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Tuesday
Oct302007

Paper: Dynamo: Amazon’s Highly Available Key-value Store

Update 2: Read/WriteWeb has a good article talking about the scalability issues of relational databases and how Dynamo solves them: Amazon Dynamo: The Next Generation Of Virtual Distributed Storage. But since Dynamo is just another frustrating walled garden protected by barbed wire and guard dogs, its relevance is somewhat overstated.

Update: Greg Linden has a take on the paper where he questions some of Amazon's design choices: emphasizing write availability over fast reads, a lack of indexing support, use of random distribution for load balancing, and punting on some scalability issues.

Werner Vogels, Amazon's avuncular CTO, just announced a new paper on the internal database technology Amazon uses to handle tens of millions customers. I'll dive into more details later, but I thought you'd want to read it hot off the blog. The bad news is it won't be a service. They are keeping this tech not so secret, but very safe. Happily, it's another real-life example to learn from. As many top websites use a highly tuned key-value database at their core instead of a RDBMS, it's an important technology to understand.

From the abstract you can get a feel for what the paper is about: 

Reliability at massive scale is one of the biggest challenges we
face at Amazon.com, one of the largest e-commerce operations in
the world; even the slightest outage has significant financial
consequences and impacts customer trust. The Amazon.com
platform, which provides services for many web sites worldwide,
is implemented on top of an infrastructure of tens of thousands of
servers and network components located in many datacenters
around the world. At this scale, small and large components fail
continuously and the way persistent state is managed in the face
of these failures drives the reliability and scalability of the
software systems.

This paper presents the design and implementation of Dynamo, a
highly available key-value storage system that some of Amazon’s
core services use to provide an “always-on” experience. To
achieve this level of availability, Dynamo sacrifices consistency
under certain failure scenarios. It makes extensive use of object
versioning and application-assisted conflict resolution in a manner
that provides a novel interface for developers to use.

My first impressions after reading the paper:

  • Wow. But crap, I'll never be able to build anything like that. This is really competition through better infrastructure. Take that Google :-)
  • Their purposeful embracing of probability and manged centers of uncertainty must be dizzying for those from a RDBMS background. In a RDBMS it's all right angles. You write something and it's assumed consistent, correct, and durable. Now, how do you do this at scale across multiple data centers under failure conditions? There's the rub. So Amazon says writes must go through and we will deal with the complexities that model generates. They version objects and merge them later. Who does that? I love it, because when delve into these problems you realize you need this type of functionality, but it's too complex, so you back away and continue trying to force a square peg in a round whole. To have no fear to go where your requirements leads you is real engineering.
  • Can you imagine finding a problem in that system? I'd love to be a fly in those debugging sessions. But infrastructure takes on self-consciousness of its own when dealing with complex problems, so you just have to deal with knowing you don't know anymore.

    A lot of this thinking is driven by the CAP conjecture which states it's impossible for a web service to simultaneously guarantee consistency, availability, and partition-tolerance. When you get over your initial "that can't be true" reaction and embrace it, you get something like Dynamo.

    I'd really love to hear what you guys think about Dynamo.
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