Storage issues in cloud when creating multiple instances

In a cloud hosting environment (amazon, rackspace,) you can create multiple instances. Let's say I have a database server (mysql,) and other persistent data.

If I create more instances, what happens to the data ? 开发者_Go百科Ex.

1 Instance -> user table (in a db)

I make another 3 instances

4 Instances -> each one has it's one user table

Errors: if someone adds data to the table on instance 3 how does instance nr 4 see it ? If I merge the instances back to one, which instance data does it keep ?

Thank you

---

I would suggest having one (or more) dedicated database servers that all the instances connect to. If you are using Amazon Web Services check out their RDS service ( http://aws.amazon.com/rds/ )

That way you don't need to worry about replication - if you do want each server running it's own db instance you'll have to look into replication - for MySQL this is a good guide: http://dev.mysql.com/doc/refman/5.0/en/replication.html

I would strongly recommend the former solution for the database. Replication is tricky to get right and can be a nightmare to maintain

If you are using static data eg images I would recommend using amazon's S3 service for uploading to ( http://aws.amazon.com/s3/ ) - that way all your servers are getting their data from a single point instead of having to replicate over servers, which is always going to end up a less scalable solution