Chapter 1 Reliable, Scalable, and Maintainable Applications

Three concerns that are important in most software system: Reliability, Scalability and Maintainability
## Scalability
load parameters: numbers to describe load. e.g., (Twitter) The distribution of followers per user is a key load parameter for discussing scalability. Hybrid mode of both approaches: Most users' tweets continued to be fanned out to home timelines, small number of celebrities are fetching tweets separately and merged with that user's home timeline when it is read
use percentiles to measure performance
If you are working on a fast-growing service, you will need to rethink your architecture on every order of magnitude load increase -- or perhaps more often than that.
Keep your database on a single node (scale up) until scaling cost or high-availability requirements forced you to make it distributed.