[OpenSIPS-Users] [OpenSIPS-Devel] [Fwd: Re: [NEW] Virtual DB module]

[Dan Pascu]

On Thursday 30 July 2009, Razvan Pistolea wrote:
> 2. Synchronization between dbs will be lost.   
>    
> 3b.        use database managers (that know how to merge databases)   
> 3c.        use a cluster db   

That sounds good in theory, but in practice fails in so many ways. I've worked 
with this for years and even after so much time bidirectional database 
replication is extremely fragile and fails easily. Just one example:

You start writing to db1, it performs the operation just fine, but right after 
it finished and is about to return you the success response to your query you 
lose the connection. If never see the answer, assume it failed and go on to 
write the record to db2, which succeeds as well and also returns the answer. 
Now you have the same record in both databases and when they will try to 
replicate from each other they'll fail. The problem gets even worse with 
multiple databases that replicate from each other.

This is not just a theoretical example. I've seen this on a constant basis 
when performing a simple operation like heartbeat stop on the master to move 
the services to the slave, while somebody writes into the database as the same 
time (like for example opensips writing accounting requests for something as 
modest as 1.5 calls per second). This is so common, that you can consider 
yourself lucky if replication is not broken between the 2 databases when you 
stop one to activate the other, without taking measures to stop the influx of 
write operations to them during the switch.

-- 
Dan