High availability email server...

[Andrew Morgan]

On Tue, 1 Aug 2006, Daniel Eckl wrote:

> Well, I don't have cluster knowledge, and so of course I simply believe you 
> that a good cluster system will never have file locking problems.
> I already stated this below!
>
> But how will the cluster affect application level database locking? That was 
> my primary question and you didn't name this at all.
>
> A database file which is in use is practically always inconsistent until it's 
> being closed by the database application.
>
> That's why databases can be corrupt after an application crash and have to be 
> reconstructed.
>
> When you have two applications changing the same database file, you have a 
> never ending fight, because every application thinks, the database is 
> inconsistent, but it's just in use by another application. And every app will 
> try to reconstruct it and so break it for the other app(s).
>
> It's like letting two cyrus master run on the same single node! It will break 
> in my opinion.
>
> Can you shed some light on this subject?

I think the point here is that the situation you describe already occurs 
all the time on a stand-alone Cyrus server.  There are multiple imapd 
processes accessing the mailboxes.db database concurrently.  If you are 
using Berkeley DB, it has an API to manage concurrent access.  I assume 
the same is true of skiplist and the other backend formats.  I don't know 
enough about the Berkeley DB internals to explain how it actually works, 
but it does.  :)

 	Andy