I am glad to be part of a team who deployed Drupal as an Intranet platform for a Fortune 20 telecom company. This is the first time that Drupal was used on an open surce LAMP stack to create a corporate Intranet for over 90,000 employees. The project was a complete rewrite of the existing intranet that had been in operation for over half a decade. It was accomplished in less than 12 months from conception to full deployment, ramping up from just a one member team to a 17 person group with only 5 full time developers
There are lot of projects done with Drupal around the globe that supports a large user base but most of them optimize and scale anonymous users. In this project Drupal was scaled for registered users and on top of that they get personalized information. Drupal core had minimal patches applied and we developed in-house modules to optimize page execution. Additional optimization caching techniques were applied to cache pages, blocks, views for registered user via memcache, opcode cache etc. As an example, a >1Mb personalized front page with over 200 database queries was optimized to load in less than 800ms.
The infrastructure is on an active-passive multi site platform utilizing 10 web servers per site in a virtualized environment. The DB is MySQL tuned for performance. Consultants on the project were Lullabot (for custom modules) and Percona (for MySQL tuning). The site concept was designed by Treehouse.
The intranet servers offers personalized news and multimedia content, integrates with other company systems bringing in content personalized to the user and much more functionality that any intranet would typically have.