Fifth Annual Tcl/Tk Workshop, 1997
[Technical Program]
| Pp. 189–190 of the Proceedings |  |
Brent Welch
Stephen Uhler
{bwelch,suhler}@eng.sun.com
Sun Microsystems
Laboratories
2550 Garcia Ave. MS UMTV29-232
Mountain View, CA 94043
This demo shows a Tcl-based Web server that can be
extended with application-specific modules to support a variety of
applications. The server is written as a pure Tcl script[Ouster94][Welch95]
that is modular, extensible, and embeddable. Clients use a regular
Web browser to make HTTP requests to the Tcl server. The server can
be directly embedded into existing applications, or it can act as a
stand-alone server that includes application-specific modules. The
server web-enables your application so it can be accessed through a
standard Web browser.
The architecture of the server allows interception of
URL requests at two levels. At the top-level, the application can implement
a whole sub-tree of the URL hierarchy. In this case the application
is free to implement whatever semantics it wants for those URLs. The
server provides an Direct module that works at this level. It maps
URL requests directly into calls on Tcl procedures. The name of the
URL determines the name of the Tcl command, and query data from forms
is bound to procedure arguments. The return value of the Tcl procedure
is returned as HTML data to be displayed in the browser.
For example, the URL /mail/bugreport is bound to Mail_/mail/bugreport,
and this procedure sends its form data to the registered webmaster
for the server. Note that it is not possible to invoke arbitrary Tcl
commands in the server via this interface.
The second level of URL processing supports handlers
for different document types. This level is supported by the Doc module
that implements file-based URLs. The file name suffix is used to select
a MIME content type [MIME93], and handlers can be defined for different
content types. This level is used to support standard CGI, server-side
includes, and imagemaps. Applications can define new file content types
and register handlers by defining appropriately named Tcl commands.
The application/x-tcl-subst content type is used for
hybrid HTML+Tcl documents. These pages are HTML templates that include
in-line Tcl commands. The server processes the templates in Safe-Tcl
interpreters. Side effects from the Tcl processing control the application,
and the results are inserted into the Web page viewed on the clients.
HTML forms allow input, and specialized displays can use Tclets or
Java applets.
The server supports a series of page accesses by the
same client, which are called a session. A session is identified by
query data posted with the URL. Each session maps to an instance of
a Safe-Tcl interpreter. The HTML+Tcl templates are processed in the
interpreter for the session. Creating a session creates an interpreter
and installs application-specific aliases. The aliases are intended
to be the primary Tcl commands used in the HTML+Tcl templates.
Sessions are further subdivided into groups (for lack
of a better name) that collect session state information. A group maps
to a Tcl array within the per-session interpreter. A group may be associated
with a page or shared by different pages, and a page could have several
groups. Query data can be bound to groups, automatically updating the
Tcl arrays based on query data.
The server supports other standard features like logging
and authentication mechanisms. These are implemented by modules that
can be tuned for application-specific needs.
The system includes an authoring environment for the
HTML+Tcl templates that is based on the WebTk Tcl/Tk HTML editor [Welch96].
Content developers are presented with a simple property sheet interface
to aliases that are used in the templates. They can develop the surrounding
HTML in a WYSIWYG environment and easily add and tune the calls to
the aliases. Open APIs are used to create the aliases and integrate
them into the authoring environment.
The demo features SNMP device management [SNMP] via
a Web browser. The server uses the Scotty SNMP Tcl extension to access
and control SNMP-based network devices [Schoenwaelder95]. This application
uses the HTML+Tcl templates. It defines a content type, application/x-snmp,
that processes the query data in an SNMP-specific way and then uses
the regular template mechanism. It uses sessions to record which device
is being accessed. Groups within a session are used to control the
data-driven SNMP module. SNMP uses a database, or MIB, that describes
the device. Session groups are used on different pages to record what
part of the MIB is being displayed and how it is displayed. The application
includes a general MIB browser that lets you navigate the MIB to examine
different parts of the SNMP state of the device.
The Tcl Web server is designed to be a flexible mechanism
that facilitates web-access to new and existing applications. It leverages
the embeddability and extensibility of Tcl so it is easy to add the
server to your application. The architecture supports a variety of
ways to map URL accesses to the application. In particular, the HTML+Tcl
templates for dynamic page generation provide a rich infrastructure
for building the web-based interface to your application.
The server is available for evaluation purposes.
We hope to have a freely available version available soon. Please contact
the authors for more information, or check the server's web site.
- MIME93
- RFC-1521. MIME (Multipurpose Internet Mail
Extensions). N. Borenstein.
- Ousterhout94
- J. Ousterhout, Tcl and the Tk Toolkit, Addison-Wesley,
April 1994, ISBN 0-201-63337-X.
- Schoenwaelder95
- Schoenwaelder, Langendorfer, Tcl Extensions for Network Management
Applications. Proc. of the 3rd Tcl/Tk Workshop.
- SNMP
- RFC 1155, RFC 1157. Simple Network Management Protocol.
- Welch95
- B. Welch, Practical Programming in Tcl and Tk. Prentice
Hall, June 1997 (2nd Ed.), ISBN 0-13-616830-2.
- Welch96
- B. Welch and S. Uhler. Tcl/Tk HTML Tools, Proc. of the
4th Tcl/Tk Workshop. July 1996. pp 173-182.
Last Modified: 03:24pm PDT, June 02, 1997
|
Technical Program