wkday]

set htmlc [HtmlCalendar #auto]
set html_txt [$htmlc emit -container {table class calendar} -current_weekday 3 \
                          -cell_function cal_cell_attributes ]
puts $html_txt

with some CSS styling would print