Rollback segments explained

I recently read this in a book about data warehousing:

Source System Rollback Segments

When extracting from a relational source, extracts that take a long time can be problematic. If an extract asks for all records updated in the last 24 hours, the system must locate the appropriate set of records. This means that no user can change the updated_date field while your query is being processed. As transactions flow in during your query, they are queued up in a separate place called a rollback segment, to be applied once your request is finished. If your query takes too long, this queue gets too large and runs out of space. The system then kills your job and processes the transactions that have been queued up. In general, the folks responsible for the transaction system don't like this kind of behavior.

Now to be fair, the book was published in 1998 and is not specifically about Oracle. Does anyone know whether there has ever been a commercial DBMS that worked anything like this, or did they just make it up?

The Consultant on Backups

Our correspondent overheard The Consultant sorting out the backup requirements for the new system:

Consultant: You have a 6 hour window overnight, now as the queues get longer under heavy loading the end of day queue clearing will run into that 6 hours. Your backup window will start to get squeezed, so we need to know the minimum time to back-up this amount of data, including the time to shut down and start up the databases.

Technical guy: Why?

Consultant: Because it has to take place in that 6 hour window.

Technical guy: Why is that?

Consultant: Because then the users will come back on line and want to use the system.

Technical guy: So?

Consultant: They can't use the system if it's down to be backed up.

Technical guy: We'll use an online backup and they can do whatever they like.

Consultant: Well, if I was in auditing I'd fire you right now. You simply have to shut a database down to back it up. It's the only way you can get a consistent backup.