[Glass] Class Migration

[Otto Behrens via Glass]

Hello Dario,

When your database is small, automatic migrations will be fine. But we like
to control when and how the migrations take place, because we like to take
the site down so that we do not have active users on the system while
migrations take place. I don't know how it's possible anyway to be sure how
your system will behave when migrations have run partially (you then have
the situation with some objects migrated and others not: how will the
system behave?)

We often override the method #migrateFrom:instVarMap: to initialize new
instVars or to transform some instVars that have been renamed or changed
structure. What is important is that the you clean up these methods after
migration because it may cause harm if you migrate the same class again.
You can also write the migrateFrom:instVarMap: in a way that it is
idempotent, for example:

migrateFrom: oldObject instVarMap: aMap
  super migrateFrom: oldObject instVarMap: aMap.
  someInstVar ifNil: [ someInstVar := OrderedCollection new ]

We write this code in Pharo and put it in an extension package. This code
obviously only runs in GemStone.

To invoke migrations, we have written a class that does the work, similar
to GsDeployer, which is what I think you can start with. If you want to use
some of the code we've written, you can just say.

There are several other issues to deal with when the system becomes
complex, some are:
- Fixing data problems before doing the migration
- Renaming classes
- Controlling the order of migrating some classes before others
- Ensuring that all instances are migrated when you're done

Let me know if this helps or if you need more information.

Cheers
Otto

On Sat, Jan 30, 2016 at 5:24 PM, Trussardi Dario Romano via Glass <
glass at lists.gemtalksystems.com> wrote:

> Ciao,
>
> i need to manage the 'migrations' of class and relative instances.
>
> I read the Prog-guide3-1 Class Creation, Versions, and Instance Migration
> .
>
>
> Now my question is how to manage the migration when i development in Pharo,
>
>  and load the new structure into GLASS with the relative package using
> GemTools or Tode.
>
> I do some test with GemTools migrate option and filetree packages load.
>
>
>
> Question:
>
> someone implemented a support into Pharo to handle these problematic
>
> and porting the 'migration' into Gemstone?
>
> I think of something that when I change classes in Pharo,
>
> record - define  the statements that then allow gemstone to applying m
> igration.
>
>
> Thanks for any considerations,
>
> Dario
>
>
>
>
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