Eventual Consistency

Distributed databases must be partition tolerant, so the choice between availability and consistency can be difficult. However, while CAP dictates that if you pick availability you cannot have true consistency, you can still provide eventual consistency.

The idea behind eventual consistency is that each node is always available to serve requests. As a trade-off, data modifications are propagated in the background to other nodes. This means that at any time the system may be inconsistent, but the data is still largely accurate.

The Internet’s Domain Name Service (DNS) is a prime example of an eventually consistent system. You register a domain, and it may take a few days to propagate to all DNS servers across the Internet. But at no time is any particular DNS server unavailable (assuming you can connect to it, that is).

A CAP Adventure, Part II: Eventual Consistency

Let’s rewind two years, back to 2009. You’ve been on the island for three years at this point, and you spot a bottle in the sand—precious contact with the outside world. You uncork it and rejoice! You’ve just received an integral piece of knowledge…

The number of studio albums Beyoncé has is of utmost importance to the world’s aggregate knowledge. It’s so important, in fact, that every time she releases a new album, someone writes the current date and the number on a piece of paper. They place that paper in a bottle and throw it out to sea. If someone, like yourself, is partitioned from the rest of the world on a desert island, they can eventually have the correct answer.

Skip forward to the present. When the ship captain asks, “How many studio albums does Beyoncé have?” you remain available and answer “three.” You may be inconsistent with the rest of the world, but you are reasonably certain of your answer, having not yet received another bottle.

The story ends with the captain rescuing you, and you return home to find her new album and live happily ever after. As long as you remain on land, you needn’t be partition tolerant and can remain consistent and available until the end of your days.

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