OLTP systems, by their very nature, produce a very high transactional volume. Any disruption to this volume is extremely visible and costly. A primary goal with running a highly-available service, such as a database, is to minimize downtime. So for any database instance that is a critical component, there should be a copy of the server configured in such a manner that near-immediate promotion to production status is possible.
Any server that needs direct access to the database, whether it be a queue system, application server, or web frontend, is sensitive to database overload. One way of diffusing this risk is to set up one database copy for every two to four directly-connected servers. These copies are only usable for reads ...
