repoze.vhm 0.9

repoze.vhm README

Overview

This package provides middleware and utilities for doing virtual hosting within a WSGI/Repoze environment. It is particularly useful within a ‘repoze.zope2’ environment, where it may be used as an alternative to the classic “VirtualHostMonster”:http://www.zope.org/Members/4am/SiteAccess2/info method of doing virtual hosting.

Virtual Hosting in a Nutshell

“Virtual hosting” enables dynamic applications to be served from within a larger URL namespace, independent of the physical location of the script files used to serve the application, or the precise layout of objects within the application. In particular, the application and the server collaborate to generate URLs for links in the application, such that the links preserve the “apparent” location of the application.

The simplest case requires no effort at all: links rendered as relative paths from within pages work nicely. However, such links begin to be problematic quickly, e.g. when the page is serving as the default index view for its folder, and the URL does not end in a ‘/’. In that case, the browser interprets the links relative to the folder’s parent, and chaos ensues.

CGI Environment Variables

As used for applications running “inside” Apache (e.g., using ‘mod_python’), there follwing environment variables are of interest when doing virtual hosting:

‘SERVER_NAME’ – the name which the server believes it has.

‘HTTP_HOST’ – the apparent hostname of the server (i.e., as

passed in the ‘Host:’ header)

‘SERVER_PORT’ – the apparent port of the server

‘SCRIPT_NAME’ – any path prefix used by Apache to dispatch to

the application (as defined via the ‘ScriptAlias’ directive).

‘PATH_INFO’ – the remainder of the path, after removing any parts

used in dispatch.

‘repoze.vhm#xheaders’ WSGI Filter

When configured as WSGI middleware, this filter will convert the path information in the environment from the “X-Vhm” headers added to the request into the “standard” CGI environment variables outlined above. It will also place repoze.vhm-specific environment variables into the WSGI environment for consumption by repoze.zope2 (or another application which chooses to use its services).

If this filter is placed into the pipeline in front of a Zope 2 application, the standard Virtual Host Monster object (‘/virtual_hosting’) may be deleted, as it is no longer necessary. However, it does not need to be deleted; repoze.vhm will work if it is present.

The filter requires no configuration; it can be added to any pipeline via its egg name: “egg:repoze.vhm#vhm_xheaders”.

‘repoze.vhm#path’ WSGI Filter

As a fallback for proxies which cannot add headers to proxied requests, this filter implements the same path-based virtual hosting syntax used by the Zope2 Virtual Host Monster. Because this syntax is quite arcane (so much that there is a web-app for generating the rewrite rules!), this filter is not recommended except for environments which cannot be configured to add headers (e.g., Apache has ‘mod_rewrite’ enabled, but cannot be changed to enable ‘mod_headers’).

When configured as WSGI middleware, this filter will convert the path information in the environment from the classic “Zope2 virtual hosting mungned URL” into the “standard” CGI environment variables outlined above. It will also place repoze.vhm-specific environment variables into the WSGI environment for consumption by repoze.zope2 (or another application which chooses to use its services).

If this filter is placed into the pipeline in front of a Zope 2 application, the standard Virtual Host Monster object (‘/virtual_hosting’) may be deleted, as it is no longer necessary. However, it does not need to be deleted; repoze.vhm will work if it is present.

The filter requires no configuration; it can be added to any pipeline via its egg name: “egg:repoze.vhm#vhm_path”.

repoze.vhm Virtual Hosting Model

This model (based on a “suggestion of Ian Bicking’s”, http://blog.ianbicking.org/2007/08/10/defaults-inheritance/), passes virtual hosting information from the proxy / web server to the application by adding extra headers to the proxied request:

‘HTTP_X_VHM_HOST’ – indicates the apparent URL prefix of the

root of the application (concatenating ‘wsgi.url_scheme’, ‘SERVER_NAME’, ‘SERVER_PORT’, and ‘SCRIPT_NAME’ variables; the equivalent of Zope2’s ‘SERVER_URL’).

‘HTTP_X_VHM_ROOT’ – path of the object within the application

which is supposed to function as the “virtual root”.

When serving an application from “within” Apache via mod_wsgi, we can just set the environment directly:

<Directory /path/to/wsgiapp>
  SetEnv HTTP_X_VHM_HOST http://www.example.com
  SetEnv HTTP_X_VHM_ROOT /cms
</Directory>

If you are serving repoze.zope2 via a proxy rewrite rule, you may pass this information by adding additional headers. E.g., a sample Apache configuration for the example above might be:

<VirtualHost *:80>
  ServerName www.example.com
  RewriteEngine on
  RewriteRule ^/(.*) http://localhost:8080/$1 [P,L]
  RequestHeader add X-Vhm-Host http://www.example.com
  RequestHeader add X-Vhm-Root /cms
</VirtualHost>

In either of the above example cases, the effect on repoze.zope2 when repoze.vhm’s filter is in the WSGI pipeline is the same: the apparent root of “http://www.example.com” will be the default view of the object that has a physical path of “/cms”. Additionally, paths in URLs generated by Zope will not start with ‘/cms’, and the scheme and hostname in URLs will be “http://www.example.com” as opposed to “http://localhost:8080”.

The “vhm host” header may contain further path information as necessary; further path information can (and will, in the case of repoze.zope2) be respected by downstream applications to root an application at a non-server-root path

<Directory /path/to/wsgiapp>
  SetEnv HTTP_X_VHM_HOST http://www.example.com/further/path
  SetEnv HTTP_X_VHM_ROOT /cms
</Directory>

In this case, URLs generated by Zope will begin with “http://www.example.com/further/path”. This syntax replaces the “inside out” virtual hosting syntax (‘_vh_’ segment markers in the URL) as described in the “Virtual Host Monster” documentation.

The “vhm host” and “vhm root” headers can be used independently (the system will operate as you would expect in the absence of one or the other).

‘repoze.vhm’ Library API

Because the existing Zope 2 virtual hosting machinery does not rely on the “standard” CGI variables, the application dispatcher needs to “fix up” the environment to match Zope’s expectations. ‘repoze.vhm’ offers the following functions to aid in this fixup:

‘repoze.vhm.utils.setServerURL’ – convert the standard CGI

virtual hosting environment into the form expected by Zope2 (adding the ‘SERVER_URL’ key).

‘repoze.vhm.utils.getVirtualRoot’ – return the virtual root

path (‘repoze.vhm.virtual_root’) as set by the middleware.