====== Interchange: How vlink/tlink communicates with the Catalog ======

**Interchange** is an Open Source perl application server targeted at
e-commerce web applications.(([[http://www.icdevgroup.org]]))\\
This document describes how the web server communicates with the 
**Interchange** application server.

Interchange version: 5.7.7\\
Webserver: Apache2


===== Interchange Link Program =====

The Interchange **link program** links the **Interchange** application
server to the web server via CGI.
Interchange link programs come in three flavours:

^  Interchange Link Programs  ^^^
^ Type   ^ Communicates via  ^ Comment  ^
| vlink  | Unix domain socket  | Local communication via a "socket file"  |
| tlink  | Inet domain socket  | Webserver and Interchange server can be on different hosts  |
| tlink.pl  | Inet domain socket  | tlink program (perl version)  |

The unix socket (vlink) or host/port (tlink) to be connected to
is configured into the link program by the **compile_link** perl script.
((Dig into the compile_link script for the default values.))

More information:\\
[[http://www.icdevgroup.org/xmldocs/glossary/link-program.html]]


===== The complete flow chart =====

  - The browser requests: **<nowiki>http://webhost.somedom.com/mycatalog/index.html</nowiki>**
  - **Apache2** processes the URL:
    - **/mycatalog** matches a **ScriptAlias** in the config file: **/etc/apache2/sites-available/webhost**<code>
ScriptAlias /mycatalog /export/home/appbox/cgi-bin/ic/mycatalog
</code>
    - **Apache2** invokes the CGI-program: **/export/home/appbox/cgi-bin/ic/mycatalog**((In this example the **Interchange** server and Catalogs are installed in a user's home directory:\\ /export/home/appbox))
  - The **mycatalog** link program connects to the **interchange** daemon via a UNIX- or INET domain socket.((**mycatalog** is a copy of **vlink** or **tlink**.))
  - The **interchange** daemon searches the **interchange.cfg** for a matching URL:((The **interchange.cfg** may include multiple config files.\\ E.g. (Debian package):\\ **include /var/lib/interchange/catalog.d/*.cfg**))<code>
FullURL  yes
Catalog  mycatalog  /export/home/appbox/catalogs/mycatalog  webhost.somedom.com/mycatalog
</code>
  - The **interchange** daemon uses the **cgi extra path information** to process the page:
    * URL: **<nowiki>http://webhost.somedom.com/mycatalog/index.html</nowiki>**
    * CGI extra path info: **/index.html**
    * CATROOT: **/export/home/appbox/catalogs/mycatalog**
    * Page to be processed: **CATROOT/pages/index.html**
  - The **interchange** daemon returns the processed page data to **Apache2**
  - **Apache2** passes the data to the web browser
  - The web browser displays the page.


====== . ======
Copyright (c) 2013 Integrated Services; Tux4u.nl\\
Author: Marjan Waldorp; ic-vlink-tlink2catalog 2013-07-17