Site Reliability Engineering (SRE) combines software and systems engineering to build and run large-scale, massively distributed, fault- tolerant systems. SRE ensures that Google's services—both internally critical and externally-visible systems—have reliability, uptime appropriate to users' needs and a fast rate of improvement. Additionally SRE’s keep an ever-watchful eye on systems capacity and performance. Much of the software development focuses on optimizing existing systems, building infrastructure and eliminating work through automation.