"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.
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 can be 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 [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.
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.
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 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.request.Request.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>
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.
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).