Making a system work in one datacenter is hard. Now imagine you move to two datacenters. Now imagine you need to support multiple geographically distributed datacenters. That’s the journey described in another excellent and thought provoking paper from Google: High-Availability at Massive Scale: Building Google’s Data Infrastructure for Ads.

The main idea of the paper is that the typical failover architecture used when moving from a single datacenter to multiple datacenters doesn’t work well in practice. What does work, where work means using fewer resources while providing high availability and consistency, is a natively multihomed architecture:

Our current approach is to build natively multihomed systems. Such systems run hot in multiple datacenters all the time, and adaptively move load between datacenters, with the ability to handle outages of any scale completely transparently. Additionally, planned datacenter outages and maintenance events are completely transparent, causing minimal disruption to the operational systems. In the past, such events required labor-intensive efforts to move operational systems from one datacenter to another

The use of “multihoming” in this context may be confusing because multihoming usually refers to a computer connected to more than one network. At Google scale perhaps it’s just as natural to talk about connecting to multiple datacenters.

Google has built several multi-homed systems to guarantee high availability (4 to 5 nines) and consistency in the presence of datacenter level outages: F1 / Spanner: Relational Database; Photon: Joining Continuous Data Streams; Mesa: Data Warehousing. The approach taken by each of these systems is discussed in the paper, as are the many challenges is building a multi-homed system: Synchronous Global State; What to Checkpoint; Repeatable Input; Exactly Once Output.

The huge constraint here is having availability and consistency. This highlights the refreshing and continued emphasis Google puts on making even these complex systems easy for programmers to use:

The simplicity of a multi-homed system is particularly valuable for users. Without multi-homing, failover, recovery, and dealing with inconsistency are all application problems. With multi-homing, these hard problems are solved by the infrastructure, so the application developer gets high availability and consistency for free and can focus instead on building their application.

The biggest surprise in the paper was the idea that a multihomed system can actually take far fewer resources than a failover system:

In a multi-homed system deployed in three datacenters with 20% total catchup capacity, the total resource footprint is 170% of steady state. This is dramatically less than the 300% required in the failover design above

What’s Wrong With Failover?