Information on this page is taken from Designing Data-Intensive Applications by Martin Kleppmann.
A system is reliable when it continues to work correctly even in the face of adversity. The things that can go wrong are called faults, and systems that anticipate faults and can cope with them are called fault-tolerant or resilient.
Fault vs. Failure
A fault is usually defined as one component of the system deviating from its spec, whereas a failure is when the system as a whole stops providing the required service to the user. It is impossible to reduce the probability of a fault to zero; therefore it is usually best to design fault-tolerance mechanisms that prevent faults from causing failures.
Reliability is sometimes seen as the probability a system will fail in a given period.
Types of Faults
Hard disk crash, faulty RAM, and power blackout are all examples of hardware faults. Traditionally, redundancy is added to individual hardware components to help reduce system failure rate. Modern cloud platforms also commonly use software fault-tolerance techniques.
Systematic errors within the system are hard to anticipate and tend to cause many more system failures than uncorrelated hardware faults. There is no quick solution to the problem of systematic faults in software. Lots of small things can help: carefully thinking about assumptions and interactions in the system; thorough testing; process isolation; allowing processes to crash and restart; measuring, monitoring, and analyzing system behavior in production.
Humans design and build software systems, and the operators who keep the systems running are also human. Even when they have the best intentions, humans are known to be unreliable. The best systems combine several approaches to help reduce human errors:
- Design systems in a way that minimizes opportunities for error. For example, well-designed abstractions, APIs, and admin interfaces make it easy to do "the right thing" and discourage "the wrong thing."
- Decouple the places where people make the most mistakes from the places where they cause failures. In particular, provide fully featured non-production sandbox environments where people can explore and experiment safely, using real data, without affecting real users.
- Test thoroughly at all levels, from unit tests to whole-system integration tests and manual tests.
- Allow quick and easy recovery from human errors, to minimize the impact in the case of a failure. For example, make it fast to roll back configuration changes, roll out new code gradually (so that any unexpected bugs affect only a small subset of users), and provide tools to recompute data (in case it turns out that the old computation was incorrect).
- Set up detailed and clear monitoring, such as performance metrics and error rates.
- Implement good management practices and training.
Reliability vs. Availability
Availability is the time a system remains operational to perform its required function in a specific period. If a system is reliable, it is available. However, if it is available, it is not necessarily reliable. In other words, high reliability contributes to high availability, but it is possible to achieve a high availability even with an unreliable product by minimizing repair time and ensuring that spares are always available when they are needed.