Copy of Put Back-of-the-Envelope Numbers in Perspective

A distributed system has compute nodes, connected via a network. There is a wide variety of available compute nodes and they can be connected in many different ways. Back-of-the-envelope calculations help us to ignore nitty-gritty details of the system (at least at the design level) and to focus on more important aspects.

An example of the back-of-the-envelope calculation could be:

• Number of concurrent TCP connections a server can support
• Latency of a cross-continental link connecting two data centers

Choosing an unreasonable number for such calculations can irk the interviewer and send a wrong message that we are not aware of any real system.

Overview of the system

Let’s consider an example of an online classified platform, similar to OLX which is the world’s leading classifieds platform in growth markets and is available in more than 40 countries and over 50 languages.

Our platform will allow people to buy, sell or exchange a wide variety of used goods and services by enabling them to post a listing through their mobile phone or on the web in a fast and convenient manner.

Server requirements

Before we dig deep into the server requirements and assess different design options, let’s first look at some numbers to get an idea of the time taken by some of the routine computer operations.

The following tables shows the typical latency, throughput numbers, and server capabilities to guide our back-of-the-envelope calculations.

Note: Some of these numbers may be a little outdated considering all the advancements in modern day computers. However, they are enough to give us some idea about the time taken by some of the routine computer operations.