Eventual Consistency Explained for Techies

Preamble: Have a look at my previous article titled “Eventually Consistency Explained for non-Techies”

Eventual and Weak Consistency

SimpleDB is eventually consistent. Eventual consistency is a version of weak consistency — you may not see the latest writes committed to the system.

Imagine that you have a system of N nodes. Of these, W nodes are involved in any write sent to the system and R nodes are contacted on any read from the system. Strong consistency can be achieved if R+W > N. In other words, if the read sets and write sets overlap, the read can discover the most recent write to the system.

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The “Consistency, Not Accuracy” Principle

Preamble: Read my post “The CAP Theorem distilled”

In my previous post, I started talking about the “Consistency, Not Accuracy” Principle (a.k.a. The CNA Principle)

Essentially, in order to scale your web site and to keep running amidst unpredictable network and system outages, you need to have a replicated, fault-tolerant data store that accepts reads and writes in multiple locations. One replica might be in California and another might be in Virginia. If California were to fall into the great Pacific, your web site should still work and your users should be none the wiser.

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About Me
A blog describing my work in building websites that millions of people visit. I'm a senior member of LinkedIn's Distributed Data Systems team. I previously held technical and leadership roles at Netflix, Etsy, eBay & Siebel Systems.
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