Engineering scalability into an application

Question

What does it mean to say - Engineering scalability into applications. Are there design patterns that would make an application more scalable? This question is mainly in the cont

Answer 1

Scaleability means that if the load/data can be measured by some metric N, i.e. number of users, total number of transactions done daily, etc., with some fixed response requirement t, that the application can be reconfigured to handle an arbitrary N given an increase in resources of O(f(n)) where f(n) is a linear or close to linear function of N within the same response time t.

Typically this means that the application uses a distributed architecture so that more servers, application servers, webservers, database servers can be added linearly to handle more users. I.e. to handle twice as many users, you would need to add twice the database servers, webservers, machines, etc.

Even theoretically this is not usually possible because distributing the requests usually requires a tree-like structure so that the scaling factor is O(n * log(N)). In practice, because you can use a large branching factor in the tree and the distribution cost is small compared to the overall transaction cost, the log(N) factor is not significant.