Fundamentals
Little's Law
L = λW: concurrency equals arrival rate times time in system. Holds regardless of arrival distribution.
Why interviewers ask about it
The fastest way to size thread pools and connection pools. 10,000 QPS at 50 ms means 500 concurrent requests - if your pool is 50, you have found the bottleneck before writing any code.
Related terms
This term is part of the free System Design interview preparation module - browse the full glossary for every definition.