yuppie wrote:
Since CMF 1.5.1 CMFSetup/GenericSetup supports extension profiles. They
are quite useful, but their current format has some fundamental issues:
I posted something in a similar vein to this on plone-developers last
night, about using GenericSetup profiles for product installation and
un-installation. The basic premise is that people install Plone (which
has a base profile) and the install and un-install products, trying them
out. They do this on sites they intend to keep (even if they're just
test instance) and may not want to re-create the site from scratch
(including content!) each time they decide they don't like some simple
product.
See
http://www.nabble.com/Installing-products-using-GenericSetup-tf1995512.html
1.) Extension profiles are hard to create and maintain
+1 - I've just been learning to do this and intend to document it, but
there are some conceptual barriers if nothing else. The thing that gets
to me is that when you want to apply an extension profile (i.e. get the
extra functionality it installs) you click "run all import steps" after
applying the profile. It seems very counter-intuitive (and potentially
dangerous) to re-run e.g. all workflow installation for all products
when I only wanted the new workflows provided by my new product that
registered that new extension profile.
You need a lot of knowledge about the import handlers. Only if you know
how their non-purging mode is implemented and which update directives
are available you can write good extension profiles. There are no tools
that help you to create extension profiles.
Bearing in mind I don't understand purging and non-purging modes, take
that as a yes :)
2.) Sites using extension profiles are hard to create and maintain
You have to know which profiles depend on other profiles.
This was another problem I hit. It seemed to me profiles were being
applied alphabetically by the Plone ZMI setup form, but who knows ;-)
It's hard to
control the order in which profiles are applied. The Comparison tab
becomes useless if the site uses several extensions that are not part of
the 'initial_configuration' snapshot. (Sometimes I create a new site to
get a clean snapshot of the applied profiles and delete that site again.
A quite expensive procedure.) A snapshot contains no information which
original profiles it 'contains'. The tool has no control on the applied
profiles. There is no way to uninstall profiles.
This is the other big one - we can't reasonably tell people to make
extension profiles for their add-on products if users won't be able to
remove them again cleanly. The irony is that the information contained
in the GS XML files should make it way easier to figure out how to do
clean un-installs compared to the Extensions.Install.uninstall() method
idiom.
And now forget everything you know about extension profiles. Imagine
they were like this:
Extension profiles describe how to transform base profiles. They know
which profiles they can transform (AKA dependencies). They are applied
to an other profile, *not* to the site.
That sounds sensible.
Only complete profiles (base profiles or extended base profiles) are
applied to the site. The handlers only need to understand complete
configuration files.
That sounds easier, but also a bit scary. If I want to just install some
product after a site has been populated with content, applying a
complete profile implies that it may stomp on changes I've made manually
afterwards (people click around the ZMI after reading some how-to all
the time). And what about being able to re-install (i.e. reset to
original state as defined by the extension profile) and un-install
(remove/reset the state that existed before the profile) a single
product only?
The setup tool knows how to merge profiles without modifying the site.
You can compare your snapshot with any valid combination of profiles you
like. There might even be a function that searches for the profile
combination that has the smallest diff to your snapshot and finds this
way the best dependencies list.
And now imagine we have a tool that can *create* extension profiles
based on the current customizations.
This would allow to provide many new features:
- customization snapshots:
Snapshots would just contain the customizations and a list of
dependencies, not a complete base profile.
- install/uninstall:
Basically any setup change is applied by recreating the site from
scratch. At least for some expensive to create items like indexes we
have to make sure we don't delete/recreate them if they were not
modified. Uninstall would be no different procedure, we just don't
reapply a specific profile.
This is the bit that worries me. Recreating the site from scratch sounds
like it'd be a nightmare. What if some bug causes portal_setup to
effectively wipe people's sites or people's settings? What about the
cases where the XML vocabulary known to the setup handlers don't cover
all possible things people do in the ZMI (as indeed they don't do now).
I still see a real-world need for 'importVarious' type handlers that do
installation the old-fashioned way with Python.
It also sounds quite expensive for complex sites, even with basic
optimisations.
- migrations:
There are different kinds of migrations. Updates in the base profiles
would be applied using the install procedure.
Again, that sounds sensible for site-wide migrations. It sounds a bit
less easy in cases where you need contingencies ("add this property if
it didn't exist during migration, but if the user added it manually,
don't stomp on their value") or when you want to migrate only the
changes implied by a particular extension profile ("leave the
customisations to the base profile alone, but apply the changes that
have since been made to extension profile X because product X was upgraded).
Martin
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