Core features of CouchDB

CouchDB is an open-source, document-oriented NoSQL database that focuses on ease of use, scalability, and reliability. Here are the core features of CouchDB:

1. Document-Based Storage

• JSON Documents: CouchDB stores data as JSON (JavaScript Object Notation) documents. Each document is self-contained and can store data in a flexible format, including nested structures.
• Schema-free: There is no fixed schema for documents. This makes CouchDB suitable for applications that require flexible and evolving data models.
• Unique _id: Each document has a unique identifier (_id) that serves as its primary key, allowing easy retrieval.

2. ACID Compliance

• Atomicity, Consistency, Isolation, Durability: CouchDB guarantees ACID compliance for each individual document. This ensures that even in the case of failures, a document will either be fully written or not at all.
• Multi-Version Concurrency Control (MVCC): CouchDB uses MVCC to ensure concurrent reads and writes without causing conflicts. This is especially useful for distributed systems.

3. Built-in Replication

• Master-Master Replication: CouchDB supports multi-master replication, meaning that data can be synchronized between multiple CouchDB instances, either within the same server or across multiple geographically distributed nodes.
• Continuous Replication: You can set up continuous replication between CouchDB databases, making it easy to sync data across devices, servers, or environments in real-time.
• Conflict Resolution: When conflicts occur during replication (e.g., when a document is updated in multiple locations), CouchDB stores multiple versions of a document and allows applications to resolve conflicts manually or automatically.

4. RESTful HTTP API

• Simple HTTP Interface: CouchDB is designed to be accessed over HTTP, and most operations (create, read, update, delete) are done via standard HTTP methods (GET, PUT, POST, DELETE).
• JSON Over HTTP: Data is exchanged in JSON format, making it easy to integrate with web technologies and other HTTP-based applications.
• Easy Integration: Because CouchDB uses RESTful APIs, it integrates well with modern web applications and client-side technologies like JavaScript, Node.js, and mobile apps.

5. MapReduce Views

• Views for Querying: CouchDB allows you to define MapReduce views using JavaScript. You write map functions that process documents and emit keys and values, which are then indexed by CouchDB.
• Flexible Querying: Views provide a powerful, flexible way to query your data. You can use views to retrieve documents based on complex conditions and even perform aggregation operations.
• Indexing: CouchDB indexes views, making subsequent queries faster. The MapReduce paradigm can be used to aggregate, filter, or transform data according to your needs.

6. Fault Tolerance

• Replication for Reliability: CouchDB’s replication mechanism ensures that data is reliably distributed across multiple nodes or machines, ensuring fault tolerance and high availability.
• Automatic Failover: In the event of a failure or crash, CouchDB is designed to handle the recovery process and ensures minimal data loss.
• Crash Recovery: CouchDB ensures that data is not lost due to crashes by using a write-ahead log (WAL) and MVCC.

7. Scalability and Distributed Architecture

• Clustering: CouchDB can be scaled horizontally by running multiple instances in a cluster. Data can be distributed across different nodes, allowing CouchDB to handle large datasets and high traffic loads efficiently.
• Sharding: CouchDB supports data sharding, which allows documents to be distributed across multiple servers in a cluster, providing scalability for large applications.
• Replicated Databases: Databases themselves can be replicated, so you can have multiple copies of the same database running on different nodes, ensuring high availability and fault tolerance.

8. Attachment Support

• Storing Binary Files: CouchDB allows you to store not only JSON documents but also binary files as attachments to documents (e.g., images, PDFs). These attachments are stored efficiently and can be retrieved along with their respective documents.
• Streaming: CouchDB allows you to stream large attachments directly to and from clients without requiring the entire document to be loaded into memory.

9. Eventual Consistency

• Distributed Nature: CouchDB prioritizes availability and partition tolerance (according to the CAP theorem), meaning that in a distributed environment, it may return stale data temporarily but will eventually synchronize data across all nodes to ensure consistency.
• Eventually Consistent Reads: If a read is requested from a node that has not yet received the latest updates, it may return older data. However, this is eventually reconciled across nodes.

10. Authentication and Security

• Authentication: CouchDB supports basic authentication, allowing users to authenticate using a username and password. It also supports session-based authentication.
• Access Control Lists (ACLs): CouchDB provides support for permissions and access control. You can assign roles to users and restrict access to databases, documents, and views.
• SSL Support: CouchDB supports SSL encryption for secure communication over HTTP, making it suitable for production environments.

11. Versioning

• Document Revision: CouchDB supports document versioning using the _rev field. This allows for managing document revisions and handling conflicts during concurrent updates.
• Conflict Resolution: When two nodes update the same document independently, CouchDB tracks and stores the conflict, allowing the application to handle or resolve the conflict manually.

12. MapReduce for Custom Queries

• Map and Reduce Functions: CouchDB provides a flexible way to query data using MapReduce functions written in JavaScript. You can use map functions to filter data and reduce functions to perform aggregation, allowing for powerful custom queries.

13. Full-Text Search (via Plugins)

• While CouchDB does not natively support full-text search, it can integrate with external plugins (like CouchDB-lucene or CouchDB-Fulltext), enabling full-text search capabilities on the data stored in the database.

14. Multi-Document Operations

• Bulk Operations: CouchDB allows you to perform bulk operations, such as bulk inserts or updates, through batch processing. This is useful for optimizing the performance of large-scale insert or update operations.
• Atomic Bulk Writes: CouchDB guarantees atomicity for multi-document updates when using the bulk API.