[elephant-devel] Quick sketch of type evolution system

[Henrik Hjelte]

I am dreaming up something I would like to see, it is perhaps obvious and I
haven't thought out all details but anyway:

Schemas are stored in the elephant database. Each class has an associated
schema class that stores a number of class-versions. Each class-version has
a number and a list of slot definitions.

Each persistent class instance has a way to identify the schema and actual
class-version. For example a class-version-nr, and the schema can be
identified from the class-name. (Optional feature: it would be nice if there
was a way to rename classes and keep the persistent data, so link the
version 1 of my-renamed-class to version 7 of my-old-class)

When a class definition is updated, a new class-version is created. New
instances get the latest class-version-nr.
For each retrieval of an instance, or slot-value of an instance, the
version-nr of the instance is checked. If it is outdated, a generic function
is called to update it. Something along class-evolve, but I like long names
like update-persistent-instance-for-class-redefinition. It is up to the user
to keep code to update between versions. If you want to do lazy updates, you
will have to keep the old update code in your system for ever.

There could be a convenience function called update-all-instances that takes
a class-name and does upgrades, after calling it the class-versions objects
of older versions may get a known-to-be-obsolete value so
you will be able to know that you can drop update code safely.

With this schema, you would never actually read from old versions. Every
time you query an instance of an old version, the whole instance should be
updated slot for slot. So the slot-value-using-class suggested would be
unnecessary, there would be no need to upgrade slot types in this function,
it should already be done using
update-persistent-instance-for-class-redefinition.

But, since I personally don't have the time to do this grand plan (now at
least), I don't object to the slot-value-using-class updates Leslie
proposes. It does not feel like a complete and perfect solution, but won't
do no big harm either. But for me, until Elephant gets a complete and proper
schema evolution facility I won't trust it to handle lazy updates to slots
or objects. So a performance switch to turn it off will be nice. Proper
schema handling is perhaps elephants greatest weakness?

Best wishes,
Henrik Hjelte



On Jan 4, 2008 11:43 AM, Leslie P. Polzer <leslie.polzer at gmx.net> wrote:

>
> > to clarify one of the points that are important to me,
> > I have prepared this working sketch:
> >
> > [...]
>
> Any reason why no one commented on this?
> I'd appreciate some feedback.
>
>  Leslie
>
> --
> My personal blog: http://blog.viridian-project.de/
>
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>
>
>


-- 
Henrik Hjelte
henrik.hjelte at stix.to
+46703993945
http://stix.to
Lästmakarg 18-20 (IQube)
Box 7438
S-103 91 Stockholm
Sweden
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