During my learning years at previous job, we did launch our secondary DC but it was in active/passive rather than active/active. We were worried about DB replication, latency and implementing sharding for transactional systems. At the very end, we had solid ideas for DB Sharding but couldn't implement it for various reasons.
At current job, I was lucky to start everything from scratch. We designed it for active/active using Amazon EC2, ELB, MySQL RDS and Dyn Inc's Dynect CDN Manager for our first iteration. We did migrate completely to different architecture in iteration two more about it later.
Our goal was to do all our processing of ad optimization and hand out the hints to our ad server within 100 ms. To do this we had to reduce latency issues as much as possible so we decided to take advantage of cloud solutions and geo load balancing.
Amazon Elastic Load Balancer (ELB) supports only load balancing with in the region and we had to look for options to handle the geo load balancing. After lot of research we found Dyn Inc solution was very cost efficient and had products that met our needs. We used Dynect CDN Manager since ELB doesn't have static IP address. Also we defined rules in CDN to send 100% of our west traffic to Amazon US-WEST ELB, 100% of east traffic to US-EAST ELB and the rest we split traffic to both regions.
Behind Amazon ELB we took advantage of multiple zones in each region purely for DR than for geo-load balancing, we had equal number of web servers in all zones, used central RDS in each region for DB writes, read replica's in each zone for reads. It looks easy when you read but it was interesting and tough journey for my team as they had to build and implement the entire solution in 3 months.
Note: Our support from Dyn Inc is really good and sometimes wonder whether our sales contact John D'Amato is sales or support person.
I am thinking of next topic to be either cloud providers or DB Sharding. As I mentioned earlier though we started with DB sharding moved to BigData.
Hope you enjoyed my first post!