Maintainability

Operability Make it easy for operations teams to keep the system running smoothly.
Simplicity Make it easy for new engineers to understand the system, by removing as much complexity as possible from the system. (Note this is not the same as simplicity of the user interface.)
Evolvability Make it easy for engineers to make changes to the system in the future, adapting it for unanticipated use cases as requirements change. Also known as extensibil‐ity, modifiability, or plasticity.

Operability

Monitoring the health of the system and quickly restoring service if it goes into a bad state
Tracking down the cause of problems, such as system failures or degraded per‐formance
Keeping software and platforms up to date, including security patches
Keeping tabs on how different systems affect each other, so that a problematic change can be avoided before it causes damage
Anticipating future problems and solving them before they occur (e.g., capacity planning)
Establishing good practices and tools for deployment, configuration manage‐ment, and more
Performing complex maintenance tasks, such as moving an application from one platform to another
Maintaining the security of the system as configuration changes are made
Defining processes that make operations predictable and help keep the produc‐tion environment stable
Preserving the organization’s knowledge about the system, even as individual people come and go

Providing visibility into the runtime behavior and internals of the system, with good monitoring
Providing good support for automation and integration with standard tools
Avoiding dependency on individual machines (allowing machines to be taken down for maintenance while the system as a whole continues running uninter‐rupted)
Providing good documentation and an easy-to-understand operational model (“If I do X, Y will happen”)
Providing good default behavior, but also giving administrators the freedom to override defaults when needed
Self-healing where appropriate, but also giving administrators manual control over the system state when needed
Exhibiting predictable behavior, minimizing surprises