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ZEO_Replication

ZEO replication for school servers

I want school zope/CMF servers to always be available so that if one is down for any reason, another can take over, and when the original is back up, the second sends all the updates and both are synced again

Editorial (Jan 25, 2002) ZEO (which used to cost $20,000 until released as open source) is a very elegant piece of work. Yet it does not do everything and it took me a while to figure out the areas where it is useful (namely, for me: allowing several computers to share the execution load but share the ZODB, and as an environment for non-zope command line programming on the ZODB).

Replication and External Mounting products exist but again one must figure out the contexts where it makes sense to apply them. I thought replication meant that two computers would continuously sync and serve the same database but with 2 copies instead of the default 1 copy in standard zeo. That is currently not the case (and I don't know what the commercial version will do) ... it replicates for use in case the first server is out of service (which requires software to monitor and trigger these events). External Mounting offers the hope of mounting several ZODB on the same zope service but without undo capability and possibly other interesting side effects.

Having taken a Reality Check, I now serenely respect ZEO and look forward to doing something similar to what that Brazilian Consultant wants to do with 2200 schools sharing ZODB with a ZEO web :-) (see reference below)

Issues

  • Must have same exact Zope installation, products, external methods on each Zope Client
  • SQL access permission needed for all ZEO clients. Use ip address or hostname when you define the Z Database connection not localhost
  • specify FORCE_PRODUCT_LOAD at one of your Zeo Clients, and new installed products will show up
  • If using exUserFolder with postgresql source, that will not replicate by itself... which can be an advantage if authentication sources should vary from school to school
  • Howto handle multiple storages with zeo and "here:http://lists.zope.org/pipermail/zodb-dev/2000-July/000355.html
  • ZEO Advantage: Command line scripts for crontab for example to pack the database

References
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