Stage 1 · Lesson 3 of 10
Availability, redundancy and SLOs
is the share of time a system does its job. It is usually quoted in nines: 99.9% allows about 43 minutes of downtime a month, and 99.99% about 4 minutes. When a request passes through several components in a row, their availabilities multiply.
Three services each at 99.9% give roughly 99.7% together. , spare copies running side by side, raises availability, as long as the copies do not fail together. To remove single points of failure, ask of every box on the diagram: what happens when this one dies?
Three terms describe the promise you make. An (service level indicator) is the measurement, such as the share of requests under 300 ms. An SLO (service level objective) is the target you commit to internally, such as 99.9% over 30 days. An SLA (service level agreement) is the contract, with penalties if you miss it.
Common trappromising five nines while depending on a single database.
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Resources for this stage
Start with· follow one of these
- CoursePaidGrokking System Design Fundamentals
Design Gurus · Beginner · Subscription
Load balancing, API gateway, proxies, caching, CAP and Bloom filters: week 1 of the plan in order
- GitHub repoFreeThe System Design Primer
Donne Martin · Intermediate
Free: its scalability, latency, availability, CDN, load balancer and cache sections
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