What Are Key Value Stores and When Should You Use Them?

Definition
Key-value stores map unique keys to opaque values with constant-time lookups. The simplest database model: GET(key) returns value, SET(key, value) stores it. This simplicity enables extreme performance at massive scale.
The Problem They Solve
Relational databases excel at complex queries but struggle with simple high-throughput access patterns. Reading one row by primary key still requires query parsing, planning, and index traversal. Key-value stores strip away everything except the lookup, achieving sub-millisecond latency and 100,000+ operations per second on a single node.
When To Use Them
Session storage: Fast reads, automatic expiration, no relationships needed. Caching: Reduce load on slower databases for frequently accessed data. Rate limiting: Atomic counters with expiration. Feature flags: Rapid configuration changes across services. Distributed locks: Coordinate access across multiple services. The pattern is consistent: simple access patterns requiring extreme speed.
When Not To Use Them
Key-value stores sacrifice querying flexibility. You cannot search by value content, join across keys, or run aggregations. If you need to find "all users in California" without knowing their keys, a key-value store forces you to scan every record or maintain a separate index. Use relational or document databases when query patterns are complex or unknown upfront.
Key Design Principles
Keys are typically strings encoding entity relationships: session:user_123:abc789 or cache:product:456. This namespacing prevents collisions and enables pattern-based operations. Keep keys under 256 bytes since keys are stored repeatedly in indexes and replicas.

💡 Key Takeaways

✓Key-value stores trade query flexibility for raw speed, achieving sub-millisecond latency and 100,000+ ops/sec per node

✓Use for simple access patterns: caching, sessions, rate limiting, feature flags, distributed locks

✓Avoid when you need to query by value content, join data, or run aggregations

✓Keys should be hierarchical strings under 256 bytes with clear namespacing

✓The simplicity is the feature, not a limitation, as it enables horizontal scaling

📌 Interview Tips

1When asked about database choice, identify if the access pattern is simple key lookup, as key-value stores are often overlooked for obvious use cases like sessions and caching

2Discuss the trade-off explicitly: what query capabilities you lose for the performance gains

3Mention specific numbers: sub-ms latency, 100K+ ops/sec, to show you understand the performance characteristics

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