36 lines
1.7 KiB
Markdown
36 lines
1.7 KiB
Markdown
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noatcards = True
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isdraft = False
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# Availability vs consistency
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## CAP theorem
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[Source: CAP theorem revisited](http://robertgreiner.com/2014/08/cap-theorem-revisited)
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In a distributed computer system, you can only support two of the following guarantees:
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- Consistency - Every read receives the most recent write or an error
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- Availability - Every request receives a response, without guarantee that it contains the most recent version of the information
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- Partition Tolerance - The system continues to operate despite arbitrary partitioning due to network failures
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_Networks aren't reliable, so you'll need to support partition tolerance. You'll need to make a software tradeoff between consistency and availability._
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#### CP - consistency and partition tolerance
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Waiting for a response from the partitioned node might result in a timeout error. CP is a good choice if your business needs require atomic reads and writes.
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#### AP - availability and partition tolerance
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Responses return the most recent version of the data, which might not be the latest. Writes might take some time to propagate when the partition is resolved.
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AP is a good choice if the business needs allow for [eventual consistency](https://github.com/donnemartin/system-design-primer#eventual-consistency) or when the system needs to continue working despite external errors.
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## Source(s) and further reading
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- [CAP theorem revisited](http://robertgreiner.com/2014/08/cap-theorem-revisited/)
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- [A plain english introduction to CAP theorem](http://ksat.me/a-plain-english-introduction-to-cap-theorem/)
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- [CAP FAQ](https://github.com/henryr/cap-faq)
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