Handling Schema Changes

[Jon Stewart]

I'd like to bounce a problem off the braintrust here, if I may:

our environment, which I'm certain isn't unique, manages a replicated db environment that receives asynchronous updates. My question is: what's the best way to manage database schema changes?

For example, if table A contains
column 1
column 2

and contains records in that format, but we determine that table A needs to change to
column 1
column 2
column 3

the associated software won't store records correctly until it is updated to the new schema, but it can't be updated until the new schema is available. So if I disable writes to the database while changing the schema, those writes will have to be translated to the new schema.

Is there a general procedure for handling this type of scenario? It seems like everyone would have to address this, but I can't seem to find a decent explanation of the deployment cycle.

Thanks.

jon