Grokking Database Fundamentals for Tech Interviews
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Key-Value Stores
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Key-value stores are a type of NoSQL database designed for simplicity and speed. They store data as key-value pairs, where a key acts as a unique identifier, and the value holds the associated data. These databases are ideal for use cases requiring fast read and write operations, such as caching, session storage, and real-time data applications.

Data Modeling in Key-Value Databases

In key-value databases, data is stored as pairs, like this:

  • Key: A unique identifier, such as a string or number.
  • Value: The associated data, which can be anything—text, JSON, binary, or even a file.

Example

Consider the example below.

  • Key: Student_Name, Value: John
  • Key: Age, Value: 19
  • Key: City_Name, Value: Washington
Image

Here, the key uniquely identifies the user, and the value stores the user's cart items in JSON format.

Key-value databases work well when:

  1. You need fast access to data.
  2. Relationships between data are minimal or can be handled in the application code.
  3. Each key-value pair is independent of others.

1. Redis

Redis (Remote Dictionary Server) is an in-memory key-value store known for its speed and versatility.

  • Features:

    • Stores data in memory for extremely fast access.
    • Supports multiple data types like strings, hashes, lists, sets, and sorted sets.
    • Provides advanced features like pub/sub messaging, Lua scripting, and automatic eviction policies.

  • Example Use Case:

    • Caching: An e-commerce platform uses Redis to cache product information. When a user searches for a product, Redis retrieves the data quickly, reducing database load.

2. Riak

Riak is a distributed key-value store designed for high availability and fault tolerance.

  • Features:

    • Distributed architecture ensures data replication and fault tolerance.
    • Implements eventual consistency, meaning all replicas converge to the same state over time.
    • Scalability through horizontal scaling (adding more servers).

  • Example Use Case:

    • Session Management: A gaming platform uses Riak to store user sessions. Even during server failures, users’ session data remains available due to Riak’s replication features.

Use Cases of Key-Value Stores

1. Caching

Key-value stores are excellent for caching frequently accessed data to improve application performance.

Example:

  • A weather app caches recent weather data in Redis to serve user requests faster.

2. Session Storage

Web applications often store user session data in key-value stores to manage login states efficiently.

Example:

  • An online store uses Redis to track active user sessions, ensuring quick authentication and session updates.

3. Real-Time Analytics

Key-value stores handle high-velocity data for real-time applications like analytics dashboards.

Example:

  • A stock market application uses Redis to update and retrieve live stock prices in milliseconds.

4. Leaderboards

For applications like gaming or fitness trackers, key-value stores manage leaderboards efficiently.

Example:

  • A fitness app uses Redis sorted sets to rank users based on their daily step count.

Performance Considerations

Key-value stores are designed for speed and scalability, but there are trade-offs:

Advantages

  1. Fast Read/Write Operations: Data is accessed via a unique key, eliminating complex queries.
  2. Scalability: Most key-value stores support horizontal scaling, making them suitable for large-scale applications.
  3. Flexibility: Values can hold various data types, from simple strings to complex objects like JSON.

Limitations

  1. Limited Querying: They lack advanced querying capabilities like joins and filtering, requiring application-side logic for such operations.
  2. Not Ideal for Complex Relationships: Key-value stores are unsuitable for scenarios involving intricate relationships between data.
  3. Memory Usage: For in-memory stores like Redis, data size is constrained by available memory.

Key-value stores provide a powerful solution for fast, scalable, and straightforward data storage. With systems like Redis and Riak, they excel in caching, session management, and real-time analytics, making them indispensable for modern, high-performance applications.

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