Introduction to key-value stores

Key-value stores are distributed hash tables (DHTs).A distributed hash table (DHT) is a decentralized storage system that provides lookup and storage schemes similar to a hash table, storing key-value pairs. Source: A key is generated by the hash function and should be unique. In a key-value store, a key binds to a specific value and doesn't assume anything about the structure of the value. A value can be a blob, image, server name, or anything the user wants to store against a unique key.


Let’s list the requirements of designing a key-value store to overcome the problems of traditional databases.

Functional requirements

The functional requirements are as follows:

  • Configurable service: Some applications might have a tendency to trade strong consistency for higher availability. We need to provide a configurable service so that different applications could use a range of consistency models. We need tight control over the trade-offs between availability, consistency, cost-effectiveness, and performance.

  • Ability to always write: The applications should always have the ability to write into the key-value storage.

  • Hardware heterogeneity: The system shouldn’t have distinguished nodes. Each node should be functionally able to do any task.

Non-functional requirements

The non-functional requirements are as follows:

  • Scalable: Key-value stores should run on tens of thousands of servers distributed across the globe. Incremental scalability is highly desirable, and the system should be able to handle a large number of users.

  • Available: We need to provide continuous service, so availability is very important. This property is configurable.

  • Fault tolerance: The key-value store should operate uninterrupted despite failures in servers or their components.

Now that you understand the functional and non-functional requirements of a key-value store, what do you think are the key differences between key-value stores and traditional databases?

In what scenarios are key-value stores particularly advantageous?

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