Hows does Drupal 6 interact with MySQL for connections and transactions? Does connection pooling get used? How are transactions handled? At what level are these things managed by Drupal vs being handed off to be handled by MySQL?
I did a good amount of searching on the web and within Stack Overflow, but mainly, I only found articles for tweaking Drupal performance and scaling needs.
From Acquia support team,
The number of connections would vary based on activity but you can boil it down as you mention here, one request per user request. There is no concept of connection pooling or persistent connections in Drupal.
Sometimes it helps to get a handle on the Database Abstraction Layer (how Drupal talks to the database) and the bootstrap process (See http://api.drupal.org/api/drupal/includes--bootstrap.inc/6) for a more detailed walk of how it works.