Scaling horizontally across different machines

Because Node is so efficient, most websites or applications can accommodate all of their scaling needs in the vertical dimension. As we learned from Eran Hammer's experiences at Walmart, Node can handle enormous levels of traffic using only a few CPUs and an unexceptional volume of memory.

Nevertheless, horizontal scaling can still be the right choice, even if only for architectural reasons. Having one point of failure, no matter how robust, still entails some risk. The parking lot problem is another consideration that Walmart likely faces—during shopping holidays, you will need many thousands of parking spots, but during the rest of the year this investment in empty space is hard to justify. In terms of servers, the ability to dynamically scale both up and down argues against building fixed vertical silos. Adding hardware to a running server is also a more complicated process than spinning up and seamlessly linking another virtual...