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Virtual Hosting

“Virtual hosting” is, loosely, the act of serving a Pyramid application or a portion of a Pyramid application under a URL space that it does not “naturally” inhabit.

Pyramid provides facilities for serving an application under a URL “prefix”, as well as serving a portion of a traversal based application under a root URL.

Hosting an Application Under a URL Prefix

Pyramid supports a common form of virtual hosting whereby you can host a Pyramid application as a “subset” of some other site (e.g. under http://example.com/mypyramidapplication/ as opposed to under http://example.com/).

If you use a “pure Python” environment, this functionality is provided by Paste’s urlmap “composite” WSGI application. Alternately, you can use mod_wsgi to serve your application, which handles this virtual hosting translation for you “under the hood”.

If you use the urlmap composite application “in front” of a Pyramid application or if you use mod_wsgi to serve up a Pyramid application, nothing special needs to be done within the application for URLs to be generated that contain a prefix. paste.urlmap and mod_wsgi manipulate the WSGI environment in such a way that the PATH_INFO and SCRIPT_NAME variables are correct for some given prefix.

Here’s an example of a PasteDeploy configuration snippet that includes a urlmap composite.

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[app:mypyramidapp]
use = egg:mypyramidapp#app

[composite:main]
use = egg:Paste#urlmap
/pyramidapp = mypyramidapp

This “roots” the Pyramid application at the prefix /pyramidapp and serves up the composite as the “main” application in the file.

Note

If you’re using an Apache server to proxy to a Paste urlmap composite, you may have to use the ProxyPreserveHost directive to pass the original HTTP_HOST header along to the application, so URLs get generated properly. As of this writing the urlmap composite does not seem to respect the HTTP_X_FORWARDED_HOST parameter, which will contain the original host header even if HTTP_HOST is incorrect.

If you use mod_wsgi, you do not need to use a composite application in your .ini file. The WSGIScriptAlias configuration setting in a mod_wsgi configuration does the work for you:

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WSGIScriptAlias /pyramidapp /Users/chrism/projects/modwsgi/env/pyramid.wsgi

In the above configuration, we root a Pyramid application at /pyramidapp within the Apache configuration.

Virtual Root Support

Pyramid also supports “virtual roots”, which can be used in traversal -based (but not URL dispatch -based) applications.

Virtual root support is useful when you’d like to host some resource in a Pyramid resource tree as an application under a URL pathname that does not include the resource path itself. For example, you might want to serve the object at the traversal path /cms as an application reachable via http://example.com/ (as opposed to http://example.com/cms).

To specify a virtual root, cause an environment variable to be inserted into the WSGI environ named HTTP_X_VHM_ROOT with a value that is the absolute pathname to the resource object in the resource tree that should behave as the “root” resource. As a result, the traversal machinery will respect this value during traversal (prepending it to the PATH_INFO before traversal starts), and the pyramid.url.resource_url() API will generate the “correct” virtually-rooted URLs.

An example of an Apache mod_proxy configuration that will host the /cms subobject as http://www.example.com/ using this facility is below:

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 NameVirtualHost *:80

 <VirtualHost *:80>
   ServerName www.example.com
   RewriteEngine On
   RewriteRule ^/(.*) http://127.0.0.1:6543/$1 [L,P]
   ProxyPreserveHost on
   RequestHeader add X-Vhm-Root /cms
 </VirtualHost>

Note

Use of the RequestHeader directive requires that the Apache mod_headers module be available in the Apache environment you’re using.

For a Pyramid application running under mod_wsgi, the same can be achieved using SetEnv:

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 <Location />
   SetEnv HTTP_X_VHM_ROOT /cms
 </Location>

Setting a virtual root has no effect when using an application based on URL dispatch.

Further Documentation and Examples

The API documentation in pyramid.traversal documents a pyramid.traversal.virtual_root() API. When called, it returns the virtual root object (or the physical root object if no virtual root has been specified).

Running a Pyramid Application under mod_wsgi has detailed information about using mod_wsgi to serve Pyramid applications.

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