Colin Putney <cputney at wiresong.ca> writes:

> On 2009-12-31, at 6:58 AM, David T. Lewis wrote:
>
>>> 2) How does one Data Migration in Smalltalk? I just know it from Rails,
>>> where one has migrations, but how's that done in the Persistence layers
>>> of some Smalltalk. E.g what happens if I add or remove instance
>>> variables?
>> 
>> I don't know enough about Rails to understand the question, but in
>> general it is straightforward to add and remove instance variables
>> in Smalltalk.
>
> Hi Friedrich,
>
> David didn't understand the question because data migration is
> generally a non-issue in Smalltalk. When you add or remove an instance
> variable, the instances of that class are automatically migrated for
> you. There's a class called ClassBuilder that handles this as part of
> creating the new class. It builds the new class, locates all the
> instances of the old class, creates equivalent instances of the new
> class, copies instance state to the new instances and updates all
> object reference to point to the new instances. Finally, the old
> instances are discarded and available for garbage collection. 

Well that' why I asked because I found nothing about it in the videos
I've seen. So either it it "trivial" or so difficult that nobody dared
to do it ;-)

>
> The one area where data migration does become an issue is when objects
> have been serialized and stored outside an image. Then reading them
> back into an image requires migration. There are various methods of
> doing this, and they tend to depend on the particular needs of your
> application. Of course, if you're using Gemstone, there's not much
> need to store objects outside an image. :-) 
I'm not yet decided, but it seems Gemstone is the most elaborate in that
area and it seems to be rock solid. I'm not that much a fan of things
which have not shown (yet) that they can handle that kind of stuff. Just
imagine, I´d decide to go on "own" way just to figure out. Well
Friedrich, you did not get it right.... 

Regards
Friedrich

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