While most companies have the luxury of taking their time to evaluate the viability of using cloud infrastructure services before they implement them, Zynga had to go to the cloud because it had no other choice. Zynga CTO Allan Leinwand said FarmVille, one of the social-gaming company's most popular products, grew to 25m daily active users within five months of its launch.

Zynga's infrastructure team has found itself in a situation where growth of its user base quickly outpaced provisioning time of infrastructure components, including data center space, power and cooling, CentOS servers and internet transit network capacity.

"We couldn't continue to grow traditional retail (or) wholesale data centers fast enough," he said. Leinwand delivered an opening keynote at this Thursday's DatacenterDynamics conference in San Francisco, in which he explained how the company scales its infrastructure.

In 2009, when FarmVille led to the inflection point of not being able to provision capacity fast enough, the company moved to public cloud, using VM instances through Amazon Web Services' Elastic Compute Cloud (EC2) and automating provisioning. The approach worked, but, as Leinwand put it, Zynga was only replacing capital expenses with operating ones.

The next step was to build its own private cloud. The cloud was built on several thousand x86 servers using XenServer and KVM hypervisor technology.

Contrary to the common private-cloud thinking, where multiple VMs run on each physical host, the "zCloud" has one virtual instance per physical server. The company is using private cloud because of its flexibility, which allows it to spin VMs up and down without shutting down physical servers and bringing them back up when they're needed.

"We do have an application that can consume an entire system," Leinwand said.

Through RightScale ÔÇô platform it was using to manage its Amazon cloud ÔÇô Zynga integrated the private cloud with AWS and created Amazon-like security zones on the private infrastructure. It took them less than six months to get the zCloud up and running.

Capacity of the private cloud has grown tremendously over the past two years. The number of physical servers grew by 75 times between January 2009 and January 2011.

In the planning stages, the team studied the Amazon infrastructure it was using and spent a lot of time architecting the games to minimize impact from performance issues public clouds frequently have. A recent public IaaS outage that took down numerous high-profile websites did not have any "material" effect on Zynga, Leinwand said.

Today, as it is preparing for an initial public offering, Zynga has 250m monthly users. The biggest chunk of that user base (about 88m) plays CityvVille, which is followed by Empires &Allies (about 44m), and FarmVille (about 38m), according to AppData.com.

Its other games include Zynga Poker, FrontierVille, Caf├® World, and Mafia Wars.