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The Calculus of Service Availability
- yarapavan 9y agoImplication 1: Rule of the extra 9 A service cannot be more available than the intersection of all its critical dependencies. If your service aims to offer 99.99 percent availability, then all of your critical dependencies must be significantly more than 99.99 percent available. Internally at Google, we use the following rule of thumb: critical dependencies must offer one additional 9 relative to your service—in the example case, 99.999 percent availability—because any service will have several critical dependencies, as well as its own idiosyncratic problems. This is called the "rule of the extra 9." If you have a critical dependency that does not offer enough 9s (a relatively common challenge!), you must employ mitigation to increase the effective availability of your dependency (e.g., via a capacity cache, failing open, graceful degradation in the face of errors, and so on.)