[daisy] Daisy multi-server replication using export/import?

[Mindaugas Idzelis]

In an effort to increase availability of the Daisy server what is the best 
way to implement fail-over? We would want to have at least 2 servers that 
can be switched to and from quickly in case one goes down. 

At first, we were going to use our daily backups of our production server 
and restore them to the failover server. This would work, but our backups 
are not performed every day since they take too long (although, we may 
start doing this since Daisy 2.0 backups lock the repository for far 
shorter a time than in 1.5) Plus, the synchronization story is not good 
since backups are full-replace instead of incremental. The fail-over 
repository would have to be "read-only."

In reading more about Daisy 2.0, I discovered the import/export feature. 
It seems that the "import tool is smart enough to not update documents if 
there are no changes, so that no unnecessary update events are caused." 
This seems like it would be a good way to set up some replication between 
servers. Every night, we could "export" a copy of the production 
repository to failover server. If the production server ever goes down, 
users could then use the fail-over. 

My questions are  - what happens when changes colide? For example, lets 
say that a user makes a change to a document on the production database 
and then it goes down. The user then logs in to the failover server, sees 
that their document is out-of-date (since this would only be synced 
nightly) and (perhaps foolishly) makes changes to the same document. When 
the production server comes back up, we want to sync all the changes that 
were done to the failover server back to production. I assume that we 
could just export the documents from the failover to production, and only 
the changed documents will be updated. I assume that if a new document 
were added to the fail-over server, it would transfer to production just 
fine. Also seems like documents that were updated on the fail-over server 
but not in production would transfer fine too. But the question is, what 
happens to the document that was changed on both servers?

If anyone has any advice or if there are any better ideas for replication 
out there please let me know. 


Thanks,

Min Idzelis