# Apache CouchDB — Seamless Multi-Master Sync Database

> Apache CouchDB is a document-oriented NoSQL database that uses JSON for documents, JavaScript for queries, and HTTP for its API. Its signature feature is multi-master replication, enabling offline-first applications that sync reliably across distributed nodes.

## Install

Save in your project root:

# Apache CouchDB — Seamless Multi-Master Sync Database

## Quick Use
```bash
docker run -d -p 5984:5984 
  -e COUCHDB_USER=admin -e COUCHDB_PASSWORD=password 
  apache/couchdb:3
# Access Fauxton web UI at http://localhost:5984/_utils
```

## Introduction
CouchDB was designed for reliability and ease of replication. Its HTTP-based API and multi-master sync protocol make it a natural fit for mobile, IoT, and edge computing scenarios where nodes go offline and need to reconcile data later. The database is built in Erlang for fault tolerance.

## What CouchDB Does
- Stores schema-free JSON documents accessible via a RESTful HTTP API
- Replicates data bidirectionally between any number of CouchDB instances
- Resolves conflicts deterministically and exposes them for application-level resolution
- Provides MapReduce views and Mango queries for flexible data retrieval
- Includes Fauxton, a built-in web administration interface

## Architecture Overview
CouchDB uses an append-only B-tree storage engine that never overwrites data, making it crash-resistant. Each document carries a revision tree for conflict detection during replication. The replication protocol uses a changes feed to propagate updates incrementally between nodes. In clustered mode, CouchDB distributes data across shards using consistent hashing with quorum-based reads and writes.

## Self-Hosting & Configuration
- Deploy via Docker, native packages for Linux and macOS, or Snap
- Configure in `local.ini` or via the HTTP `_node/_local/_config` endpoint
- Set up clustering by joining nodes through the `/_cluster_setup` wizard
- Enable SSL by providing certificate and key paths in configuration
- Tune compaction settings to reclaim disk space from append-only storage

## Key Features
- Multi-master replication works across data centers and offline devices
- HTTP API means any language with an HTTP client can interact with CouchDB
- Append-only storage ensures durability and crash recovery without journals
- Mango query language provides declarative JSON-based queries with indexing
- Built-in Fauxton UI for database management without external tools

## Comparison with Similar Tools
- **MongoDB** — richer query language and aggregation; CouchDB excels at multi-master replication
- **PouchDB** — JavaScript client database that syncs with CouchDB using the same protocol
- **RethinkDB** — real-time push queries; CouchDB focuses on reliable replication and offline sync
- **Couchbase** — enterprise-focused with memory-first architecture; CouchDB is simpler and fully open source
- **Firebase Firestore** — managed cloud sync database; CouchDB is self-hosted and open source

## FAQ
**Q: How does CouchDB handle conflicts?**
A: It picks a deterministic winner based on revision history depth but preserves all conflicting revisions. Applications can query conflicts and merge them as needed.

**Q: Is CouchDB good for real-time applications?**
A: CouchDB provides a changes feed for event-driven patterns, but it is optimized for eventual consistency and replication rather than real-time pub/sub.

**Q: Can CouchDB scale horizontally?**
A: Yes. CouchDB 2.x and 3.x support clustering with automatic sharding and configurable quorum levels.

**Q: What language is CouchDB written in?**
A: Erlang, chosen for its strong concurrency model and fault-tolerance properties.

## Sources
- https://github.com/apache/couchdb
- https://docs.couchdb.org/en/stable/

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Source: https://tokrepo.com/en/workflows/4a643015-3b64-11f1-9bc6-00163e2b0d79
Author: AI Open Source