What types of NoSQL databases exist and when should they be used?

There are several main types of NoSQL databases, each suitable for different tasks:

1. Document-Oriented Databases (Document Stores)

   • Store data in document formats (e.g., JSON, BSON).
   • Examples: MongoDB, CouchDB.
   • Used when data has a flexible structure and changes frequently.

2. Column-Family Stores (Column-Oriented Databases)

   • Store data by columns, which is efficient for analytics and large data volumes.
   • Examples: Apache Cassandra, HBase.
   • Suitable for systems with high data throughput and the need for quick aggregation.

3. Graph Databases

   • Store data as nodes and relationships.
   • Examples: Neo4j, JanusGraph.
   • Ideal for complex relationship tasks, such as social networks, recommendations.

4. Key-Value Stores

   • Store key-value pairs.
   • Examples: Redis, Riak.
   • Used for caching, sessions, and fast key-based operations.

The choice of NoSQL database type depends on data structure requirements, access speed, scalability, and application specifics.