Scalable Software Systems

Scalability has long been one of the hallmarks of quality IT systems. When we hear this term we usually associate it with an upward scalability of the system. Generally, it seems to be about how much more throughput and load our system can sustain through additonal RAM, CPU or extra machines. Often, however, downwards scalability is just as interesting, that is, the behaviour of the system when only very few resources are available. Such flexibility is very useful, for instance, if a complex system has to go through a variety of differently sized development, test and acceptance environments before it is provisioned in the live system.