Why are distributable systems more scalable then centralized systems?

Question

Why are distributable systems more scalable then centralized systems? And What practical limit to the scalability can occur?

Thank you very much in advance!!!

Answer 1

First, understand clearly what is meant by "scalability", and then think of the system in analogous terms, e.g. as a toll-plaza on the turnpike.

Answer 2

That statement is wrong in general. Actual results will depend on the system design.

However the implication is that a centralized system will have some resource shared by a large number of nodes - some data storage for example - and nodes will have to synchronize access to that resource. Once a huge number of nodes uses the same resource overhead for accessign that resource becomes intolerably high and this halts the system.

In case of a properly designed distributed system different groups of nodes will use different instances of those shared resources and resource instances will exchange data once in a while. In this case the practical limit is higher - when new nodes are added they don't immediately share the same single resource instance (instead they use one of multiple instances) and thus they don't cause a system halt that fast. Of course after some limit those shared resource instances will have to spend a lot of time exchanging data between each other and this will put a limit on the distributed system scalability.