How to write robust database application?

1. Fault tolerant design is cheaper than fault tolerant application and fault tolerant application is significantly cheaper than fault tolerant database and more robust. Database has not knowledge what is critical and what is not critical, what can be lost with less cost, and what must not be lost.
2. Database is good tool for data processing, and wrong tool for communication - A design and implementation of communication server is more simply, more robust than master-master replication based communication solution. Design servers, design protocols, design services - don't lost your time with master-master replication and collision solutions.
3. More smaller servers, databases are more robust, cheaper, practical than one super server, super database. Try to minimize dependency between components - when one component fails, all other should to work without significant problems some time (or critical services should to continue). Strong dependency  increases risk of total blackout. Think about failures. Decrease strong dependency, when it is possible and practical. Better data duplication than total blackout.
4. Be sure, so you well understand, what kind is your application - for lot of application some 24x7 solution uselessly increase a complexity, cost, maintenance cost, time. The most simple solution is usually the best. But attention on too simply solutions!
5. Think about deployment - strong dependency between components increase a risk of total failures, when you deploy your solution. Try to deploy any component without strong dependency on other component - you should be able to do rollback any individual component.