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Lesson 9 of 17

16. Consistency and CAP

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The picture to keep in your head
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16.1 What does "consistent" actually mean?
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16.2 What is strong consistency?
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16.3 What is eventual consistency?
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16.4 What are read-your-writes and monotonic reads?
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16.5 What is a network partition?
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16.6 What does the CAP theorem actually say?
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16.7 What do CP and AP systems look like in practice?
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16.8 What is PACELC?
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16.9 How do quorums give tunable consistency?
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16.10 How do you choose consistency for each feature?
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Memory card and where the analogy breaks
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Practice: five levels
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Interview questions
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Solutions
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Sources
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Code lab
16. Consistency and CAP

16.8 What is PACELC?

3 min read+10 XP

At the bank: CAP talks about the day the line is cut. But every day, phoning the other counter before each withdrawal adds a few minutes. A bank must also decide: on normal days, do we phone first (consistent, slower) or skip the call (faster, slightly risky)?

In the system: Daniel Abadi's PACELC (2012) completes the picture:

If there's a Partition, choose Availability or Consistency; Else (normal operation), choose Latency or Consistency.

PACELC: during a partition choose A or C; otherwise choose latency or consistency.

Figure: even with no failures, waiting for other replicas to agree takes round trips.

The "else" part matters more day to day, because partitions are rare but every request pays latency. A strongly consistent write that must reach a replica in another region waits at least one cross-region round trip. With an assumed 120 ms Mumbai–Frankfurt round trip, that's at least 120 ms per write, compared with a couple of milliseconds for a local write.

In Abadi's paper, Dynamo-style systems (Dynamo, Cassandra, Riak) in their default settings are PA/EL: during partitions they stay available, and normally they favour low latency. Fully ACID systems that coordinate every write (VoltDB is one example in the paper) are PC/EC: they pay in latency and errors to stay consistent.

Memory code: "On bad days, choose answer or accuracy; on good days, choose speed or accuracy."