This report from LMSPulse (formerly Moodle News) focuses on auto-scaling architectures for Moodle using cloud services. It's an update of a post from MediaAgility that has since disappeared. There's a 6.5 minute video and a graphic (which is probably the easiest way to see the architecture). The diagram illustrates the use of a load balancer to distribute requests across two instances (the idea is you scale by adding instances), one of which handles email notifications. These are backed by syncronized database instances, and all are connected to cloud asset storage (S3 buckets) distributed via a content distribution network (CDN).