Maurits van Rees wrote:
Now you've set my mind racing... :) I've got a revised counter-proposal
below that I'm starting to quite like.... very interested in your thoughts!
class Migration(object):
implements(IMigration)
def __init__(self, id, to_version, profile):
self.id = self
self.to_version = version
self.profile = profile
Let's compare this to my untested code in
https://svn.plone.org/svn/collective/collective.generations/trunk
from Products.collective.generations import SchemaManager
sm = SchemaManager(
minimum_generation = 0,
generation = 5,
package_name = 'Products.eXtremeManagement'
)
I think the name SchemaManager is a bit misleading, though. Is it
managing an Archetypes schema? :)
We are still relying on naming conventions. Also, the minimum_generation
thing is for zope.generation's auto-migration thing, which I don't think
can (or should) have an analogue in Plone.
The other problem here is that users don't really think of
"generations", they think of versions, which are sequential things.
We could solve this by being a bit more explicit:
class IVersionManager(Interface):
"""Keep track of the current version for a product
"""
current_version = schema.TextLine("The current version")
product_name = schema.TextLine("Describe the product")
migration_steps = schema.Tuple("An ordered list of migration steps")
class IMigrationStep(Interface):
"""Describes a migration
"""
to_version = schema.ASCIILine("Migrates to version - displayed to
the user")
def __call__(portal):
"""Execute this migration step
"""
class VersionManager(object):
implements(IVersionManager)
def __init__(self, current, name, steps=[]):
self.current_version = current
self.product_name = name
self.steps = steps
class GSMigrationStep(object):
implements(IMigrationStep)
def __init__(self, version, profile):
self.to_version = version
self.profile = profile
... now, in e.g. Poi, I'd do:
version = VersionManager(
name=u"Poi",
steps=[
MigrationStep("1.1", "profile-Products.Poi:1.1"),
MigrationStep("1.2", "profile-Products.Poi:1.2"),
MigrationStep("1.3", "profile-Products.Poi:1.2"),
]
)
provideUtility(version, name=u"Products.Poi") # do this with ZCML
So, we have one global utility, named after the product by convention.
We specify the name of the product to display in the migration UI.
The migration steps are listed explicitly, in order. The "latest"
version is the last one in the list (remember, it specifies "versions
being migrated to"). That makes sense - it's the last thing you can
migrate to.
We reference a profile by name (we could simplify by putting the product
name in the VersionManager, and/or defaulting the profile name to
profile=${product_name}:${version}, but I think it makes sense to be
able to specify one explicitly.
When we migrate current version to 1.3, say, it goes through the list
from the current/pre-migration version, and does each step up until and
including the last one.
Adding a new migration step means adding another element to that list,
which should be pretty simple.
To me, this is easier to understand and manager than sequential ints
that have no tie to the actual version number I'm releasing, and will
make more sense to users.
Of course, you may then end up with "pseudo-versions" if you have a need
for migrations in-betwen releases, but I think this is a minor need, and
that people should be making more releases anyway. :) But we do need to
make sure it's possible to "force migrate" from a version.
The missing pieces now are:
- a local utility which keeps track of the current/last migrated
version of a package
- some standard way of refreshing this if a product is re-installed
from scratch
- UI for this, including the ability to force-migrate from a given version
In general, I don't like encoding meaning into names of things. It's too
easy to get it wrong. Also, I find strings that contain version numbers
really hard to read and easy to get wrong: "migrate-1.1a2-2.0rc1" - yipes!
then, you register a profile, like Products.Poi:migrate-1.0-2.0 and
Products.Poi:migration-2.0-2.1
That would be something like this:
<genericsetup:registerProfile
name="3"
title="Migration profile for eXtremeManagement generation 2 to 3."
description="This profile contains configuration changes that
are applied during the migration from
eXtremeManagement generation 2 to 3, which is part
of the change from 1.1 to 1.5."
directory="migrations/v1.5"
provides="Products.GenericSetup.interfaces.MIGRATION"
/>
Note that Products.GenericSetup.interfaces.MIGRATION does not
currently exist. It is just an idea. But that is not really an issue
here.
It can be collection.migration.interfaces.IMigrationProfiles. No need
for it to be in CMF Core. :)
No utilities need to be registered with the collective.generations
approach. That one SchemaManager should be enough. Maybe that can be
done in zcml though if wanted.
I don't see any benefit of doing this with ZCML at this point, just more
work for you. :)
It ought to be an explicit action in Install.py when reinstall=True, but
that sounds sensible, yes.
Extensions/install.py could have something like this:
from Products.collective.generations import UpgradeManager
from Products.collective.generations import SchemaManager
sm = SchemaManager(
minimum_generation = 0,
generation = 5,
package_name = 'Products.eXtremeManagement'
)
def install(context):
um = UpgradeManager(context) # or getUtility/getToolByName
um.install(sm)
It may make more sense if this was either an adapter or a utility,
rather than a helper class. I think a utility makes more sense, even if
we have to pass in the context:
migrations = getUtility(IMigrationManager)
migrations.migrate_to_current_version(portal, "Products.Poi")
And during a reinstall it is probably enough to evolve from the
current iteration to the maximum defined generation, using this
method:
um.evolve_maximum(sm)
I do not know if this works, but I *am* beginning to like the
approach. :-)
I'm beginning to like it less. :) The more I think about it, the more
wrong it seems to tie this to an arbitrary, incremented integer. Users
and developers think in terms of releases, which have unique and
sequential versions. There's lots of scope for things to go subtly
pear-shaped during migration, and being explicit and keeping things
simple seems a good general principle.
Also, I'm not sure I see the need for the min/max generation stuff
(which is easier to do with integer generations, for sure). From what i
remember, zope.generation does it so that you can do auto-migration on
Zope startup for "Zope won't even run properly if you don't do this"
kind of migrations. That's fine for fundamental Zope things, but I don't
think auto-migration is *ever* a good idea in the context of Plone, let
alone technically difficult (when do you trigger it?).
Besides that, the only migration that ever makes sense to invoke
explicitly is from "current version" to "latest version", with a
fallback option of explicitly choosing the "current version" if the
system's lost track. This is how Plone's migration has worked for years,
and I think this is how people expect it to be.
Thoughts?
Martin
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