CouchDB Technology Intelligence

What is CouchDB?

Apache CouchDB is an open-source NoSQL document database that uses JSON for data storage, JavaScript for queries, and HTTP for its API. Created by Damien Katz in 2005 and now an Apache Software Foundation project, CouchDB pioneered the concept of a database accessible entirely through RESTful HTTP endpoints.

CouchDB's philosophy embraces "offline-first" design and eventual consistency. Its unique multi-master replication allows databases to sync across servers, data centers, or even to mobile devices. When conflicts occur during sync, CouchDB preserves all versions, letting applications resolve conflicts appropriately.

The database is built in Erlang, a language designed for fault-tolerant, distributed systems. This foundation enables CouchDB's reliability and clustering capabilities. CouchDB influenced many modern databases including Couchbase (a fork) and PouchDB (JavaScript implementation for browsers).

CouchDB Architecture

Document Model: Schema-free JSON documents with unique IDs. Documents can contain nested objects, arrays, and attachments (binary files). Revision tracking enables conflict detection during replication.

HTTP API: Every operation uses standard HTTP methods. GET retrieves documents, PUT creates/updates, DELETE removes. Curl and any HTTP client work as database clients. No special drivers required.

Views & MapReduce: JavaScript functions define views. Map functions emit key-value pairs from documents. Reduce functions aggregate results. Views are incrementally built and cached as B-trees.

Mango Queries: MongoDB-style JSON query syntax added in CouchDB 2.0. Declarative queries without writing JavaScript. Automatic index selection. Familiar to developers from other document databases.

Multi-Master Replication: Bidirectional sync between any CouchDB instances. Continuous or one-shot replication. Conflict-free for non-overlapping changes. Deterministic winner selection for conflicts with full version history preserved.

Clustering: CouchDB 2.0+ supports clusters with automatic sharding. Quorum reads/writes for consistency. Nodes can join/leave dynamically. Built-in cluster management via Fauxton UI.

CouchDB Use Cases

Offline-First Applications: PouchDB in browsers syncs with CouchDB servers. Mobile apps work offline and sync when connected. Field service apps, mobile surveys, and disconnected workflows.

Multi-Site Synchronization: Replicate data between geographic locations. Each site operates independently. Eventual consistency across data centers. Disaster recovery built-in.

Document Storage: CMS backends, medical records, and legal documents. Attachments store binary files with documents. Full revision history for compliance.

Event Logging: Append-only workloads with immutable documents. IoT sensor data collection. Audit trails with guaranteed preservation.

API Backend: Direct HTTP API for mobile and web apps. No backend code for simple CRUD operations. Validation functions enforce business rules.

Semi-Structured Data: Documents with varying schemas. Product catalogs with different attributes. Content management with flexible metadata.

CouchDB Examples

HTTP API Operations

# Create a database
curl -X PUT http://localhost:5984/mydb

# Create a document
curl -X PUT http://localhost:5984/mydb/doc1 \
  -H "Content-Type: application/json" \
  -d '{"name": "John", "email": "[email protected]", "roles": ["admin", "user"]}'

# Get a document
curl http://localhost:5984/mydb/doc1

# Mango query - find users by role
curl -X POST http://localhost:5984/mydb/_find \
  -H "Content-Type: application/json" \
  -d '{
    "selector": {"roles": {"$elemMatch": {"$eq": "admin"}}},
    "fields": ["_id", "name", "email"],
    "limit": 10
  }'

# Create a view
curl -X PUT http://localhost:5984/mydb/_design/users \
  -H "Content-Type: application/json" \
  -d '{
    "views": {
      "by_email": {
        "map": "function(doc) { if(doc.email) emit(doc.email, doc.name); }"
      }
    }
  }'

# Start replication
curl -X POST http://localhost:5984/_replicate \
  -H "Content-Type: application/json" \
  -d '{"source": "mydb", "target": "http://remote:5984/mydb", "continuous": true}'

CouchDB Benefits

HTTP-Native: RESTful API needs no drivers. Any language with HTTP works. Curl for testing, debugging, and automation. Familiar HTTP semantics.

Offline-First Sync: Industry-leading replication protocol. PouchDB enables browser/mobile offline. Conflict resolution preserves all data.

ACID Semantics: Full ACID compliance per document. Crash-safe append-only storage. Consistent views even during writes.

Built-in Web UI: Fauxton administrative interface. Database browsing, query testing, replication setup. No additional tools required.

Erlang Foundation: Built for distributed, fault-tolerant systems. Hot code upgrades. Process isolation prevents cascading failures.

Schema Flexibility: No migrations for document structure changes. Store any JSON. Evolution-friendly data model.

Attachments: Binary files stored with documents. Images, PDFs, and files alongside metadata. Single retrieval for document and attachments.