In Lynda.com’s Drupal Essential Training video series, I said:
“…you shouldn’t use Drupal if it’s going to be an extremely high-traffic or mission-critical site. If it’s necessary to have this site online to save lives, you should probably use something else or at least have something else as a backup. If you’re going to run something that’s going to have millions and millions and millions of page views, probably Drupal is not the right solution — although I should mention, Drupal does run very popular sites.” –“Choosing Drupal” video, timecode approx. 5:00
This assertion was questioned by Stephanie Pakrul (stephthegeek) of TopNotchThemes in her review this past September. Now it’s being discussed again in this thread on Drupal.org — although I have to say that that particular thread is throwing off more heat than light. :-/
I originally made that statement based on past criticisms of PHP and MySQL… but now am coming to think that my understanding of these bottlenecks might be obsolete. Alexa says that theonion.com — probably the highest-traffic Drupal site at the moment — hovers around the 3,000 mark in terms of most trafficked sites. Pretty impressive! Sears.com, which is solidly in the top 1,000, has only about three times as much traffic. That’s not much of a difference.
So let’s open the discussion here: How scalable is Drupal?
(Error in penultimate paragraph corrected — see comments.)