For years a war has been fought in the software architecture trenches between the ideal of decentralized services and the power and practicality of centralized services. Centralized architectures, at least at the management and control plane level, are winning. And Google not only agrees, they are enthusiastic adopters of this model, even in places you don't think it should work.

Here's an excerpt from Google Lifts Veil On “Andromeda” Virtual Networking, an excellent article by Timothy Morgan, that includes a money quote from Amin Vahdat, distinguished engineer and technical lead for networking at Google:

Like many of the massive services that Google has created, the Andromeda network has centralized control. By the way, so did the Google File System and the MapReduce scheduler that gave rise to Hadoop when it was mimicked, so did the BigTable NoSQL data store that has spawned a number of quasi-clones, and even the B4 WAN and the Spanner distributed file system that have yet to be cloned.

 

"What we have seen is that a logically centralized, hierarchical control plane with a peer-to-peer data plane beats full decentralization,” explained Vahdat in his keynote. “All of these flew in the face of conventional wisdom,” he continued, referring to all of those projects above, and added that everyone was shocked back in 2002 that Google would, for instance, build a large-scale storage system like GFS with centralized control. “We are actually pretty confident in the design pattern at this point. We can build a fundamentally more efficient system by prudently leveraging centralization rather than trying to manage things in a peer-to-peer, decentralized manner.”

The context of the article is Google's impressive home brew SDN (software defined network) system that uses a centralized control architecture instead of the Internet's decentralized Autonomous System model, which thinks of the Internet as individual islands that connect using routing protocols.

SDN completely changes that model as explained by Greg Ferro: