Question

G’day,

I was reading the item Quantify in the book “97 Things Every Software Architect Should Know” (sanitised Amazon link) and it got me wondering how to quantify scalability.

I have designed two systems for a major British broadcasting corporation that are used to:

1. detect the country of origin for incoming HTTP requests, or
2. determine the suitable video formats for a mobile phones’s screen geometry and current connection type.

Both of the designs were required to provide scalability.

My designs for both systems are scalable horizontally behind caching load-balancing layers which are used to handle incoming requests for both of these services and distribute them across several servers which actually provide the service itself. Initial increases in service capacity are made by adding more servers behind the load-balance layer, hence the term horizontal scalability.

There is a limit to the scalability of this architecture however if the load balance layer starts having difficulty coping with the incoming request traffic.

So, is it possible to quantify scalability? Would it be an estimate of how many additional servers you could add to horizontally scale the solution?

Answer 1

I think it is possible in some contexts – for example scalability of a web application could be quantified in terms of numbers of users, numbers of concurrent requests, mean and standard deviation of response time, etc. You can also get into general numbers for bandwidth and storage, transactions per second, and recovery times (for backup and DR).

You can also often give numbers within the application domain – let’s say the system supports commenting, you can quantify what is the order of magnitude of the number of comments that it needs to be able to store.

It is however worth bearing in mind that not everything that matters can be measured, and not everything that can be measured matters. 🙂