DynamoDB: An Inside Look Into NoSQL – Part 1

In our earlier posts (here and here), we introduced the Hadoop ecosystem & explained its various components using a real world example of the retail industry. We now possess a fair idea of the advantages of Big Data. NoSQL datastores are being used extensively in real-time & Big Data applications, which is why a look into its internals would help us make better design decisions in our applications.

NoSQL datastores provide a mechanism for retrieval and storage of data items that is modeled in a non-tabular manner. Simplicity of design, horizontal scalability and control over availability form the motivations for this approach. NoSQL is governed by the CAP theorem in the same way RDBMS is governed by ACID properties.


NoSQL Triangle (Credit)

From the AWS stable, DynamoDB is the perfect choice if you are looking for a NoSQL solution. DynamoDB is a “fast, fully managed NoSQL database service that makes it simple and cost-effective to store and retrieve any amount of data, and serve any level of request traffic. Its guaranteed throughput and single-digit millisecond latency make it a great fit for gaming, ad tech, mobile and many other applications.” Since it is a fully managed service, you need not worry about provisioning & managing of the underlying infrastructure. All the heavy-lifting is taken care for you.

Majority of the documentation available on the Net are how-to-get-started guides with examples of DynamoDB API usage. Let’s look at the thought process and design strategies that went into the making of DynamoDB.

“DynamoDB uses a synthesis of well known techniques to achieve scalability and availability: Data is partitioned and replicated using consistent hashing, and consistency is facilitated by object versioning. The consistency among replicas during updates is maintained by a quorum-like technique and a decentralized replica synchronization protocol. DynamoDB employs a gossip based distributed failure detection and membership protocol. Dynamo is a completely decentralized system with minimal need for manual administration. Storage nodes can be added and removed from DynamoDB without requiring any manual partitioning or redistribution.” You must be wondering – “Too much lingo for one paragraph”. Fret not, why fear when I am here 🙂 Let’s take one step at a time, shall we!

Requirements and Assumptions

This class of NoSQL storage system has the following requirements –

  • Query Model: A “key” uniquely identifies a data item. Read and write operations are performed on this data item. It must be noted that no operation spans across multiple data items. There is no need for relational schema and DynamoDB works best when a single data item is less than 1MB.
  • ACID Properties: As mentioned earlier, there is no need for relational schema and hence ACID (Atomicity, Consistency, Isolation, Durability) properties are not required. The industry and the academia acknowledge that ACID guarantees lead to poor availability. Dynamo targets applications that operate with weaker consistency if it results in high availability.
  • Efficiency: DynamoDB needs to run on commodity hardware infrastructure. Stringent SLA (Service Level Agreement) ensure that latency and throughput requirements are met for the 99.9% percentile of the distribution. But everything has a catch – the tradeoffs consist of performance, cost, availability and durability guarantees.

In subsequent articles, we will look into Design Considerations & System Architecture.

Article authored by Vijay Olety

References

  1. http://www.allthingsdistributed.com/files/amazon-dynamo-sosp2007.pdf

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