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Purpose

As the OpenSocial spec matures and containers begin to
support multiple versions, there needs to be a way for gadgets to
specify multiple source gadget XML files for various situations.  This
will be handled by a single manifest file that identifies to the
container which gadget XML file to utilize based on supported
features.

Objective

This proposal should satisfy the need for a manifest while
allowing for backwards compatibility of existing gadgets already
deployed in production.

Specifics:

Location of the manifest:

When uploading a gadget, the developer may specify either the URL of a
gadget XML file OR the URL of a gadget manifest file.  The manifest
file will point to the actual gadget XML file URL based on version and
features required/supported.  For backwards compatibility purposes,
the gadget XML file may also contain a reference back up to its
manifest file URL.  In this way containers that do not support the
manifest feature will still function with their existing gadget XML
sources, and newer containers that do support the manifest feature can
take advantage of its ability to specify multiple gadgets for
different scenarios.  This will be done via an optional "gadget-
manifest" feature specified in ModulePrefs.

Manifest Example
<Module>
  <ModulePrefs title="Manifest sample gadget" description="Example
showing usage of manifest">
    <Require feature="opensocial-1.0"/>
        <Optional feature="gadget-manifest">
          <Param name="manifest">http://example.org/samplegadget/
manifest.xml</Param>
        </Optional>
  </ModulePrefs>
  ...
<Module>

(manifest file at http://example.org/samplegadget/manifest.xml

<GadgetManifest>

<Gadgets>
        <Gadget os-version="1.0" gadget-version="2.1" status="released"
src="http://example.org/samplegadget/gadget.xml" />
        <Gadget os-version="1.0" gadget-version="2.2" status="development"
src="http://example.org/samplegadget/gadget-dev.xml" />
        <Gadget os-version="0.8" src="http://example.org/samplegadget/
gadget.os08.xml" />
        <Gadget os-version="1.1"  src="http://example.org/samplegadget/gadget.
11.xml">
                <Features>
                        <feature name="templates" required="true" />
                        <feature name="html-6" required="false" />
                </Features>
        </Gadget>
</Gadgets>

</GadgetManifest>

Processing:

If the container supports the gadget-manifest feature, it attempts to
download the manifest file from the referenced location.  It then
selects the source most appropriate to its supported opensocial
version and features.  This is true if the manifest file is directly
referenced as the source, or if a gadget file is referenced which
contains a gadget-manifest features.  Older containers which do not
support the gadget-manifest feature will simply ignore this directive
and utilize the referenced gadget XML file.

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