[Zope] Migration Strategy

[Paul Winkler]

On Mon, Oct 20, 2003 at 11:28:51AM -0600, Edward Pollard wrote:
> We're moving some content from Zope 2.6 to 2.6.2, and I'm wondering 
> what the best way to do this would be.

usually just copy the Data.fs into the new instance and away you go...

> In the fantasy world that is my mind, I think I'd be looking for a way 
> to safely lock the ZODB in the 2.6 instance, so we can migrate to 2.6.2 
> while still serving (unchangeable) content from 2.6.
> 
> Does such a concept exist?

Not that I know of, but maybe something like this would work:

- make the Zope 2.6.2 instance the only publically accessible instance, 
  even though it has no content yet.

- set up the old 2.6.0 instance in read-only mode (there are docs on 
  how to do this), and run it on a port that is not accessible to
  the outside world.

- serve stuff from 2.6.2 if found, else from 2.6.0 
  A pure-Zope approach might be to do this:
  modify standard_error_message on the 2.6.2 instance to do (pseudocode):
  if the error is NotFound:
      make an http request with the same headers to the 2.6.0 instance
      (you will need an ExternalMethod to do that)
      if 404: 
          re-raise the NotFound error
      else:  
          return the result of the request from 2.6.0

  An alternative, probably higher-performance, approach is to use an 
  Apache rewrite rule that tries to get stuff from 2.6.2, 
  and rewrites the request to 2.6.0 only if the response is 404.
  This might require a cgi script, not sure.
  I don't know how to do this off the top of my head, but some quick googling
  turns up some likely-looking documentation.

  All of the above should work if you're just serving content, but 
  if people in the outside world can add or modify zope objects, you'd 
  need to be sure that it actually works. I suspect this will be tricky.
  E.g. what do you do when object foo exists on 2.6.0 and wants to
  be modified? You have to be sure that it gets copied to 
  2.6.2 first.

  There are other potential complications. e.g. catalog searches.

- Once all that's done, use export/import or ZSyncer to gradually 
  move things to the Zope 2.6.2 instance.


Seems like a lot of work, I'll just keep copying my storage when
I upgrade :-)

-- 

Paul Winkler
http://www.slinkp.com
Look! Up in the sky! It's UGLY-DISTRACTION NURSE!
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